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2.1: How Light Interacts with Matter

Difficulty Level: At Grade Created by: CK-12

Unit Overview

Contents

  • (Optional) Clear Sunscreen: Pretest
  • (Optional) Clear Sunscreen: Posttest

Name_______________

Date_______________

period_______________

Clear Sunscreen: Pretest

1. In what ways are “nano” sunscreen ingredients similar and different from other ingredients currently used in sunscreens? For each of the four categories below, indicate whether “nano” sunscreen ingredients are “similar” or “different” to organic and inorganic ingredients and explain how.

Organic Ingredients (e.g. PABA) Inorganic Ingredients (e.g. Classic Zinc Oxide used by lifeguards)
Chemical Structure Similar or Different Similar or Different
How: How:
Kinds of Light Blocked Similar or Different Similar or Different
How: How:
Way Light is Blocked Similar or Different Similar or Different
How: How:
Appearance on the Skin Similar or Different Similar or Different
How: How:

2. Briefly describe one benefit and one drawback of using a sunscreen that contains “nano” ingredients.

3. What determines if a sunscreen appears white or clear on your skin?

4. How do you know if a sunscreen has “nano” ingredients?

Name_______________

Date_______________

period_______________

Clear Sunscreen: Posttest

1. In what ways are “nano” sunscreen ingredients similar and different from other ingredients currently used in sunscreens? For each of the four categories below, indicate whether “nano” sunscreen ingredients are “similar” or “different” to organic and inorganic ingredients and explain how.

Organic Ingredients (e.g. PABA) Inorganic Ingredients (e.g. Classic Zinc Oxide used by lifeguards)
Chemical Structure Similar or Different Similar or Different
How: How:
Kinds of Light Blocked Similar or Different Similar or Different
How: How:
Way Light is Blocked Similar or Different Similar or Different
How: How:
Appearance on the Skin Similar or Different Similar or Different
How: How:

2. Briefly describe one benefit and one drawback of using a sunscreen that contains “nano” ingredients.

3. What determines if a sunscreen appears white or clear on your skin?

4. How do you know if a sunscreen has “nano” ingredients?

Introduction to Sun Protection

Contents

  • Summary of Radiation Emitted by the Sun: Student Handout
  • Clear Sunscreen Initial Ideas: Student Worksheet
  • Ultra-Violet (UV) Protection Lab Activity: Student Instructions & Worksheet

Summary of Radiation Emitted by the Sun: Student Handout

Chart of Different Kinds of Solar Radiation
Radiation Type Characteristic Wavelength \begin{align*}(\lambda)\end{align*} Energy per Photon \begin{align*}\%\end{align*} of Total Radiation Emitted by Sun Effects on Human Skin Visible to Human Eye?
UVC \begin{align*}\sim 200-290 \;\mathrm{nm}\end{align*} (Short-wave UV) High Energy \begin{align*}\sim 0\%\end{align*} (\begin{align*}< 1\%\end{align*} of all UV) DNA Damage No
UVB \begin{align*}\sim 290-320 \;\mathrm{nm}\end{align*} (Mid-range UV) Medium Energy

\begin{align*}\sim.35\%\end{align*}

(\begin{align*}5\%\end{align*} of all UV)

Sunburn DNA Damage Skin Cancer No
UVA \begin{align*}\sim 320-400 \;\mathrm{nm}\end{align*} (Long-wave UV) Low Energy

\begin{align*}\sim 6.5\%\end{align*}

(\begin{align*}95\%\end{align*} of all UV)

Tanning Skin Aging DNA Damage Skin Cancer No
Visible \begin{align*}\sim 400-800 \;\mathrm{nm}\end{align*} Lower Energy \begin{align*}\sim 43\%\end{align*} None Currently Known Yes
IR \begin{align*}\sim 800-120,000 \;\mathrm{nm}\end{align*} Lowest Energy \begin{align*}\sim 49\%\end{align*} Heat Sensation (high \begin{align*}\lambda\end{align*} IR) No

Graph of Radiation Emitted by the Sun by Wavelength

Name_______________

Date_______________

period_______________

Clear Sunscreen Initial Ideas: Student Worksheet

Write down your initial ideas about each question below and then evaluate how confident you feel that each idea is true. At the end of the unit, we’ll revisit this sheet and you’ll get a chance to see if and how your ideas have changed.

1. What are the most important factors to consider in choosing a sunscreen?

How sure are you that this is true?

Not sure

How sure are you that this is true?

Kind-of-Sure

How sure are you that this is true?

Very Sure

End of Unit Evaluation
2. How do you know if a sunscreen has “nano” ingredients?

How sure are you that this is true?

Not sure

How sure are you that this is true?

Kind-of-Sure

How sure are you that this is true?

Very Sure

End of Unit Evaluation
3. How do “nano” sunscreen ingredients differ from most other ingredients currently used in sunscreens?

How sure are you that this is true?

Not sure

How sure are you that this is true?

Kind-of-Sure

How sure are you that this is true?

Very Sure

End of Unit Evaluation

Name_______________

Date_______________

period_______________

Ultra-Violet (UV) Protection Lab Activity: Student Instructions & Worksheet

Introduction

It is important to protect our skin from damaging UV radiation, but how do we know how well we are protecting ourselves? Is wearing a light shirt at the beach as effective as wearing sunscreen? Is it better protection? Do thicker, whiter sunscreens protect us better than transparent sprays? Can we tell how well something will block UV by looking at its appearance?

Research Question

In this lab you will be investigating the following research question:

  • Does the appearance of a substance (its opacity) relate to its ability to block UV light?

Opacity

The opacity of a substance is one way to describe its appearance. Opacity is the opposite of how transparent or “see-through” something is; for a completely opaque substance you can not see through it at all. Opacity is a separate property than the color of a substance –for example you can have something that is yellow and transparent like apple juice or something that is yellow and opaque like cake frosting.

Hypothesis

Do you think that UV blocking ability relates to a substance’s opacity? Would you expect transparent or opaque substances to be better UV blockers? If you are right, what implications does this have for how you will protect yourself the next time you go to the beach? Write down your best guesses to answer these questions and explain why you think what you think.

Materials

  • Assorted white substances varying in opacity (for example: different sunblocks, sunscreens, sungels, glass pieces, white tee-shirts of varying thickness, white tissue paper, white paper of varying thickness, laundry detergent, white paint, white face makeup)
  • Eight paper cups
  • One micro spoon
  • Sunscreen Smear Sheet
  • Black construction paper (for judging opacity of white substances)
  • UV light source
  • UV sensitive bead testers
  • UV bead color guide
  • Cotton swabs (for apply sunscreen to the Sunscreen Smear Sheet)
  • Alcohol wipes (for cleaning sunscreen off the Sunscreen Smear Sheet)

Procedure

Part I: Choose Your Samples

Goal: Choose a group of substances from the ones provided by your teacher that you think will best help you determine if opacity is related to UV blocking.

  • Obtain eight small paper cups. Obtain a small sample of each of the substances you have chosen. Label each cup with the name of the substance.
Tip: Try to choose substances that vary in their opacity and that you would expect to vary in their blocking ability.

Part II: Judge the Opacity

Goal: To make observations about the appearance (opacity) of the substances you chose, using your eyes as the instruments.

  • Obtain a Sunscreen Smear Sheet. Place it on top of a black sheet of paper.
  • Label one square with the name of each substance you are going to test.
  • Use the micro spoon to measure out the first substance (make sure to use an equal amount of all the other substances).
  • Then use the cotton swab to smear the substance onto the Sunscreen Smear Sheet, evenly covering a whole square with a thin layer. (For solid substances, just place them on top of the sheet).
  • How well can you see through the substance to the black sheet of paper?
  • Use the Opacity Guide on the next page to rank each sample on a \begin{align*}1\end{align*} to \begin{align*}5\end{align*} scale.
Use \begin{align*}5\end{align*} to represent no opacity (you cannot see the substance at all).
Use \begin{align*}1\end{align*} to represent complete opacity (you can’t see any black through the sample).
  • Record your observations into the Data Chart in this packet.
  • Repeat for each of your substances.

Part III: Test the UV Blocking

Goal: Use UV-sensitive beads to determine how effective your chosen substances are in blocking UV-light.

  • Obtain \begin{align*}3\end{align*}UV bead testers:
    • Bead Tester “C1” for Control 1. This bead will always be kept out of the UV light and will show you the lightest color that the bead can be. Keep this in the envelope until you need it.
    • Bead Tester “C2” for Control 2. This bead will always be exposed to the UV light and should always change color to let you know that the UV light is reaching the bead. This bead will show you the darkest color that the bead can reach.
    • Bead Tester “E” for Experimental. Keep this in its envelope so that it is not exposed to any UV light while you are not using it.

Checking Bead Tester C1 and C2

  • Use UV bead color guide to record the initial bead color number \begin{align*}(2-10)\end{align*} of C1 on your data chart.
  • Expose C2 to the UV light for \begin{align*}30 \;\mathrm{sec}\end{align*}. and quickly compare it to the UV bead color guide. Record the bead color number \begin{align*}(2-10)\end{align*} on your data chart.

Using Bead Tester E with Your Substances

  • To test the UV blocking of a substance, hold Bead Tester E under the square for that substance on the Sunscreen Smear Sheet. (For solid substances, just hold Bead Tester E directly behind them).
  • Expose Bead Tester E (covered by the substance) and Bead Tester C2 (uncovered) to your UV lamp (or direct sunlight) for \begin{align*}30\;\mathrm{secs}\end{align*}.
  • Take both Bead Testers out of the light, uncover Bead Tester E, and observe any changes to the color of the beads using the UV bead color guide. Record the bead color number \begin{align*}(2-10)\end{align*} for both E and C2 on your data chart.
  • Repeat for each of your substances.

Data Chart

Initial C1 Bead Color Number________

Initial C2 Bead Color Number_________

Substance Name(Include SPF if applicable) Appearance (Describe) Opacity (1 to 5 rating) Color of UV bead “E” (2 to 10 rating) Color of UV bead “C2” (2 to 10 rating) Observations and Notes

Analysis

Now you need to analyze your data to see if it helps to answer the research question:Does the appearance of a substance (opacity) relate to its ability to block UV light? One of the ways that scientists organize data to help them see patterns is by creating a visual representation. Below you will see a chart that you can use to help you analyze your data.

To fill in the chart, do the following for each substance that you tested:

  1. Find the row that corresponds to its opacity.
  2. Find the column that corresponds to its UV blocking ability.
  3. Draw a large dot \begin{align*}\bullet\end{align*} in the box where this row and column intersect.
  4. Label the dot with the name or initials of the substance.

After you have filled in the chart, answer the analysis questions that follow.

UV Blocking Ability \begin{align*}\rightarrow\end{align*} No Blocking \begin{align*}(10)\end{align*} Low Blocking \begin{align*}(8)\end{align*} Medium Blocking \begin{align*}(6)\end{align*} High Blocking \begin{align*}(4)\end{align*} Total Blocking \begin{align*}(2)\end{align*}
Opacity \begin{align*}\downarrow\end{align*}
\begin{align*}5\end{align*} Fully Transparent
\begin{align*}4\end{align*}
\begin{align*}3\end{align*}
\begin{align*}2\end{align*}
\begin{align*}1\end{align*} Fully Opaque

1. Look at the visual representation of your data that you have created and describe it. Note any patterns that you see. Remember that seeing no pattern can also give you important information.

2. What pattern would you expect to see if there is a relationship between the appearance of a substance (opacity) and its ability to block UV light? Draw the pattern by coloring in the grid below.

3. Does your chart match the pattern you would expect to see if there is a relationship between opacity and UV blocking ability?

\begin{align*}\Box\end{align*} Yes

\begin{align*}\Box\end{align*} No

\begin{align*}\Box\end{align*} I’m not sure

4. What does this answer mean in practical terms? What does it tell you about how well you can judge the effectiveness of sun protection by looking at its appearance? How might this affect your sun protection activities?

5. Do you think that increasing the number of substances you tested would change your answer? Why or why not?

6. How confident are you that the answer you came up with is correct? Do you think that increasing the number of substances you tested would change how sure you are of your answer? Why or why not?

Conclusions

1. Answer the research question:

\begin{align*}\Box\end{align*} Yes, there is a relationship.

\begin{align*}\Box\end{align*} No, there is not a relationship.

\begin{align*}\Box\end{align*} I’m not sure if there is a relationship.

2. This is how the evidence from the experiment supports my answer: (Make sure to be specific and discuss any patterns you do or do not see in the data.)

3. Identify any extra variables that may have affected your experiment:

4. How could you control for these variables in future experiments?

5. What changes would you make to this experiment so that you could answer the research question better?

6. All experiments raise new questions. Sometime these come directly from the experiment and others are related ideas that you become curious about. What is a new research question that you would want to investigate after completing this experiment?

All About Sunscreens

Contents

  • Overview of Sunscreen Ingredients: Student Handout
  • Light Reflection by Three Sunscreens: Student Worksheet
  • Sunscreen Ingredients Activity: Student Instructions & Worksheet
  • Summary of FDA Approved Sunscreen Ingredients: Student Handout
  • Reflecting on the Guiding Questions: Student Worksheet

Overview of Sunscreen Ingredients: Student Handout

Organic Ingredients Inorganic Ingredients (Nano) Inorganic Ingredients (Large)
Atoms Involved Carbon, Hydrogen, Oxygen, Nitrogen Zinc, Titanium, Oxygen Zinc, Titanium, Oxygen
Structure (not drawn to scale)

Individual molecule

Cluster \begin{align*}\sim 100 \;\mathrm{nm}\end{align*} in diameter

Cluster \begin{align*} > 200 \;\mathrm{nm}\end{align*} in diameter
Interaction with UV light

Absorb specific \begin{align*}\lambda\end{align*}

Absorb all UV \begin{align*} < \end{align*} critical \begin{align*}\lambda\end{align*}

Absorb all UV \begin{align*} < \end{align*} critical \begin{align*}\lambda\end{align*}
Absorption Range Parts of UVA or UVB spectrum Broad spectrum UVA and UVB Broad spectrum UVA and UVB
Interaction with Visible light None

Minimal Scattering

Much Scattering
Appearance Clear Clear White

Name_______________

Date_______________

period_______________

Light Reflection by Three Sunscreens: Student Worksheet

Introduction

Three sunscreens were tested for reflection (back-scattering) with different wavelengths of light:

  • One contains nanosized inorganic ingredients
  • One contains traditional inorganic ingredients
  • One contains organic ingredients

A graph was created to show the percent of light reflected by each sunscreen at different wavelengths and is included in this packet.

Instructions

Use the graph to answer the following questions for each sunscreen in the chart on the next page:

  1. Will it appear white or clear on your skin? How do you know?
  2. What size (approximately) are the molecules / clusters?
  3. Can we tell how good a UV blocker it is from this graph? Why/ why not?
  4. Which one of the sunscreens is it? How do you know?
Light Reflection by Three Sunscreens Chart
Appearance Size UV Blocking Identity (w/ reason)
# 1
# 2
# 3

Light Reflected by Three Sunscreens

Sunscreen Ingredients Activity: Student Instructions & Worksheet

Most of us (hopefully) apply sunscreen to protect us from the sun when we are going to be outside for a long time. But how many of us have ever stopped to read the bottle to see what we are putting on our bodies? What kinds of chemicals are used to block the sun rays? Do different sunscreens use different ingredients to block the sun? How might the different ingredients used affect us? In this activity you’ll take a look at several sunscreens to see what we are putting on our bodies when we use these products.

Materials

  • Five different bottles of sunscreen.

To get a diverse group of sunscreens try to use more than one brand. Also see if you can find the following:

  • One sunscreen with a high SPF (30-50).
  • One sunscreen with a low SPF (5-15).
  • One sunscreen designed for skiers or surfers.
  • One sunscreen for sensitive skin or babies.
  • One sunscreen that has zinc oxide \begin{align*}(ZnO)\end{align*} or titanium dioxide \begin{align*}(TiO_2)\end{align*} as an ingredient.

Note: the proper scientific name for \begin{align*}TiO_2\end{align*} is “titanium (IV) oxide”, but the older name “titanium dioxide” is more commonly used.

Instructions

Look at the back of one of the bottles. You should see a list of the “active ingredients” in the sunscreen. These are the ingredients that prevent sunlight from reaching your skin (“inactive ingredients” are added to influence the appearance, scent, texture and chemical stability of the sunscreen.) Also look to see what kind of protection the sunscreen claims to provide. Does it provide UVB protection? UVA protection? Does it claim to have “broad spectrum” protection? What is its SPF number? Does it make any other claims about its protection? Record your observations for each sunscreen in the data chart and then answer the questions that follow.

Data Chart
Brand Active Ingredients SPF UVB? UVA? Broad Spectrum? Price
#1
#2
#3
#4
#5

Questions

1. How many different active ingredients did most of the sunscreens have?

2. What were the most common active sunscreen ingredients you saw? Are these organic or inorganic ingredients?

3. Did any of the sunscreens you looked at have active ingredients that were very different from the rest? If so, what were they?

4. Were you able to find a sunscreen with inorganic ingredients in it? If so, which one(s) contained them?

5. How many of your sunscreens claimed to have UVA protection? UVB protection? Broadband protection?

6. Why do you think that many sunscreens have more than one active ingredient? Why can’t they just put in more of the “best” one?

7. You have just looked at a sample of the different chemicals you are putting on your skin when you use sunscreen. Does this raise any health concerns for you? If so, what are some of the things you might be concerned about and why?

8. Where could you go to find out more information about possible health concerns?

Summary of FDA Approved Sunscreen Ingredients: Student Handout

\begin{align*}\lambda\end{align*} range (nm) Protection Against Protection Against Possible Allergies Other Issues
UVB (280-320 nm) UVA (320-400 nm)
Organic Ingredients
PABA derivatives
Padimate O (Octyl dimethyl PABA) 295-340 Good Little Yes -
PABA (p-aminobenzoic acid) 200-320 Good Little Yes Greasy, Stains
Cinnamates
Octinoxate (Octyl methoxycinnamate) (OMC) (Parasol MCX) 295-350 Good Little Yes -
Cinoxate 280-310 Good Little Yes -
Salicylates
Homosalate 295-340 Good Little Yes -
Octisalate (Octyl salicylate) 295-330 Good Little Yes -
Trolamin salicylate 260-355 Good Little Yes -
Benzophenones
Oxybenzone (Benzophenone-3) 295-375 Good Some Yes -
Sulisobenzone (Benzophenone-4) 260-375 Good Some Yes Hard to solubilize
Dioxybenzone (Benzophenone-8) 250-390 Good Some Yes Hard to solubilize
Other Organics
Ensulizole 290-340 Good Little Yes -
Octocrylene 295-375 Good Little Yes -
Menthyl anthranilate (Meradimate) 295-380 Good Some Yes -
Avobezone (Parsol 1789) (Butyl methoxydibenzoyl methane) 295-395 Good Good Yes If not well formulated, loses potency
**NEW** Ecamsule (Mexoryl SX) 310-370 Some Good Yes Water-soluble
Inorganic Ingredients
Titanium Dioxide up to 365 Good Good No -
Zinc Oxide up to 380 Good Good No -

Name_______________

Date_______________

period_______________

Reflecting on the Guiding Questions: Student Worksheet

Think about the activities you just completed. What did you learn that will help you answer the guiding questions? Jot down your notes in the spaces below.

1. What are the most important factors to consider in choosing a sunscreen?

What I learned in this activity:

What I still want to know:

2. How do you know if a sunscreen has “nano” ingredients?

What I learned in this activity:

What I still want to know:

3. How do “nano” sunscreen ingredients differ from most other ingredients currently used in sunscreens?

What I learned in this activity:

What I still want to know:

How Sunscreens Block: The Absorption of UV Light

Contents

  • Absorption of Light by Matter: Student Reading
  • Absorption Summary: Student Handout
  • Reflecting on the Guiding Questions: Student Worksheet

Absorption of Light by Matter: Student Reading

Absorption is one of the ways in which light can interact with matter. In absorption, the energy of the light shining on an object (or a gas or liquid) is captured by the substance’s molecules and used to move them from a ground (low energy) state to higher (excited) energy states.

Absorption by a Single Atom

Absorption can only occur when the energy packet carried by one photon of light is equal to the energy required to bring about transition between states. Since the energy of a photon is directly related to its frequency (by the formula \begin{align*}E=hf\end{align*} where \begin{align*}E\end{align*} is the photon’s energy, \begin{align*}f\end{align*} is its frequency, and \begin{align*}h\end{align*} is Plancks’ constant: \begin{align*}6.26 \times 10^{-34} \;\mathrm{J \ s}\end{align*}), then a given transition can only be caused by one specific frequency of light.

The energy of a photon must be exactly the same as the energy difference between levels for it to be absorbed [1]

The frequency of a photon is inversely related to its wavelength (by the formula \begin{align*}\lambda=c/f\end{align*} where \begin{align*}\lambda\end{align*} is the wavelength, \begin{align*}f\end{align*} is the frequency and \begin{align*}c\end{align*} is the speed of light in a vacuum). This means that the energy of the energy transition determines what frequency and hence what wavelength of light can be absorbed. The greater the energy of transition, the smaller the wavelength of light that can be absorbed.

What exactly are these different energy states that the atom can be in? Well, in atoms they correspond to the types of electron configurations in orbitals that are allowed by the rules of quantum mechanics. You may be familiar with transitions between electronic states if you’ve studied light emission by gasses. The main difference between the emission and the absorption of light is that in one case the light energy is being given off and in the other it is being captured, but in both cases the light energy of the photon absorbed or emitted corresponds to the energy difference between the two electronic states.

For example, Figure 2 shows several different energy levels and one possible transition for a hydrogen atom (in a gas). Each of the horizontal lines is an electronic state that the atom can be in and each the vertical line shows a possible “jump” or transition that could occur. Each possible transition (like the one shown) has a characteristic energy, and thus a specific wavelength of light that must be absorbed for it to occur. If we shine a flashlight at a sample of hydrogen atoms in a gas and measure the wavelengths of light that get through at the end, we would see an absorption spectrum like the one shown in the bottom of Figure 3. What you see is the full color spectrum, except for the “absorption lines”, the specific wavelengths of light whose energy was absorbed because it corresponds to the energy difference between states for helium atoms. Similarly, if we stimulate a hydrogen atom to emit light, it will do so only at these same characteristic wavelengths whose energy (which is proportional to its frequency given by \begin{align*}E=hf\end{align*}) corresponds to the energy between possible electron states for the hydrogen atom.

Energy states and one transition for the hydrogen atom (Left) Emission (top) and absorption (bottom) spectra for the hydrogen atom [2] Each line corresponds to a single transition (Right)

Absorption by Molecules

When we start dealing with molecules instead of atoms, the situation gets more complicated. When we talk about atoms being bonded together, it is not as if they are solidly glued to one another. The “glue” that hold the atoms together in a molecule is the attractive forces between the electrons and the different nuclei (intramolecular forces.) As electrons are constantly in motion, the strength of these attractive forces fluctuates over time allowing the nuclei to move back and forth and giving rise to molecular vibrations. As with electronic states, there are only certain vibrational modes which are possible. One simple example of molecular vibration can be seen in formate \begin{align*}(COOH^-)\end{align*}, a molecular anion. The six possible vibration modes for formate are shown in Figure 4.

Within each vibrational state, there are also multiple ways that the atoms in the molecule can rotate. These are called rotational states and again there are a limited number of possible rotational states that the molecule can be in. When we consider vibrational and rotational states, we realize that even without exciting any electrons (ground state), there are a bunch of different energy states that a molecule can be in. Not surprisingly, this is also true for the excited state. So now we have a situation in which the molecule can transfer from any of the ground states to any of the excited states and instead of a single energy of transition, we have a large group of energy gaps able to absorb light energy.

6 Vibrational modes for formate ion [3] (Left) Multiple Transition Energies for Electrons in Molecules (Right)

The multiple energy transitions possible correspond to a range of photon energies and thus frequencies of light that can be absorbed. Comparing Figure 5 with Figure 2 we can see that where there was a single transition for an atom, there are multiple ones for a molecule. Each arrow corresponds to one transition. Since we now have clusters of energy levels (due to the different variations in rotational and vibrational energies possible), we have a cluster of multiple transitions with similar energies.

Thus light absorbing molecules produces multiple, closely spaced absorption lines which combine to form an absorption curve. Absorption curves can be described both by their absorption range and peak absorption wavelength as shown in Figure 6. Not that absorption is not uniform across the range; it is greatest at the peak and drops off rapidly. So absorption at the edges of the range is not very good.

Absorption lines and curve spectrum for an organic molecule (Range = ) (Peak = )

Absorption by Ionic Compounds

In inorganic compounds, there is no discrete atom or molecule to talk about. Instead the electrons belong to the positive nuclei as a collective group. Because so many atoms are close together and involved, there are a very large number of possible energy levels for electrons in both the ground and excited states as shown in Figure 7. Because there are so many possible energy states packed so closely together, we assume that electrons can have virtually any energy within each state and change between them very easily. Thus we call the group of possible energies for the ground states the “valence band” and the group of possible energies for the excited state the “conduction band”.

The large number of tightly packed energy levels possible for ground and excited electron states and possible transitions(Left) Absorption versus frequency graph for an inorganic compound. The smallest of light that can be absorbed corresponds has an energy equal to the band gap energy(Right)

Electrons can transition from any energy value in the valence band to any energy value in the conduction band. The energy spacing between the two bands is called the “band gap” and is the minimum amount of energy that the substance can absorb. This corresponds to the minimum frequency (and maximum wavelength) of light that the substance can absorb as shown in Figure 8. Now instead of seeing an absorption peak, we see almost complete absorption up to a cut-off wavelength (which corresponds to the energy of the band gap). For example, zinc oxide \begin{align*}(ZnO)\end{align*} has a particular band gap (minimum energy that can be absorbed). Using \begin{align*}E=hf\end{align*} and \begin{align*}c=\lambda f\end{align*} (where \begin{align*}h\end{align*} is Planck’s constant and \begin{align*}c\end{align*} is the speed of light) we can calculate that this corresponds to light with a wavelength of \begin{align*}380\;\mathrm{nm}\end{align*}. Thus light with higher frequencies and energies is almost completely absorbed and light with lower frequencies and energies is not absorbed. If we think about absorption in relation to the wavelength of light (instead of the frequency) our graph gets reversed (remember that wavelength and frequency are inversely related by \begin{align*}c=\lambda f\end{align*}). This leads to an absorption spectra with the characteristic sharp drop shown in Figure 9. Thus the minimum frequency corresponds to a maximum wavelength up to which the inorganic sunscreen ingredients can absorb.

Absorption Spectrum for an Inorganic Compound . Light with wavelengths less than is almost completely absorbed. Light with wavelengths greater than is not absorbed at all.

References

(Internet resources accessed December 2005)

Absorption Summary: Student Handout

Atoms Organic Molecules Inorganic Compounds
Structure (not drawn to scale)

Nucleus with electron cloud

Individual molecule

Cluster of ions

Energy Levels
Absorption Spectrum

Absorbs specific \begin{align*}\lambda\end{align*}

Absorbs specific \begin{align*}\lambda\end{align*} range

Absorbs all UV \begin{align*} < \end{align*} critical \begin{align*}\lambda\end{align*}

UV Protection Minimal Parts of UVA or UVB spectrum Broad spectrum UVA and UVB

Name_______________

Date_______________

period_______________

Reflecting on the Guiding Questions: Student Worksheet

Think about the activity you just completed. What did you learn that will help you answer the guiding questions? Jot down notes in the spaces below.

1. What are the most important factors to consider in choosing a sunscreen?

What I learned in this activity:

What I still want to know:

2. How do you know if a sunscreen has “nano” ingredients?

What I learned in this activity:

What I still want to know:

3. How do “nano” sunscreen ingredients differ from most other ingredients currently used in sunscreens?

What I learned in this activity:

What I still want to know:

How Sunscreens Appear: Interactions with Visible Light

Contents

  • Scattering of Light by Suspended Clusters: Student Reading
  • Ad Campaign Project (ChemSense Activity): Student Instructions
  • Sunscreens & Sunlight Animations: Student Instructions & Worksheet
  • Reflecting on the Guiding Questions: Student Worksheet

Scattering of Light by Suspended Clusters: Student Reading

Dust clusters from a passing car scatter sunlight (Left) Without the dust clusters we can not see the sun rays (right)

What is Scattering?

Scattering is a phenomenon in which light is redirected in different directions by small clusters of atoms suspended in some other substance. A common example of scattering is when you shake out a dusty object in a sunny room - the dust seems to sparkle in the air. This effect occurs because the dust is scattering the sunlight, which then reaches your eyes. Scattering also explains why snow and salt are white, and why the sky is blue. In each of these situations, the light is being redirected many times before it reaches our eyes. This is why the process is called multiple scattering.

What is wrong with this picture?[1]

In many cartoons we often see the “light” from a flashlight in the dark, but this is a false image because we can not see this light unless there are clusters there to scatter it towards our eyes. Try shining a flashlight at a wall in a dark room. Can you see the beam of light between the wall and the flashlight (like in the picture above)? Now sprinkle some baby powder in the air while you shine the beam. Can you see the beam now?

How Does Scattering Occur?

When lots of clusters of one material are suspended in another material (for example drops of water in the air, or active sunscreen ingredients in the lotion) light has a chance to interact with these many clusters. The interaction bends the light in many different directions. After this, it will then continue traveling in the suspension medium until it reaches another cluster. If the light is bent multiple times in multiple directions, we call this multiple scattering.

Scattering of light by a suspended cluster (Left) Multiple scattering of light by suspended cluster (Right)

While on the micro-level scattering redirects the light in many different directions, on the macro-level this combines to produce one of two results: the light is sent back in the general direction from which it came at various angles (back scattering) or the light continues in the same general direction it was moving, but at various angles (front scattering).

Does Scattering Always Happen?

Whether or not scattering will occur depends on many factors. For clusters suspended in a medium some of the most important factors are: the identity of the clusters, the identity of the suspending medium and the cluster size. Scattering happens most when the clusters have a diameter that is half as big as the wavelength of light involved. So a \begin{align*}200 \;\mathrm{nm}\end{align*} cluster would scatter \begin{align*}400 \;\mathrm{nm}\end{align*} light the most and it would scatter \begin{align*}300-500 \;\mathrm{nm}\end{align*} light quite a bit as well. The amount of scattering continues to decrease as wavelengths become much bigger or much smaller than \begin{align*}400 \;\mathrm{nm}\end{align*}, as shown in figure 5.

Graph of Scattering for a Cluster in Sunscreen

References

(Accessed December 2005)

Ad Campaign Project: Student Instructions

Overview

Sunsol, the prominent sunscreen maker, has just decided to launch a new product into the market. The sunscreen will use a zinc oxide \begin{align*}(ZnO)\end{align*} nanopowder as its only active ingredient, and will be formulated to go on clear and non-greasy. Sunsol is very excited about its new product, and wants to launch a full ad campaign to promote it to consumers who may not be familiar with the idea of a clear sunscreen that offers full spectrum protection.

Sunsol feels that it is very important for their potential customers to understand both how \begin{align*}ZnO\end{align*} interacts with light to protect people’s skin and how the size of the particles affects the sunscreen’s appearance. For this reason, they have decided that the ad campaign should center on an animated commercial that shows how traditional \begin{align*}ZnO\end{align*} and \begin{align*}ZnO\end{align*} nanopowders interact with UV and visible light.

Sunsol has invited several creative teams––including yours––to use the ChemSense Animator to create animations showing how the different sized \begin{align*}ZnO\end{align*} particles suspended in the sunscreen will scatter visible light differently.

The Request

Sunsol is requesting a total of \begin{align*}4\end{align*} animations:

  1. Sunscreen with \begin{align*}\sim 50 \;\mathrm{nm}\ ZnO\end{align*} particles interacting with UV light.
  2. Sunscreen with \begin{align*}\sim 50 \;\mathrm{nm}\ ZnO\end{align*} particles interacting with visible light.
  3. Sunscreen with \begin{align*}\sim 300 \;\mathrm{nm}\ ZnO\end{align*} particles interacting with UV light.
  4. Sunscreen with \begin{align*}\sim 300 \;\mathrm{nm}\ ZnO\end{align*} particles interacting with visible light.

Your teacher will put you in teams and let you know which of the animations you should work on.

Animation Matrix

UV light Visible Light
\begin{align*}50 \;\mathrm{nm} ZnO\end{align*} particles \begin{align*}1\end{align*} \begin{align*}2\end{align*}
\begin{align*}300 \;\mathrm{nm} ZnO\end{align*} particles \begin{align*}3\end{align*} \begin{align*}4\end{align*}

Requirements

All animations should contain the following elements:

  • A light source (the sun)
  • A skin surface with sunscreen lotion applied
  • \begin{align*}ZnO\end{align*} particles of the required size suspended in the lotion
  • A minimum of \begin{align*}10\end{align*} frames

The UV light animations should also include:

  • At least \begin{align*}2\end{align*} UVA and \begin{align*}2\end{align*} UVB light rays interacting with the \begin{align*}ZnO\end{align*} particles (and skin when appropriate)
  • All relevant blocking mechanisms for the \begin{align*}ZnO\end{align*} particles in the sunscreen

The visible light animations should also include:

  • At least \begin{align*}5\end{align*} visible light rays interacting with the \begin{align*}ZnO\end{align*} particles (and skin when appropriate)
  • A human observer and an indication of what they see

Things to consider in your animation

  • How thick will the sunscreen be applied?
  • What concentration of particles will the sunscreen have?
  • How will you show the different blocking mechanisms?
  • How will you indicate what the human observer sees?

Evaluation

Sunsol will evaluate the animations based on the following criteria:

  • All required elements are present and accurately depicted
  • Animations show correct interaction of light rays with \begin{align*}ZnO\end{align*} particles (and skin)
  • All relevant blocking mechanisms shown (UV light only)
  • Animations clearly indicate what the observer sees and why (Visible light only)
  • All team member contributed and worked together to produce the animations

Discussion

Questions to answer about each model:

  • How does this model show absorption / scattering?
  • How does this model show what the observer sees?
  • What are its strengths? (What aspects of scattering does it show particularly well?)
  • What are its limitations? (What aspects of scattering are not shown well?)
  • Is there anything that seems inaccurately depicted?
  • What could be done (within the structure of the animation) to address some of these limitations?

Questions to answer about the group of models as a whole:

  • What do the different animations have in common? How do they show things in similar ways?
  • What things do the animations show in different ways? Are different animations better at showing different aspects of the phenomenon?
  • If different models can be used to represent a phenomenon, how do we know which one is “better”? (Models which best align with or represent the empirical data we have are better.)
Rubric for Ad Campaign Evaluation – UV Light Animations
Category Novice (1) Absent, missing or confused Apprentice (2) Partially developed Skilled (3) Adequately developed Masterful (4) Fully developed
Required Elements
  • Light source
  • Skin surface
  • Sunscreen lotion
  • Suspended \begin{align*}ZnO\end{align*} particles
  • \begin{align*}2 +\end{align*} UVA rays
  • \begin{align*}2 +\end{align*} UVB rays
  • \begin{align*}10 +\end{align*} frames
\begin{align*}0 - 2\end{align*} of the required elements are present. \begin{align*}3 - 4\end{align*} of the required elements are present. \begin{align*}5 - 6\end{align*} of the required elements are present. All \begin{align*}7\end{align*} required elements are present.
Few of the required elements are accurately depicted. Some of the required elements are accurately depicted. Most of the required elements are accurately depicted. All of the required elements are accurately depicted.
Interactions of light rays with \begin{align*}ZnO\end{align*} particles (and skin where appropriate) correctly shown Few or no key aspects of the interaction are correctly shown. Some aspects of the interaction are correctly shown. Most key aspects of the interaction are correctly shown. All key aspects of the interaction are correctly shown.
All relevant blocking mechanisms correctly shown Few or no key aspects of the blocking mechanism are correctly shown. Some key aspects of the blocking mechanism are correctly shown. Most key aspects of the blocking mechanism are correctly shown. All key aspects of the blocking mechanism are correctly shown.
Teamwork
  • All team members contributed significantly to the project
  • Group worked together to manage problems as a team
Few team members contributed to the project. Some team members contributed to the project. Most team members contributed to the project. All team members contributed to the project.
Group did not address the problems encountered. Group did not manage problems effectively. Problems in the group managed by one or two individuals. Group worked together to solve problems.
Rubric for Ad Campaign Evaluation – Visible Light Animations
Category Novice (1) Absent, missing or confused Apprentice (2) Partially developed Skilled (3) Adequately developed Masterful (4) Fully developed
Required Elements
  • Light source
  • Human observer
  • Skin surface
  • Sunscreen lotion
  • Suspended \begin{align*}ZnO\end{align*} particles
  • \begin{align*}5 +\end{align*} visible light rays
  • \begin{align*}10 +\end{align*} frames
\begin{align*}0 - 2\end{align*} of the required elements are present. \begin{align*}3 - 4\end{align*} of the required elements are present. \begin{align*}5 - 6\end{align*} of the required elements are present. All \begin{align*}7\end{align*} required elements are present.
Few of the required elements are accurately depicted. Some of the required elements are accurately depicted. Most of the required elements are accurately depicted. All of the required elements are accurately depicted.
Interactions of light rays with \begin{align*}ZnO\end{align*} particles (and skin when appropriate) correctly shown Few or no key aspects of the interaction are correctly shown. Some aspects of the interaction are correctly shown. Most key aspects of the interaction are correctly shown. All key aspects of the interaction are correctly shown.
What the observer sees and why they see is correctly shown Few or no key aspects of the observer’s view are correctly shown. Some key aspects of the observer’s view are correctly shown. Most key aspects of the observer’s view are correctly shown. All key aspects of the observer’s view are correctly shown.
Teamwork
  • All team members contributed significantly to the project
  • Group worked together to manage problems as a team
Few team members contributed to the project. Some team members contributed to the project. Most team members contributed to the project. All team members contributed to the project.
Group did not address the problems encountered. Group did not manage problems effectively. Problems in the group managed by one or two individuals. Group worked together to solve problems.

Name_______________

Date_______________

period_______________

Sunscreens & Sunlight Animations: Student Instructions & Worksheet

Introduction

There are many factors that people take into account when choosing which sunscreen to use and how much to apply. Two of the most important factors that people consider ate the ability to block UV and the visual appearance of the sunscreen (due to the interaction with visible light). You are about to see three animations that are models of what happens when sunlight (both UV and visible rays) shine on:

  • Skin without any sunscreen
  • Skin protected by \begin{align*}200 \;\mathrm{nm}\ ZnO\end{align*} particle sunscreen
  • Skin protected by \begin{align*}30 \;\mathrm{nm}\ ZnO\end{align*} particle sunscreen

Open the animation file as instructed by your teacher and explore the animations for different sunscreen and light ray options. Then choose the sunscreen option and wavelength(s) of light as indicated to answer the following questions.

Questions

1. Select the UVA and UVB wavelengths of light with no sunscreen and click the play button.

a. What happens to the skin when the UV light reaches it?

b. How is the damage caused by the UVA rays different from the damage caused by the UVB rays? (You may want to play the animation with just UVA or UVB selected to answer this question)

c. Based on what you know about the different energies of UVA and UVB light why do you think this might happen?

2. Now leave UVA and UVB light selected and try playing the animation first with the \begin{align*}30 \;\mathrm{nm}\ ZnO\end{align*} sunscreen and then with the \begin{align*}200 \;\mathrm{nm}\ ZnO\end{align*} sunscreen.

a. What kind of sunscreen ingredients are shown in each animations?

b. What happens to the UV light in the animation of \begin{align*}30 \;\mathrm{nm}\ ZnO\end{align*} particle sunscreen?

c. What happens to the UV light in the animation of \begin{align*}200 \;\mathrm{nm}\ ZnO\end{align*} particle sunscreen?

d. Is there any difference in how the UV light interacts with the \begin{align*}30 \;\mathrm{nm}\ ZnO\end{align*} particles versus the \begin{align*}200 \;\mathrm{nm}\ ZnO\end{align*} particles? Explain why this is so based on your understanding of how the sunscreens work to block UV light.

e. Is there any difference in how the two kinds of UV light interact with the sunscreens? Explain why this is so based on your understanding of how the sunscreens work to block UV light

3. Select the visible light option and play the animation for each of the sunscreen conditions. What happens to the visible light in each animation and what does the observer see?

a. Skin without any sunscreen

b. Skin with \begin{align*}200 \;\mathrm{nm}\ ZnO\end{align*} particles sunscreen

c. Skin with \begin{align*}30 \;\mathrm{nm}\ ZnO\end{align*} particle sunscreen

4. What determines what the observer sees? (Do they see the skin or the sunscreen? What color do they see?)

5. How does scattering affect what the observer sees?

6. What variables don’t change between the two animations with sunscreens?

7. What variable determines if the visible light scatters or not?

8. What would happen if we applied the large particle sunscreen in a layer only half as thick as the one shown? How would this affect its appearance? How would it affect its UV blocking ability?

9. What would happen if the observer (eye) moved 3 steps to the left to look at the skin?

10. When we make a model (such as these animations) we make tradeoffs between depicting the phenomenon as accurately as possible and simplifying it to show the key principles involved.

a. Are the different elements of the animation drawn on the same size scale? If not, which ones aren’t? How do these affect the animation’s ability to depict the scattering mechanism? Which elements in the animation are really on or close to the nanoscale? Which are on the macroscale? Which are on the cosmic scale?

b. What are some other ways these animations have simplified the model of the real world situation they describe?

c. What are some of the benefits of making a simplified model? What are some of the drawbacks?

Name_______________

Date_______________

period_______________

Reflecting on the Guiding Questions: Student Worksheet

Think about the activity you just completed. What did you learn that will help you answer the guiding questions? Jot down notes in the spaces below.

1. What are the most important factors to consider in choosing a sunscreen?

what I learned in this activity:

What I still want to know:

2. How do you know if a sunscreen has “nano” ingredients?

What I learned in this activity:

What I still want to know:

3. How do “nano” sunscreen ingredients differ from most other ingredients currently used in sunscreens?

What I learned in this activity:

What I still want to know:

Culminating Activities

Contents

  • Consumer Choice Project: Student Instructions
  • Consumer Choice Project: Peer Feedback Form
  • The Science Behind the Sunscreen: Student Quiz
  • Clear Sunscreen Final Reflections: Student Worksheet

Consumer Choice Project: Student Instructions

Introduction

SmartShopper, the consumer advocacy group, has heard a lot in the media about the new clear sunscreens with nanoparticulate ingredients coming out on the market. Consumers have been contacting them lately to ask them if these new products are better than traditional sunscreens, if they are safe to use, and how to know if a sunscreen uses nanoparticulate ingredients. To help consumers decide whether these products are right for them, SmartShopper has decided to produce a pamphlet that tells consumers all they need to know about these new products. SmartShopper also will need to take a position on whether or not they endorse the use of the sunscreens and justify this position based on a comparison of the benefits and risks backed up with science. They turn to you and your team to create this pamphlet.

Requirements

SmartShopper asks that your pamphlet makes full use of both sides of an \begin{align*}8.5 \times 11\end{align*} piece of paper folded into thirds for easy distribution (see “How to Make a Pamphlet”) and contains:

  • A brief overview of what nanoparticulate sunscreen ingredients are and how they are similar and how they are different from other active sunscreen ingredients.
  • A list of common nanoparticulate active sunscreen ingredients and how to know if your sunscreen contains them.
  • An explanation of how sunscreens with nanoparticulate ingredients work to block UV light from reaching the skin and the benefits of using them (including advantages over other sunscreen ingredients).
  • An explanation of why nanoparticulate sunscreen ingredients are clear and a diagram that illustrates the science principles involved.
  • A transmission versus wavelength graph that supports this explanation.
  • An explanation of the possible downsides / dangers of using sunscreens with nanoparticulate ingredients.
  • SmartShopper’s position on the use of sunscreens nanoparticulate ingredients (do you endorse their use?) with justification of this position based on a comparison of the benefits and risks involved.

How to Make a Pamphlet

By Hand:

Take a regular piece of \begin{align*}8.5 \times 11\end{align*} paper turn it sideways. Fold the paper into thirds and crease it firmly. This is what the pamphlet will look like when it’s done. When you unfold the paper, you can use the creases as column guides. It is good to make the front and back of your pamphlet on different pieces of paper and use a copying machine to make the pamphlet double sided in case you decide to make changes along the way.

With a Computer:

Open a new document in Microsoft Word. Go to File>Page Setup to choose a Landscape Orientation and make all of the margins \begin{align*}0.5\;\mathrm{inches}\end{align*}. Go to Format>Columns to choose 3 columns and click the check box for Line Between. You will need to either use a printer that will print double-sided or print the two sides of your pamphlet separately and use a copying machine to make them double sided.

Evaluation

SmartShopper will evaluate the pamphlets based on the following criteria:

  • All required information is present and correct
  • Scientific explanations are used to back up pamphlets claims
  • Effective use of diagram and graph to enhance explanation of why nanoparticulate sunscreen ingredients are clear
  • Convincing argument weighing all the relevant information for position taken on nanoparticulate sunscreen use
  • All team member contributed and worked together to produce the animations

Name_______________

Date_______________

period_______________

Consumer Choice Project: Peer Feedback Form

1. What are the name(s) of the student team who developed the pamphlet you are evaluating?

2. Is all of the required information present and correct?

Not at all \begin{align*}\underline{\;\;\;\;\;\;\;\;\;}\end{align*} \begin{align*}\underline{\;\;\;\;\;\;\;\;\;}\end{align*} \begin{align*}\underline{\;\;\;\;\;\;\;\;\;}\end{align*} Completely
Overview of nanoparticulate sunscreen ingredients and similarities/differencesfrom other active ingredients. \begin{align*}1\end{align*} \begin{align*}2\end{align*} \begin{align*}3\end{align*} \begin{align*}4\end{align*} \begin{align*}5\end{align*}
List of common nanoparticulate active sunscreen ingredients and how to know if your sunscreen contains them. \begin{align*}1\end{align*} \begin{align*}2\end{align*} \begin{align*}3\end{align*} \begin{align*}4\end{align*} \begin{align*}5\end{align*}
Explanation of how nanoparticulate sunscreens block UV light from reaching the skin and benefits of using them. \begin{align*}1\end{align*} \begin{align*}2\end{align*} \begin{align*}3\end{align*} \begin{align*}4\end{align*} \begin{align*}5\end{align*}
Scientific explanation and diagram of why nanoparticulate sunscreen ingredients are clear. \begin{align*}1\end{align*} \begin{align*}2\end{align*} \begin{align*}3\end{align*} \begin{align*}4\end{align*} \begin{align*}5\end{align*}
Transmission versus wavelength graph that supports the explanation. \begin{align*}1\end{align*} \begin{align*}2\end{align*} \begin{align*}3\end{align*} \begin{align*}4\end{align*} \begin{align*}5\end{align*}
Explanation of possible downsides / dangers of using sunscreens with nanoparticulate ingredients. \begin{align*}1\end{align*} \begin{align*}2\end{align*} \begin{align*}3\end{align*} \begin{align*}4\end{align*} \begin{align*}5\end{align*}
SmartShopper’s position on the use of sunscreens nanoparticulate ingredients with justification. \begin{align*}1\end{align*} \begin{align*}2\end{align*} \begin{align*}3\end{align*} \begin{align*}4\end{align*} \begin{align*}5\end{align*}

3. List any information that you think is missing or incorrect here:

4. Was the information in the pamphlet presented clearly and communicated in an appropriate way?

\begin{align*}& \text{Not at all} && \text{A little} && \text{Somewhat} && \text{A lot} && \text{Completely} \\ & 1 && 2 && 3 && 4 && 5\end{align*}

5. If not, please identify the area(s) of confusion here:

6. To what degree are scientific explanations used to back up pamphlets claims?

\begin{align*}& \text{Not at all} && \text{A little} && \text{Somewhat} && \text{A lot} && \text{Completely}\\ & 1 && 2 && 3 && 4 && 5\end{align*}

7. To what degree do the diagram and graph enhance the explanation of why nanoparticulate sunscreen ingredients are clear?

\begin{align*}& \text{Not at all} && \text{A little} && \text{Somewhat} && \text{A lot} && \text{Completely} \\ & 1 && 2 && 3 && 4 && 5\end{align*}

8. To what degree is a convincing argument made––one that weighs all of the relevant information for the position taken on nanoparticulate sunscreen use?

\begin{align*}& \text{Not at all} && \text{A little} && \text{Somewhat} && \text{A lot} && \text{Completely} \\ & 1 && 2 && 3 && 4 && 5\end{align*}

9. Describe one thing that you think the pamphlet did very well:

10. Give at least one suggestion for improving the pamphlet:

Name_______________

Date_______________

period_______________

The Science Behind the Sunscreen: Student Quiz

30 points total

1. Why is UV light a source of health concern when visible and infrared light are not? (2 points)

2. List \begin{align*}2\end{align*} kinds of damage to the body caused by UV radiation. (2 points)

3. Explain in your own words why it is important to block UVA light.(2 points)

4. How do you know if a sunscreen protects against UVA light (now and future)? (2 points)

5. How do you know if a sunscreen protects against UVB light? (1 point)

6. For each of the following absorption graphs, circle the correct answers for a) what kind(s) of light are strongly absorbed and b) whether it is an organic or inorganic sunscreen. (4 points)

7. Why do sunscreens that use nano-sized \begin{align*}TiO_2\end{align*} clusters appear clear on our skin while sunscreens that use traditional sized \begin{align*}TiO_2\end{align*} clusters appear white? (5 points)

8. How do you know if a sunscreen has “nano” ingredients? (2 points)

9. Briefly describe one benefit and one drawback of using a sunscreen that contains “nano” ingredients: (1point each, a total of 2 points)

10. In what ways are “nano” sunscreen ingredients similar and different from other ingredients currently used in sunscreens? For each of the four categories below, indicate whether “nano” sunscreen ingredients are “similar” or “different” to organic and inorganic ingredients and explain how. (1 point each, total of 8 points)

Organic Ingredients (e.g. PABA) Inorganic Ingredients (e.g. Classic Zinc Oxide used by lifeguards)
Chemical Structure

Similar or Different

How: Nano ingredients are small ionic clusters while organic ingredients are molecules.

Similar or Different

How: Nano ingredients are a kind of inorganic ingredients. Bother are ionic clusters but the nano clusters are smaller.

Kinds of Light Blocked

Similar or Different

How: Organic ingredients each block a small part of the UV spectrum (generally UVB) while nano ingredients block almost the whole thing,

Similar or Different

How: Both nano ingredients and traditional inorganic ingredients block almost the whole UV spectrum.

Way Light is Blocked

Similar or Different

How: Both nano and organic ingredients block UV light via absorption. (The specific absorption mechanism is different, but students are not expected to report this)

Similar or Different

How: Both nano and inorganic ingredients block UV light via absorption.

Appearance on the Skin

Similar or Different

How: Both nano and organic ingredients appear clear on the skin.

Similar or Different

How: Traditional inorganic ingredients appear white on the skin while nano ingredients appear clear.

Clear Sunscreen Final Reflections: Student Worksheet

Now that you have come to the end of the unit, go back and look at the reflection forms you filled out after each activity and try to answer the guiding questions below. Write down answers each question below and then evaluate how confident you feel that each idea is true.

1. What are the most important factors to consider in choosing a sunscreen?

How sure are you that this is true?

Not So Sure

How sure are you that this is true?

Kind-of Sure

How sure are you that this is true?

Very Sure

2. How do you know if a sunscreen has “nano” ingredients?

How sure are you that this is true?

Not So Sure

How sure are you that this is true?

Kind-of Sure

How sure are you that this is true?

Very Sure

3. How do “nano” sunscreen ingredients differ from most other ingredients currently used in sunscreens?

How sure are you that this is true?

Not So Sure

How sure are you that this is true?

Kind-of Sure

How sure are you that this is true?

Very Sure

Now go back to the worksheet you filled out with your initial ideas at the beginning of the unit and mark each idea with a \begin{align*}\surd\end{align*} if you still believe it is true, an \begin{align*}X\end{align*} if you don’t think that it is true and a ? if you are still unsure. Then answer the following questions.

1. What ideas do you have now that are the same as when you started?

2. What ideas are different and how?

3. What things are you still unsure about?

One-Day Version of Clear Sunscreen

Contents

  • Summary of Radiation Emitted by the Sun: Student Handout
  • Summary of FDA Approved Sunscreen Ingredients: Student Handout
  • Overview of Sunscreen Ingredients: Student Handout

Summary of Radiation Emitted by the Sun: Student Handout

Chart of Different Kinds of Solar Radiation
Radiation Type Characteristic Wavelength \begin{align*}(\lambda)\end{align*} Energy per Photon \begin{align*}\%\end{align*} of Total Radiation Emitted by Sun Effects on Human Skin Visible to Human Eye?
UVC \begin{align*}\sim 200-290 \;\mathrm{nm}\end{align*} (Short-wave UV) High Energy \begin{align*}\sim 0\%\end{align*} (\begin{align*}<1\%\end{align*} of all UV) DNA Damage No
UVB \begin{align*}\sim 290-320 \;\mathrm{nm}\end{align*} (Mid-range UV) Medium Energy \begin{align*}\sim.35\%\end{align*} (\begin{align*}5\%\end{align*} of all UV) Sunburn DNA Damage Skin Cancer No
UVA \begin{align*}\sim 320-400 \;\mathrm{nm}\end{align*} (Long-wave UV) Low Energy \begin{align*}\sim 6.5\%\end{align*} (\begin{align*}95\%\end{align*} of all UV) Tanning Skin Aging DNA Damage Skin Cancer No
Visible \begin{align*}\sim 400-800 \;\mathrm{nm}\end{align*} Lower Energy \begin{align*}\sim 43\%\end{align*} None Currently Known Yes
IR \begin{align*}\sim 800-120,000 \;\mathrm{nm}\end{align*} Lowest Energy \begin{align*}\sim 49\%\end{align*} Heat Sensation (high \begin{align*}\lambda\end{align*} IR) No

Graph of Radiation Emitted by the Sun by Wavelength

Summary of FDA Approved Sunscreen Ingredients: Student Handout

\begin{align*}\lambda\end{align*} range (nm) Protection Against Protection Against Possible Allergies Other Issues
UVB (280-320 nm) UVA (320-400 nm)
Organic Ingredients
PABA derivatives
Padimate O (Octyl dimethyl PABA) 295-340 Good Little Yes -
PABA (p-aminobenzoic acid) 200-320 Good Little Yes Greasy, Stains
Cinnamates
Octinoxate (Octyl methoxycinnamate) (OMC) (Parasol MCX) 295-350 Good Little Yes -
Cinoxate 280-310 Good Little Yes -
Salicylates
Homosalate 295-340 Good Little Yes -
Octisalate (Octyl salicylate) 295-330 Good Little Yes -
Trolamin salicylate 260-355 Good Little Yes -
Benzophenones
Oxybenzone (Benzophenone-3) 295-375 Good Some Yes -
Sulisobenzone (Benzophenone-4) 260-375 Good Some Yes Hard to solubilize
Dioxybenzone (Benzophenone-8) 250-390 Good Some Yes Hard to solubilize
Other Organics
Ensulizole 290-340 Good Little Yes -
Octocrylene 295-375 Good Little Yes -
Menthyl anthranilate (Meradimate) 295-380 Good Some Yes -
Avobezone (Parsol 1789) (Butyl methoxydibenzoyl methane) 295-395 Good Good Yes If not well formulated, loses potency
**NEW** Ecamsule (Mexoryl SX) 310-370 Some Good Yes Water-soluble
Inorganic Ingredients
Titanium Dioxide up to 365 Good Good No -
Zinc Oxide up to 380 Good Good No -

Overview of Sunscreen Ingredients: Student Handout

Organic Ingredients Inorganic Ingredients (Nano) Inorganic Ingredients (Large)
Atoms Involved Carbon, Hydrogen, Oxygen, Nitrogen Zinc, Titanium, Oxygen Zinc, Titanium, Oxygen

Structure

(not drawn to scale)

Individual molecule

Cluster \begin{align*}\sim 100 \;\mathrm{nm}\end{align*} in diameter

Cluster \begin{align*} > 200 \;\mathrm{nm}\end{align*} in diameter

Interaction with UV light

Absorb specific \begin{align*}\lambda\end{align*}

Absorb all UV \begin{align*} < \end{align*} critical \begin{align*}\lambda\end{align*}

Absorb all UV \begin{align*} < \end{align*} critical \begin{align*}\lambda\end{align*}

Absorption Range Parts of UVA or UVB spectrum Broad spectrum UVA and UVB Broad spectrum UVA and UVB
Interaction with Visible light None

Minimal Scattering

Much Scattering

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