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# 2.1: How Light Interacts with Matter

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

$\sim.35\%$

($5\%$ of all UV)

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

$\sim 6.5\%$

($95\%$ of all UV)

Tanning Skin Aging DNA Damage Skin Cancer No
Visible $\sim 400-800 \;\mathrm{nm}$ Lower Energy $\sim 43\%$ None Currently Known Yes
IR $\sim 800-120,000 \;\mathrm{nm}$ Lowest Energy $\sim 49\%$ Heat Sensation (high $\lambda$ 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
• Cotton swabs (for apply sunscreen to the Sunscreen Smear Sheet)
• Alcohol wipes (for cleaning sunscreen off the Sunscreen Smear Sheet)

Procedure

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 $1$ to $5$ scale.
Use $5$ to represent no opacity (you cannot see the substance at all).
Use $1$ 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 $3$ 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 $(2-10)$ of C1 on your data chart.
• Expose C2 to the UV light for $30 \;\mathrm{sec}$. and quickly compare it to the UV bead color guide. Record the bead color number $(2-10)$ on your data chart.

• 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 $30\;\mathrm{secs}$.
• 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 $(2-10)$ for both E and C2 on your data chart.
• Repeat for each of your substances.

Data Chart

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 $\bullet$ 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 $\rightarrow$ No Blocking $(10)$ Low Blocking $(8)$ Medium Blocking $(6)$ High Blocking $(4)$ Total Blocking $(2)$
Opacity $\downarrow$
$5$ Fully Transparent
$4$
$3$
$2$
$1$ 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?

$\Box$ Yes

$\Box$ No

$\Box$ 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

$\Box$ Yes, there is a relationship.

$\Box$ No, there is not a relationship.

$\Box$ 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?

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 $\sim 100 \;\mathrm{nm}$ in diameter

Cluster $> 200 \;\mathrm{nm}$ in diameter

Interaction with UV light

Absorb specific $\lambda$

Absorb all UV $<$ critical $\lambda$

Absorb all UV $<$ critical $\lambda$

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 $(ZnO)$ or titanium dioxide $(TiO_2)$ as an ingredient.

Note: the proper scientific name for $TiO_2$ 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?

### Summary of FDA Approved Sunscreen Ingredients: Student Handout

$\lambda$ range $(\;\mathrm{nm})$ Protection Against Protection Against Possible Allergies Other Issues
UVB $280-320 \;\mathrm{nm}$ UVA $320-400 \;\mathrm{nm}$
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 upto $365$ Good Good No -
Zinc Oxide upto $380$ Good Good No -

Name_______________

Date_______________

period_______________

### Reflecting on the Guiding Questions: Student Worksheet

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 $E=hf$ where $E$ is the photon’s energy, $f$ is its frequency, and $h$ is Plancks’ constant: $6.26 \times 10^{-34} \;\mathrm{J \ s}$), 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 $\lambda=c/f$ where $\lambda$ is the wavelength, $f$ is the frequency and $c$ 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 $E=hf$) 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 $(COOH^-)$, a molecularanion. 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 n

## Date Created:

Feb 23, 2012

Apr 29, 2014
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