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Visible Light and Matter

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Visible Light and Matter
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A mime is an actor who uses movement and facial expressions rather than words to communicate with an audience. The mime in this picture is using a mirror to apply stage makeup that will accentuate her features so she can communicate more expressively. When light strikes a mirror, it is reflected back from the shiny surface. The reflected light forms an image of whatever is in front of the mirror. Reflection is just one way that visible light may interact with matter.

Reflection

Reflection of light occurs when light bounces back from a surface that it cannot pass through. Reflection may be regular or diffuse.

  • If the surface is very smooth, like a mirror, the reflected light forms a very clear image. This is called regular, or specular, reflection. The smooth surface of the still water in the pond on the left below reflects light in this way. The reflected image of the tree in the water is almost as clear as the mime’s image in the mirror above.
  • When light is reflected from a rough surface, the waves of light are reflected in many different directions, so a clear image does not form. This is called diffuse reflection. The ripples in the water in the picture on the right below cause diffuse reflection of the blooming trees.

Regular and diffuse reflection

For more details about regular and diffuse reflection, go to this URL: http://www.physicsclassroom.com/Class/refln/U13l1d.cfm

Transmission

Transmission of light occurs when light passes through matter. As light is transmitted, it may pass straight through matter or it may be refracted or scattered as it passes through.

  • When light is refracted, it changes direction as it passes into a new medium and changes speed. The straw in the Figure below looks bent where light travels from water to air. Light travels more quickly in air than in water and changes direction. For a detailed explanation of how this happens, watch the animation at this URL:

http://www.physicsclassroom.com/mmedia/optics/bp.cfm

Bent straw in water

  • Scattering occurs when light bumps into tiny particles of matter and spreads out in all directions. In the Figure below , beams of light from car headlights are shining through fog. The light is scattered by water droplets in the air, giving the headlights a “halo” appearance.

Light scattering in fog

Q: What might be another example of light scattering?

A: When light passes through smoky air, it is scattered by tiny particles of soot.

Absorption

Light may transfer its energy to matter rather than being reflected or transmitted by matter. This is called absorption . When light is absorbed, the added energy increases the temperature of matter. If you get into a car that has been sitting in the sun all day, the seats and other parts of the car’s interior may be almost too hot to touch, especially if they are black or very dark in color. That’s because dark colors absorb most of the sunlight that strikes them.

Q: In hot sunny climates, people often dress in light-colored clothes. Why is this a good idea?

A: Light-colored clothes absorb less light and reflect more light than dark-colored clothes, so they keep people cooler.

Classifying Matter in Terms of Light

Matter can be classified on the basis of its interactions with light. Matter may be transparent, translucent, or opaque. An example of each type of matter is pictured in the Figure below .

  • Transparent matter is matter that transmits light without scattering it. Examples of transparent matter include air, pure water, and clear glass. You can see clearly through transparent objects, such as the top panes of the window below , because just about all of the light that strikes them passes through to the other side.

Light passes through transparent objects

  • Translucent matter is matter that transmits light but scatters the light as it passes through. Light passes through translucent objects but you cannot see clearly through them because the light is scattered in all directions. The frosted glass panes at the bottom of the window above are translucent.
  • Opaque matter is matter that does not let any light pass through it. Matter may be opaque because it absorbs light, reflects light, or does some combination of both.

Examples of opaque objects are objects made of wood, like the shutters in the Figure below . The shutters absorb most of the light that strikes them and reflect just a few wavelengths of visible light. The glass mirror below is also opaque. That’s because it reflects all of the light that strikes it.

Opaque objects

Summary

  • Reflection of light occurs when light bounces back from a surface that it cannot pass through. If the surface is very smooth, the reflected light forms an image.
  • Transmission of light occurs when light passes through matter. As light is transmitted, it may pass straight through matter or it may be refracted or scattered by matter.
  • Absorption of light occurs when light transfers its energy to matter rather than being reflected or transmitted by matter. The temperature of matter increases with the added energy.
  • Matter can be classified as transparent, translucent, or opaque depending on how it interacts with light.

Vocabulary

  • absorption : Interaction of light with matter in which the particles of matter absorb light energy so light neither reflects from nor passes through matter.
  • opaque : Referring to matter that does not allow visible light to pass through it because it reflects or absorbs all of the light.
  • scattering : Process in which transmitted light is spread out in all directions by particles of matter.
  • translucent : Referring to matter that transmits but scatters visible light.
  • transmission : Process in which light passes through matter.
  • transparent : Referring to matter that allows all visible light to pass through.

Practice

Make a word search or criss-cross puzzle using the vocabulary terms in this article. You can use the free puzzle maker at the following URL. Exchange puzzles with a classmate and try to solve each other’s puzzles. http://www.discoveryeducation.com/free-puzzlemaker/?CFID=2476446&CFTOKEN=27024543

Review

  1. Describe three ways that light can interact with matter.
  2. Transmitted light may be refracted or scattered. When does each process occur?
  3. Why does matter increase in temperature when it absorbs light?
  4. Compare and contrast transparent, translucent, and opaque matter.

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