visible light and matter

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 1.4. 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 1.4, because just about all of the light that strikes them passes through to the other side. 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 1.4 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 1.5. The shutters absorb most of the light that strikes them and reflect just a few wavelengths of visible light. The glass mirror 1.5 is also opaque. Thats because it reflects all of the light that strikes it.

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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 1.2 looks bent where light travels from water to air. Light travels more quickly in air than in water and changes direction. Scattering occurs when light bumps into tiny particles of matter and spreads out in all directions. In the Figure air, giving the headlights a halo appearance. Q: What might be another example of light scattering? A: When light passes through smoky air, it is scattered by tiny particles of soot.

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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 cars interior may be almost too hot to touch, especially if they are black or very dark in color. Thats 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.

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. In the Figure 1.1, the smooth surface of the still water in the pond on the left reflects light in this way. 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. In the Figure 1.1, the ripples in the water in the picture on the right cause diffuse reflection of the blooming trees.

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instructional diagrams

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questions

a clear image is produced by diffuse reflection.

a. true

-->  b. false

regular reflection occurs when the reflective surface is very smooth.

-->  a. true

b. false

when light passes from one medium into another it may

a) change direction.

b) change speed.

c) be refracted.

-->  d) all of the above

when light is absorbed, it transfers its energy to matter.

-->  a. true

b. false

what happens to light that strikes transparent matter?

a) it is reflected.

b) it is absorbed.

-->  c) it is transmitted.

d) none of the above.

matter is opaque if it

a) absorbs light.

b) reflects light.

c) transmits light.

-->  d) two of the above

diagram questions

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