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Q & A: Shiny materials reflecting light

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Most recent answer: 10/22/2007
Q:
Is that true that shiny materials can reflect radiation well? If yes, how does it work?
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- Kong (age 13)
La Salle College, Hong Kong
A:
Yes, shiny materials are good reflectors of visible light, one of many kinds of radiation. They may not be such good reflectors of other kinds of radiation, like x-rays, for example, which are just like light except they have higher frequencies.

Many different kinds of metal are shiny. Gold is a metal which stays shiny for a long time because it does not react much chemically with the air. Silver is shinier, but tarnishes easily. Many other metals, like iron or steel, aluminum, and copper are also shiny.

What makes them that way is that some of the electrons in these metals can move around very very easily. Electrons feel pushes and pulls when an electric field comes along. Light waves are made up of electric and magnetic fields. When a light wave hits a metallic surface, the electrons on the surface are pushed and pulled by the field of the incoming wave. How far do they go? They slosh around until the field that they create (after all, electrons are charged and they make their own field) cancels out the incoming field exactly. They stop when the net force is zero on them.

This means that the electric field inside a conducting metal is zero. If an incoming wave hits the material, and the electric field is zero on a plane surface at all times, you can express this as a sum of two waves -- one coming in, and an equal and opposite one coming out. The sloshing electrons in the metal radiate a wave going out that exactly matches the one coming in. If there is some resistivity to the metal, or some corrosion, the metal becomes less shiny.

There's another way a material can be shiny. If a material is transparent, but has a different index of refraction than the air, then light rays will bend when they strike the surface of the material. Some, but not all, will also be reflected from the surface at the same angle of reflection as if the material had been made out of metal. If the light starts out in the dense material (like water or glass) and hits the surface with air, if the angle is steep enough so that the law of refraction cannot be satisfied, then all of the light will bounce back into the water or glass, in a process called "total internal reflection", making that surface look shiny too.

Tom
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(published on 10/22/2007)

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