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What color is a mirror?

Date: Tue Nov 12 16:54:22 1996
Posted by: Gabrielle Cordero
Grade level: 7-9
School/Organization: No school given.
City: Ballard State/Province: Wa
Country: USA
Area of science: Other
Message ID: 847839262.Ot


What color is a mirror?

Now that is a question I never would have thought of!

First lets define what color is. What we call white light is actually made up of lots of different colors of light, literally all the colors of the rainbow. It has been known for hundreds of years that light can behave like a wave. You can think of it like a series of ripples, with a height to each ripple and a distance between the incoming ripples (called the wavelength). The color of light is determined by the wavelength: the long wavelengths make red light, somewhat shorter makes yellow, shorter even than that makes blue.

Suppose you are wearing a red shirt. That means that of all the colors of light hitting the shirt, they all get absorbed by the shirt except for red. The red light gets reflected by the shirt and into your eye. So the color of an object depends on what wavelengths of light it reflects. If it reflects all wavelengths, we say it is white. If it reflects none it is black.

A perfect mirror does actually reflect all light hitting it. So why doesn't it look white? It's because a mirror reflects light in a coherent manner; that is, the light is reflected back from the mirror depending on how the light came in. A white shirt just reflects light back everywhere in all directions. Even if red and blue light hit the shirt coming from the same direction, they may get scattered in different directions. A mirror, on the other hand, reflects the blue and red light in the same direction, and so the mirror actually builds an image of the source of the light.

So you can think of a mirror as being white, since it reflects all colors, but a smart kind of white. I guess you could say that a mirror is simply the color of whatever light source it sees!

There are many places on the web that talk about rainbows, mirrors and light. Here is a great site about rainbows and colors, which is a good place to get started. This site has some nifty diagrams about light as well.

©2008 Phil Plait. All Rights Reserved.

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