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Name: Elizabeth H.
Status: student
Age: 11
Location: N/A
Country: N/A
Date: Sunday, September 15, 2002


Question:
Which reflects more light,a white polished surface or a silver mirrored surface?


Replies:
Elizabeth,

In most cases, they would reflect very similar amounts. The biggest difference is where they reflect it. A shiny mirror reflects almost all of the light in the same direction. This makes a clear image in the mirror. A white polished surface reflects the light in all directions. This why walls in reading rooms are often painted white. The light from a few bulbs reflects everywhere.

I do not know that there is a definite rule. It would depend on the material and possibly on the color light used. A good test is sunlight. Take two shiny white plates. Cover the top of one with aluminum foil, shiny side out. Be sure the foil is very smooth and there is not any air between the plate and the foil. Put a paper grocery bag or large piece of cardboard on the ground in the sunlight on a hot day when the sun is very bright. Place both plates on the bag. After fifteen minutes, take the foil off. See which plate is hottest. The hottest plate absorbed the most light. The coolest plate reflected the most light.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


This is not as straight forward as it sounds on first reading. When light (or other electromagnetic radiation) strikes a FLAT surface it can do one or more of several things:

1. It can be REFLECTED at angle equal to the angle of incidence (usually measured as the angle from the perpendicular of the flat surface.

2. In can be TRANSMITTED through the surface. In this case the path of the light will be changed depending upon the index of refraction of the substance, (the quantitative amount is determined by a relation called Snell's Law.

3. ALL of the light can be ABSORBED by the substance, in which case the surface will appear BLACK.

4. SOME of the wavelengths of the light can be ABSORBED by the substance. If the incident light is WHITE, that is composed equally of all wavelengths of visible light, the REFLECTED light will appear colored. The color will be the COMPLEMENT of the absorbed wavelengths. That is, WHITE light MINUS the absorbed wavelengths. So if blue light is absorbed, the REFLECTED light will appear yellow; if red light is absorbed, the REFLECTED light will appear green.

5. The light can be REFLECTED and SCATTERED. That is the angle of reflection in different than the angle of incidence. If the surface is rough and randomly irregular, the REFLECTED light will appear white.

6. If the surface is irregular, but irregular in a regular way, such as a series or closely spaced scratch marks, the light will be DIFFRACTED, and the surface is called a GRATING. Then the reflected light will be the color of the rainbow. And incident white light will be decomposed into its various contributing wavelengths.

All of these six things are going on at the same time to a greater or lesser extent, so it is difficult to say with confidence whether a surface that is white and polished, or is a mirror, whether the amount of total energy reflected is greater or less for one or the other.

Vince Calder



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