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Reflection and Absorption
Name: George T.
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 2000-2001
Question:
How do different surfaces affect the amount of sunlight
reflected and absorbed?
Replies:
George,
The amount of light reflected depends on the smoothness of the surface --
smooth reflects better than rough. The amount absorbed depends on the color
of the surface -- dark colors absorb better than light; flat black absorbs
best of all.
Also, the angle at which the light strikes the object has an effect on the
amount absorbed and/or reflected. If the rays strike the surface straight
down at a 90 degree angle, absorption is favored and reflection less favored.
If the light strikes the surface at a low angle, reflection is favored over
absorption.
However, the interplay of these factors and the composition of the surface
can complicate things greatly. Consider also the difference between a black
(or white) car and a black (or white) bath towel -- or the difference between
a glass mirror and a piece of glass covered with a film of flat-black soot.
Finally, large differences can occur if the light can penetrate into the
surface. The greater the penetration, the less the reflection and the more
the absorption.
Regards,
ProfHoff
Each atom has certain energy levels it can absorb. Photons of light at or
near these levels can enter the atoms and be held for a little while. Such
energy tends to be given to other atoms, and emitted in smaller parts as low
energy wavelength. The material heats up.
Photons at energies that cannot be held are spit back out from the atom
almost immediately. A little bit may find its way deeper into the material.
Almost all is reflected back.
It just happens that energy of a photon of light is proportional to
frequency. It is frequency that determines color. The lowest frequency a
human eye responds to is red. The highest is violet. We can see all the
frequencies in between as well. A material that can absorb all the visible
frequencies except red will look red. A light that can absorb blue, violet
and everything in between will look yellow.
Another effect on absorption vs. reflection is the smoothness of a surface.
A very rough surface has many more molecules per unit area to be hit by
light. On a rough surface, a reflected photon is more likely to hit a
second atom. More light gets absorbed. Light that doesn't get absorbed
will be dispersed: reflected back randomly rather than in a narrow beam.
The best absorber is a rough, black surface. The best reflector is a very
smooth, light-colored, metal surface.
Ken Mellendorf
Your question is pretty general, and the answer is rather complicated
because many factors come into play.
Black [surface absorbs all VISIBLE light], dull [ that is not reflective]
surfaces absorb almost all the sunlight that strikes it; however, you have
to remember that a surface that is black in the visible region [ about 400
to 700 nm ] of the electromagnetic spectrum may be reflective in the
ultraviolet [shorter wavelengths]or in the infrared [longer wavelengths].
Colored surfaces absorb and/or reflect selective wavelengths of visible
light.
Polished metallic surfaces reflect a large fraction of the light that
strikes it. However, a dull metallic surface can absorb a sizeable amount of
energy that strikes it. Remember, metallic things left in the sun get very
hot. I'm not sure if this is due to absorption of visible or infrared
radiation, however.
Snow, and other white but non-reflective surfaces, also reflect and/or
scatter a large fraction of visible light that strikes it.
Vince Calder
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Update: June 2012
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