Name: Clarence L.
why does the moon appear white? If it receives light from
the sun, and we see the sun as yellow from earth, shouldn't the moon also
I am not sure I agree that we perceive the sun as yellow.
At noon, when the sun reaches you through a minimum amount of air, it is white,
even after you attenuate it by projecting it through a pinhole onto paper.
And sometimes when the moon is low on the horizon it is brownish, which is a dimmer form of yellow.
In short, the sun shines white into space, the moon is neutral gray so it reflects the same percentage in all colors, so it is a dimmer white,
and then the atmosphere you see it through (whether moon or sun) may or may not take out some of the blue, making it yellowish.
Finally, the color perception of the eye itself is quite adjustable by its surroundings, present and past.
You have to be very careful in distinguishing the spectral distribution of
light from its "color". The spectral distribution is the intensity (number
of photons / area) vs. wavelength (or frequency). "Color" refers to the
response of the eye and brain to light. They are not the same thing. Visible
light from the Sun is approximately "white" in the sense that the spectral
intensity distribution of visible radiation is essentially constant (above
the atmosphere). The apparent yellow "color" of sunlight arises from two
origins: preferential scattering of blue light by the atmosphere and the
higher sensitivity of the eye to yellow wavelengths compared to other
wavelengths of the same spectral intensity. The "whiteness" of moonlight is
due for the most part from the fact that it is faint. At low levels of
intensity the eye loses its sensitivity to "color". You can demonstrate this
be trying to distinguish different colors in a darkened room. It is
difficult because the receptors in the eye that distinguish color are not
sensitive to low intensities of light. Also recall if you have seen any
"moon-rocks" that they are dark gray -- almost black -- so the moon is a
very poor reflector of the incident light from the sun.
Another way to convince yourself that sunlight is indeed white and not yellow
is to hold a white sheet of paper out in sunlight and look at it. If it
was illuminated by yellow light, as it would be if you were using a
flashlight with a yellow lens, the paper would appear yellow; but it doesn't.
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Update: June 2012