Just saw a video about how the shadow on moon is pitch black.

That took me to the question of scattering. I have myself asked a question similar to this already.

When light from a bulb(consisting of whole blackbody spectrum) hits/interacts with the air molecules, it gets absorbed. Then, the molecules and electrons of molecules themselves vibrate with the frequency of the incident light and radiate light of their own (which is again blackbody spectrum mainly).

Now, in a room the shadow is formed dur to the blocked light from light source. But the air behind the blocking object emits light which illuminates the shadow region.

In a sense the air molecules are small light bulbs which are all around the room.

Also, scattering is mostly understood as light 'bouncing' off the atoms here and there, but I don't really like this way of thinking about it.

How correct and incorrect is my understanding regarding this. (Please point out any missing/incomplete pieces of important knowledge I seem to lack.)

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    $\begingroup$ The scattering of a black body spectrum on air molecules is, for sure, not a black body spectrum. That's why the sky is blue in the first place. $\endgroup$ Apr 6 at 17:18
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    $\begingroup$ If air absorbed light if any wavelength and re-emitted it constantly, air would not be transparent, it would be opaque white. The reason air is transparent is that the light emitted/reflected from objects does not get absorbed by the air on its way to our eyes. $\endgroup$ Apr 6 at 18:41

1 Answer 1


in a room the shadow is formed due to the blocked light from light source.


But the air behind the blocking object emits light which illuminates the shadow region.

No. The entire room will have surfaces which are illuminated and this light partially illuminates the area that is blocked from direct light. This is one of the main effects which have to emulated/simulated when 3D graphics are being created. It's light from other illuminated surfaces that lightens shadows.

Also note that air behind the blocking object does not get any direct illumination itself, so it has nothing to contribute to lighting.

In a sense the air molecules are small light bulbs which are all around the room.

I get the impression you think that the molecules in air are absorbing all (or most) of the light and re-emitting it. This is wrong. If you had a room with air and some electric light source and recorded the shadows and how the light level was there and then removed the air and did the same measurement, there would be practically no difference in partial illumination of shadows. The partial illumination in shadows is due to light scattered from other surfaces, not air.

  • $\begingroup$ When white light falls on a blue painted wall of a room, Do all components of light get absorbed by the wall and only blue is reflected? OR, Most of the white light is reflected back by the wall along with higher intensity of blue? How much is absorbed then? $\endgroup$ Apr 7 at 3:09
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    $\begingroup$ @RohitShekhawat I'm afraid there's no simple answer to those questions. Different materials absorb and reflect different frequencies of light and the emission spectrum of a material will in general be different from the spectrum of the incoming light. Note that heat is also light and some of the energy is redistributed as heat, that is as light outside the visible part of the spectrum. It's why sunlight warms a surface it illuminates, $\endgroup$ Apr 7 at 11:55
  • $\begingroup$ Iam asking in reference to partially illuminating the shadow. Does the blue wall(or other surfaces in a room) 'reflect' a spectrum of light which more or less contains white light components and hence illuminates the shadow? $\endgroup$ Apr 8 at 3:04

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