Naked Science Forum

General Science => General Science => Topic started by: guest10038 on 21/02/2010 10:48:05

Title: Why is the sky blue?
Post by: guest10038 on 21/02/2010 10:48:05
why sky seems to be blue at day time?

Mod Edit: Please phrase your posts in the form of a question.
Title: Why is the sky blue?
Post by: Madidus_Scientia on 21/02/2010 11:49:26
Particles in the air absorb light in the blue wavelength, and then re-emit it in a random direction. This has the effect that all parts of the sky will be sending some blue light towards you.

This is also why the sun appears yellow at sunset/rise (when the light is travelling through more atmosphere), much of the blue light is scattered out of the direct path between you and the sun, making it seem yellow. But the sun is actually white.
Title: Why is the sky blue?
Post by: lightarrow on 21/02/2010 13:18:19
Quote from: Madidus_Scientia on 21/02/2010 11:49:26
Particles in the air absorb light in the blue wavelength, and then re-emit it in a random direction. This has the effect that all parts of the sky will be sending some blue light towards you.

This is also why the sun appears yellow at sunset/rise (when the light is travelling through more atmosphere), much of the blue light is scattered out of the direct path between you and the sun, making it seem yellow. But the sun is actually white.
Ok, but scattering is not "absorption". Air molecules don't have absorption lines in the visible range. The reason they scatter blu-violet more than the other colours is because of Rayleigh law of scattering: the intensity of scattered light is proportional to the 4th power of the frequency:
http://en.wikipedia.org/wiki/Rayleigh_scattering
If our eye would be equally sensitive to violet and blu, we would see the sky as violet, because that corresponds to the maximum frequency of visible light, but since our eye is much more sensitive to blue instead, we actually see it as blue.
Title: Why is the sky blue?
Post by: Madidus_Scientia on 21/02/2010 19:22:59
Ah, I see