Why is it that when the sun sets, the sky changes colour?

Question

Why does it always change colour to either orange or red and not something weird like violet?

Answer 1

Because of the refractory power of our atmosphere.

When the Sun is low in the sky, the last rays are "caught" by the atmosphere and refracted. Red light has the biggest wavelength, and is refracted most strongly. Violet has the shortest wavelength, and is refracted the least. This means that red light will reach the Earth, whereas green, blue and violet will simply pass through. This colours the sky red.

Answer 2

It's NOT because of refractive dispersion (this effect exists but is very, very slight).

It IS because the sunlight is travelling through many more miles of air when it's on the horizon, compared to when it's overhead (see this graphic (http://incolor.inetnebr.com/rbrown/astro... ) to visualize why this is so.)

The light coming from the sun consists of all different wavelengths.  The short (bluish) wavelength tend to scatter away and bounce all over the sky--this in fact is why the sky is blue.  The long (reddish) wavelengths don't scatter so much, so they continue on in a straight path for a long time.

As a result, as sunlight passes through more and more miles of air, more and more of the blue light will be scattered away, and what's left over will be redder and redder.  So the light from the setting sun (which passes through about 1,000 miles of air) looks considerable redder than the light of the noonday sun (which passes through only about 100 miles of air).