Y is the sky blue?

Question

Y is the sky blue?

Answer 1

Ah yes!

This question has been asked  over 7000 times so far, and  I've prepared a simple answer, without too much science:

The technically correct answer is that the blue light is scattered by the air molecules in the atmosphere (referred to as Rayleigh scattering).  

OK, but I've known science graduates who don't understand what this means.

Here's my attempt at an answer without too much physics:

I think most people know that sunlight is made up of light of several different wavelengths, and can be split up into the colours of the rainbow. Blue light has the shorter wavelength, and red the longest wavelength.

When sunlight hits the molecules in the atmosphere, the light is absorbed; causing the molecules vibrate and and give off, or 're-emit' the light. Because the molecules vibrate in all directions, the light is emitted in all directions (called 'scattering'). The molecules in the air are much smaller than the wavelength of visible light, but because the blue wavelength is shorter and more energetic, it reacts much more with the air molecules than the red and yellow wavelengths; which tend to pass straight through.

Because the blue radiation is re-emitted from the air molecules in all directions, it seems to us looking from the ground that the blue light is coming from everywhere; hence the sky seems blue.

Near sunset, because of the low angle of the sunlight, the blue light has already brrn scattered away, and we see more of the red and yellow wavelendth, hence the colours of the setting sun.

BTW: The sky isn't blue because of a reflection of the sea. its the other way round. As well as reflecting the blue from the sky at the surface, sea water also scatters the blue light. The blue colour of the sea is a little more complicated, because as well as the water molecules scattering the blue light, the water absorbs more of the red and yellow wavelengths, leaving the blue part of the spectrum, as well as part of the green (which is why deep water can appear bluish-green).

This scattering effect is even stronger with ice; which results in the intense blue colour we see if we look down a crevasse in a glacier, or down a hole in the snow made by a ski stock..

My thanks to varoius contributers for correcting me on some details.

For complete, scientific explanations, look up 'blue sky' in Wikipedia, and follow through the references.

Answer 2

Because that's the colour it usually is. Although it changes due to the sun and clouds.

Answer 3

Contrary to popular belief the sky is not blue because water is blue the water is blue because the sky is. The sky is blue because are our atmosphere scatters blue light from the sun while leaving red light to pass through. what we see as a blue sky is that scattering of the suns light. This is also why the moon is red right before an eclipse because our atmosphere prevents the blue light from getting to the moon and therefore only red light is reaching it.

Answer 4

light scattering through Nitrogen particles...

don't you watch the fish fingers advert?

Answer 5

Particles in the atmosphere scatter blue light more than any other color in the visible portion of the electromagnetic spectrum.