why sky looks blue?

Answer 1

My standard answer to this often asked question:

This is a commonly asked question. and the correct answer is that the molecules in the air scatter the blue wavelength of the spectrum (known as the Tyndall effect, or Rayleigh scattering)..That is fine, but I know science graduates who still don't understand this explanation.

Here is my explanation, with a limited amount of physics involved: (I also improved my answer, thanks to an answer given by Yahoo member Einstein a few weeks ago)

I think most people know that white light is made up of a spectrum of different colours, which can be broken down with a prism (or in a rainbow), showing a range of different colours, from red to blue. Technically, the red light has the longer wavelength, and the blue the shorter wavelegth (there are other wavelengths such as infrared and ultraviolet that our eyes cannot see).

When the light passes through the atmosphere, the sunlight reacts with the air molecules, causing them to oscillate and emit radiation of the same frequency (the same colour) as the incoming sunlight, but the emitted light is spread over all directions (called scattering). Because the blue wavelengths are the shortest and most energetic, they react much more with the air particles, and are scattered more, wheras the red and yellow wavelengths tend to pass right through.

That is why, when we look up at the sky, the sky seems blue, and the blue colour seems to be coming from all directions. .We are seeing that scattered light, which is made up much more of the blue part of the spectrum, rather than the red and yellow. Closer to the sun, the sky is less blue, because of the red and yellow part of the spectrum coming straight through the atmosphere.

That is a simplified answer, and not scientifically complete, but is the best explanation I can think of, without using physics.
I am indebted to member Einstein, for putting me straight on this, as my original aqnswer was much too simple.