A bit of background: The white light from the sun is a mixture of all colours of the spectrum. Each colour of light ahas a wavelengths. The visible part of the spectrum ranges from red light (wavelength =720 nm), to violet (wavelength =380 nm), with orange, yellow, green, blue and indigo between. The human eye reacts most strongly to strongly to red, green and blue wavelengths. This gives us colour vision. The explanation: Light passing through a clear fluid with suspended particles is scattered. Some wavelengths like blue are scattered more strongly. The first person to notice this and experiment with it was John Tyndall in 1859. So he got to mane the effect the Tyndall Effect. He made three important observations: * From the side, (the way we see most of the sunlight in the sky) the beam can be seen by the blue light it scatters * The light seen directly at the end (looking towards the source) is red. This is the way we see the sunlight at sunset

* The scattered light is polarized. This is why polarized sun glasses make some parts of the sky seem darker.

Some early researchers (Tyndall and Rayleigh) thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the sky. Later scientist discounted this and proposed that oxygen and nitrogen molecules are the cause of the scattering.

In 1911 Einstein did the math to prove that the molecules could cause the scattering. Technically the molecules scatter light because the electromagnetic field of the light waves induces electric dipole moments in the O2 and N2 molecules,