Physics · Wave Optics · NEET
Unpolarised sunlight has electric field vibrations in all directions in the plane at right angles to its travel. When it hits an air molecule, it makes the molecule's electrons vibrate in that same plane. A vibrating (oscillating) charge sends out light in all directions EXCEPT along its own line of vibration. So when you look at 90 degrees to the sunlight, you only receive the light produced by electron vibrations that are perpendicular to your line of sight. The other vibration direction points straight at you, and that one sends nothing towards you. Only one vibration direction survives, so the scattered light is plane polarised.
Only the light coming from directions at 90 degrees to the Sun is fully polarised. Light scattered at other angles (like 45 degrees) is only partly polarised, and light scattered straight forward (0 degrees) or straight back is not polarised at all. So the sky is most strongly polarised in a band 90 degrees away from the Sun. You can check this with a polaroid (like polarised sunglasses): rotate it while looking at that part of the sky and the brightness changes.
Both give polarised light, but by different mechanisms. In scattering, light is re-emitted by vibrating electrons in air molecules, and the light emitted at 90 degrees is fully polarised. In reflection (Brewster's law), light reflecting off a surface like water or glass is fully polarised when the reflected and refracted rays are at 90 degrees to each other, which happens at the Brewster angle where tan(iB) = refractive index. Scattering explains the polarised blue sky; reflection explains glare off water.
Yes. Polarisation of any kind can only happen for transverse waves, where the vibration is perpendicular to the direction of travel. A longitudinal wave (like sound) vibrates along its travel direction and cannot be polarised. Because scattered sky light is polarised, light must be a transverse wave. This is the same reason Malus's law and Brewster's law also prove the transverse nature of light.
These are two separate effects of the same scattering. The colour is due to Rayleigh scattering: shorter wavelengths (blue) scatter much more than longer ones (scattering strength goes as 1 over wavelength to the fourth power), so the sky looks blue. The polarisation is due to the sideways electron vibration and the 90-degree geometry. So air molecules scatter sunlight, and that scattered light is both blue AND polarised, but for different reasons.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
At 90 degrees to the direction of the incoming sunlight. Light scattered at other angles is only partially polarised, and forward-scattered light is unpolarised.
An oscillating charge radiates light strongest perpendicular to its motion and radiates nothing along its own line of oscillation. So an observer standing along that line receives no light from that particular vibration, which is what removes one vibration direction and leaves the light polarised.
Look at a clear part of the sky about 90 degrees away from the Sun through a polaroid (polarised sunglasses work). Slowly rotate the polaroid. If the brightness rises and falls, the light is polarised.
No. Direct sunlight is unpolarised. Only the light that has been scattered by air molecules and viewed near 90 degrees to the Sun becomes polarised.
Malus's law (I = I0 cos^2(theta)) describes intensity after light passes through a polaroid. Scattering produces the polarised light in the first place; if you then pass that light through a polaroid, its intensity would follow Malus's law.
