Q.Write a short note on the quantum theory of light.

Four historical theories, each explaining some but not all of light's behaviour, trace the development of our modern understanding. Newton's corpuscular theory (1672) treated light as a stream of tiny, perfectly elastic, near-massless particles ('corpuscles') emitted by a source; it explained reflection and refraction as attraction or repulsion of the corpuscles by the medium, and different colours as corresponding to corpuscles of different sizes, but it wrongly predicted light should travel faster, not slower, in a denser medium, and it could not explain interference, diffraction or polarisation. Huygens' wave theory (1678) treated light as a longitudinal mechanical disturbance travelling through a hypothetical medium called 'ether' presumed to fill all space; it successfully explained reflection, refraction, interference and diffraction, but (once the ether was shown not to exist) could not explain propagation through vacuum, and -- being based on longitudinal waves -- could not explain polarisation, a property unique to transverse waves. Maxwell's electromagnetic wave theory (1864) showed light is a transverse electromagnetic wave that needs no medium at all to propagate, successfully accounting for every classical optical phenomenon including polarisation, but it could not explain …