Nature of Light

7.2.1 Corpuscular nature

The formation of shadows, and of images by mirrors and lenses, is explained by rectilinear motion of light. This led R. Descartes (1596–1650) to propose in 1636 that light is made of particles. Newton (1642–1726) developed the idea: light consists of corpuscles which are hard, elastic and massless, emitted by a source and travelling in straight lines in the absence of an external force.

• Reflection: the corpuscles strike the surface and undergo elastic collisions, so they obey the laws of reflection.
• Refraction: the particles of the medium attract the corpuscles. A denser medium exerts a larger attractive force, accelerating them along the normal to the boundary.
• Colour: light of different colours corresponds to corpuscles of different sizes.

Why the theory failed

Because a denser medium was supposed to ACCELERATE the corpuscles, the theory predicted that the speed of light in a denser medium is greater than in a rarer medium. This contradicts experiment. Interference, diffraction and polarization — discovered in the nineteenth century — could not be explained by corpuscles at all.

7.2.2 Wave nature

To get round these difficulties, C. Huygens (1629–1695) proposed in 1668 that light is a wave, caused by the vibrations of the particles of the medium. Since light travels through vacuum too, he had to assume a hypothetical all-pervading medium called ether (not the ether of chemistry). There was never any evidence for it.

• Huygens' theory explained the laws of reflection and refraction, the formation of images by mirrors and lenses, and also interference, diffraction and polarization.
• It required the speed of light in a denser medium to be SMALLER than in a rarer medium, so that light bends towards the normal — which agrees with experiment.
• The branch of optics that uses the wave nature of light to explain optical phenomena is called wave optics.
• Maxwell's equations (Std XI) showed that light is an electromagnetic wave; Maxwell too invoked ether, since every wave known at the time needed a medium. Hertz later verified experimentally that radio waves exist and travel at the speed of light.
• Michelson and Morley tried repeatedly to detect ether and always got a negative result. Einstein's special theory of relativity (1905), built on the constancy of the speed of light, ruled out both the existence and the necessity of ether. He also showed that $c$ does not depend on the motion of the source or the observer, and that nothing can travel faster than $c=3\times10^{5}$ km/s in vacuum.

7.2.3 Dual nature

In the early twentieth century it was accepted that light has a dual nature: it shows particle behaviour in some situations and wave behaviour in others. The particles of light are called photons (Chapter 14).

Comparison the board asks for

Standard results

• Polarization is the phenomenon that proves light is a TRANSVERSE wave — reflection, interference and diffraction occur for longitudinal waves too.
• Interference, diffraction and polarization together establish the wave nature of light; the photoelectric effect establishes its particle nature.