Tamil Nadu State Board: Class XII Physics

Chapter 1 : Electromagnetic Waves And Optics

The study of light began with Newton who started by assuming that light is made up of particles, themselves invisible, which he called corpuscles. On the basis of this hypothesis, he could explain the formation of images, reflection and refraction. But one phenomena that he himself noticed, which is also named after him as Newton’s rings, could not be explained using his corpuscular nature of light theory.
To explain the formation of Newton’s rings and many other phenomena classified under Interference, Diffraction and Polarization, light was hypothesized to be a wave. This hypothesis, first propounded by Christian Huygens could correctly explain not only these phenomena but also reflection, refraction and other phenomena that could not be explained by Newton’s theory. Hence Huygen’s wave theory of light was preferred over Newton’s corpuscular theory. 

The concept of Huygen’s theory was that each point along a ray behaves like a light source emitting light in all directions. The locus of all points of the light wave that are in the same phase is called wavefront. The waves emitted by each point were called secondary wavefronts. Huygen’s wave theory could explain only Interference and Diffraction of light but not Polarisation which was explained by Fresnel who consequently proved that light was a transverse wave. 

In fact, Polarisation settled the doubts about the nature of light being a transverse wave once and for all. But the question ‘what kind of waves make up light?’ still remained.Olaf Roemer had first measured the speed of light using the eclipses of Jupiter’s moons as early as 1675. His measurement ( 2.000000 x 10⁸ m/s) was amazingly close to the now known value of the speed of light - 2.9979250 x 10⁸ m/s, considering the rather primitive equipment used. This value was modified by many other experiments. It was only sometime in the late 19^th century, however, that its significance was realised. 

This was first noticed by James Clerk Maxwell who had unified Electricity and Magnetism as one phenomena under Electromagnetic theory. He had also proved that electrical and magnetic field fluctuations produced by moving charged particles travel as electromagnetic waves. He calculated their speed, in vacuum, to be nearly the velocity of light in vacuum. He presumed this to be no mere coincidence and suggested that light could be an electromagnetic wave. 

This was tested and proved correct by Heinrich Hertz sometime in 1885. Thus Maxwell’s electromagnetic theory could answer the long standing question about the nature of light - they were electromagnetic: produced by charged particles in motion. It was left to Einstein to prove that electromagnetic waves travel in vacuum overthrowing the long held belief that electromagnetic waves required a medium (ether) to propagate.
But there were more surprises to come.
It was early in the 20^th century when a new phenomena was discovered - photoelectric effect. The explanation of photoelectric effect by Einstein and the explanation of Blackbody radiation by Max Planck cast doubts over the electromagnetic wave theory of light. Both these new discoveries required light to be made up of ‘particles’ called as photons - not exactly the kind of particles that Newton had in mind, though. Indeed, in the process of explaining Blackbody radiation Planck had to assume that energy of electromagnetic radiation itself (be it heat or light or any form of radiation) is made up of particles^** called as quanta (quantum - singular), beginning a new study in Physics called Quantum Mechanics. 

Physicists weren’t late in realising that light probably had a dual nature; it behaved as if it was made up of particles (photons) under certain conditions and it also behaved like a wave under different conditions. Thus the ‘dual nature of light’ theory was born and continues to be accepted even today.
** As a matter of fact these are not particles as we usually understand, like particles of dust. 

These are discrete amounts of energy and energies less than this particular amount (which is decided on certain other factors) do not even exist ! (for a given frequency ). As a crude analogy, imagine an amount of Rs. 100 which is made up of 100 paise, each paise being the smallest permissible amount. An amount less than one paise may be imagined but they simply do not exist. Each paise is, in energy terms, a quantum of energy.