1.0 Introduction

1.1 Wavefronts
1.2 Huygens Principle
1.3 Interference of light
1.4 Intensity distribution
1.5 Phase difference

In 1637 Descartes gave the corpuscular model of light and derived Snell's law. It explained the laws of reflection and refraction of light at an interface. The corpuscular model predicted that of the ray of light (on refraction) bends towards the normal then the speed of light would be greater in the second medium.

In 1678, the Dutch physicist Christiaan Huygens put forward the wave theory of light which could explain the phenomena of reflection and refraction. However, it predicted that on refraction if the wave bend towards the normal then the speed of light would be less in the second medium. This is in contradiction to the prediction made by using the corpuscular model of light. It was later confirmed through experiment carried out by Foucault in 1850, where it was shown that the speed of light in water is less than the speed in air which confirm the prediction of wave model.

The wave theory was not readily accepted primarily because of Newton's authority and also because light could travel through vacuum and it was felt that a wave would always require a medium to propagate from one point to the other.

In 1861, Maxwell put forward his famous electromagnetic theory of light. Maxwell had developed a set of equations describing the laws of electricity and magnetism. From these equations, Maxwell could calculate the speed of electromagnetic waves in free space and he found that the theoretical value was very close to the measured value of speed of light. So, he propounded that light must be an electromagnetic wave.