Introduction
In 1637 Desartes gave the corpuscular model of light derived Snell's law. It explained the laws of reflection and refraction of light at an interface. The corpuscular model predicted that if the ray of light (on refraction) bends towards the normal then the speed of light would be greater in the second medium. This corpuscular model of light was further developed by Isaac Newton in his famous book entitled OPPTICKS and because of the tremendous popularity of this book, the corpuscular model is very often attributed to Newton.
In 1678, the Dutch physicist Christiaan Huygens put forward the wave theory of light - it is this wave model could satisfactorily explain the phenomena of reflection and refraction; however, it predicted that on refraction if the wave bends 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 much later confirmed by experiment where it was shown that the speed of light in water is water is less than the speed in air confirming the prediction of the wave model; Foucault carried out this experiment in 1850.
The wave theory was not readily accepted primarily because of Newton's authority and also because light could travel through vaccum and it was felt that a wave would always required medium to propagate from one point to the other. However, when Thomas Young performed his famous interference experiment of 1801, it was firmly established that light bid indeed a wave phenomenon. The wavelength of visible light was measured and found to be extremely small; for example, the wavelength of yellow light is about 0.5੫m. Because of the smallness of the wavelength of visible light (in comparison to the dimensions of typical mirrors and lenses), light can be assumed to approximately travel in straight lines. This is the field of geometrical optics, which we had discussed in the previous article. Indeed, the branch of optics in which one completely neglects the finiteness of the wavelength in called geometrical optics and a ray in defined as the path of energy propagation in the limit of wavelength tending to zero.
After the interference experiment of young in 1801, for the next 40 years or so, many experiments were carried out involving the interference and diffraction of light waves; these experiments could only be satisfactorily explained by assuming a wave model of light.Thus, around the middle of the nineteenth century, the wave theory seemed to be very well bestablished. The only major difficulty was that since it was through that a wave required a medium for its propagation, how could light waves propagate through vaccum.
This was explained when Maxwell put forward his famous Electromagnetic theory of light. Maxwell had developed a set of equations describing the laws of electricity and magnetism and using these equation, Maxwell could calulate the speed of electromagnetic waves in free space and the found that the theoretical value was very close to the measured value of space of light. From this, he propounded that light must be an electromagnetic wave.
Thus, according to Maxwell, light waves are associated with changing electric and magnetic field; changing electric field produces a time and space varying magnetic field and a changing electric field produces a time and space varying magnetic field and a changing magnetic field magnetic field result in the propagation of Electromagnetic waves (or light waves) even vaccum.
In this article we will first discuss the original formulation of the Huygens principle and derive the laws of reflection and refraction.
