When we think of light rays a question comes to our mind whether it light is a wave or a particle. This discussion is very interesting and has got a long history. Newton the great physicist tried to understand travel of light rays in straight line assuming that a luminous body emits very minute and weightless particles called corpuscles travelling through empty space in straight line in all directions with the speed of light and carry kinetic energy with them. Thus energy is carried by stream of particles travelling with a finite velocity , this is the basic principle behind what we call the Corpuscular theory proposed by Sir Issac Newton. This Corpuscular theory of light can fairly explain the phenomenon of reflection , refraction and rectilinear propagation of light but failed to explain phenomenon of interference , diffraction and polarization of light.
A new theory of propagation of light was suggested by Dutch physicist Christian Huygens in 1678 in which he suggested that light may be a wave phenomenon produced by mechanical vibrations of an all pervading hypothetical homogeneous medium called ether just like those in liquids and solid. This medium was supposed to be mass less with extremely high elasticity and very low density. In this theory there is a transfer of energy by wave motion without actual travelling of matter. At first wave theory of light was not accepted primarily because of Newton’s authority and also because light could travel through vacuum and waves require a medium to propagate from one place to another. Wave theory of light first begin to gain acceptance when double slit experiment of Thomas Young in 1801 firmly established that light is indeed a wave phenomenon. After this double slit interference experiment many experiments were carried out by scientists involving interference and diffraction of light which could only be explained by assuming wave model of light.
Later on in nineteenth century Maxwell put forward his electromagnetic theory and predicted the existence of electromagnetic waves and calculated the speed of EM waves in free space and fount that this value was very close to the measured value of speed of light in vacuum. He then suggested that light must be an EM wave associated with changing electric and magnetic fields which results the propagation of light or EM waves even in the vacuum. So this way mo material medium is required for the propagation of light wave travelling from one place to another. This fact established that light is a wave phenomenon.
But this is not the end of the story Hertz first observed the phenomenon of photoelectric effect in1800 according to which when light falls on metal surface , electrons are emitted from the metal surface and the kinetic energy of the electrons does not depend on the intensity of light used. This phenomenon was latter explained successfully by another great physicist Albert Einstein in 1905 by assuming light as photons the quanta of light. His theory again gave rise to the old discussion whether light is a wave or particle. Later on well established particles like electrons also shows diffraction phenomenon under suitable conditions and such effects csn be studied under wave particle duality beyond the scope of this article.
