The phenomena of interference and polarization exhibit the wave nature of light, and James Clerk Maxwell (1831 – 79), British physicist, had established by 1865 that light is, and propagates as, an electromagnetic wave.
In his interpretation of the photoelectric effect in 1905, Einstein proposed that electromagnetic radiation behaves as a series of small packets or quanta of energy, later called photons. If the frequency of radiation is v, each photon has energy hv and momentum hv/c, where c is the speed of light in free space. Einstein’s photoelectric equation was verified experimentally by Robert Andrews Millikan (1868-1953), US physicist, in 1916.
A very strong additional evidence in support of the quantum theory of radiation was the discovery (in 1923) and explanation of the inelastic scattering of X-rays or γ-rays by electrons in matter by Arthur Holly Compton (1892-1962), US physicist. This inelastic scattering in which a photon transfers part of its energy to an electron is known as the Compton effect. It is similar to the Raman effect. The Compton effect shows particle nature of electro-magnetic radiation.
Since energy and momentum are considered in classical physics as characteristic properties of particles, the photoelectric effect and Compton effect exhibit the particle nature of radiation. But, to describe the photon energy, the quantum theory needs the frequency of the radiation, which is necessarily an attribute associated with a wave in classical physics. Thus, radiation exhibits the dual, seemingly contradictory, characters of particle and wave. In an experiment, we need to use only one of the descriptions, not both at the same time.
[Note : The momentum p and energy £ of a photon are related by the equation, p = E/c, where c is the speed of light in free space.]
