Photons and light waves (Duality of Radiation and Matter)

PHOTONS AND LIGHT WAVES - (Duality of Radiation and Matter)

The wave and particle duality of radiation is easily understood by knowing a difference between a wave and and a particle.

Wave

• A wave originates due to oscillations and it is spread out over a large region of space. A wave cannot be located at a particular place and mass cannot be carried by a wave. 

• Actually, a wave is a spread out disturbance specified by its amplitude A, frequency γ, wavelength λ, phase δ and intensity I. 

• The phenomena of interference, diffraction and polarisation require the presence of two or more waves at the same time and at the same position. 

It is very clear that two or more particles cannot occupy the same. position at the same time. So one has to conclude that radiation behaves like waves. 

Particle

• A particle is located at some definite point and it has mass. It can move from one place to another. A particle gains energy when it is accelerated and it loses energy when it is slowed down. 

• A particle is characterized by mass m, velocity v, momentum p and energy E. 

• Spectra of black body radiation, Compton effect, photoelectric effect, etc. could not be explained on wave nature of radiation. 

These phenomena established that radiant energy interacts with matter in the form of photons or quanta. Therefore, Planck's quantum theory concludes that radiation behaves like particles. 

• Thus, radiation sometimes behaves as a wave and some times as a particle. Now, wave - particle duality of radiation is universally accepted.

Scattering of X-rays

Like ordinary light waves, X - rays are scattered by matter in two different ways. They are

(a) Coherent scattering or classical scattering or Thomson scattering. 

(b) Incoherent scattering or Compton scattering.

(a) Coherent scattering

In coherent scattering, X -rays are scattered by electrons without any change in their wavelengths. This type of scattering was explained by Thomson on the basis of classical electromagnetic theory.

(b) Incoherent scattering (Compton scattering)

In Compton scattering, the scattered X- ray consists of two components, one is having the same wavelength as that of the incident X-rays and the other has a slightly longer wavelength. This type of scattering was studied by Compton on the basis of particle nature of light radiation.