There are various theories regarding the nature of light. They are as follows:
1. Newton’s Corpuscular Theory: According to this theory, light consists of perfectly elastic particles called corpuscles.
2. Huygens’s Wave Theory: According to this theory, a source of light sends out disturbance or waves in all region/directions. As the wave propagates in the medium, the particles of the wave execute simple harmonic motion in different phase. For a propagation of wave, a medium is required. The medium according or Huygens is ether medium. He assumed that light is a longitudinal wave.
3. Electromagnetic Theory: According to this theory, electromagnetic waves are the transverse waves which can propagate even in vacuum with the speed equals to that of light.
4. Quantum Theory: According to quantum theory, light propagates as tiny bundles of energy called photons. A photon is said to be a massless particle having energy E = h γ.
5. De-Broglie’s dual nature of light: According to De-Broglie, light behaves in dual nature, corpuscular or particle and wave nature. Every moving particle is associated with the wave. This wave is called matter wave or De-Broglie wave. The wavelength of a moving particle can be calculated by De-Broglie’s relation: λ = h/p = h/mv
Where, h is Planck’s constant and p is momentum (product of mass and velocity) of the body.