INTERFERENCE

The phenomenon of interference and diffraction establish the wave nature of light that was given by Huygens in 1678.

Interference simply means superposition of two or more waves having same frequency travelling in a particular direction. It holds good for all waves except for the waves of large magnitude, e.g. laser waves.

When two light beams cross each other, their resultant intensity differs at the cross over point. The redistribution of intensity due to superposition is termed as interference.

If the resultant intensity is zero or less than the intensity of each light beam, we call it a destructive interference whereas if resultant intensity is more than the intensity of each light beam, it is called a constructive interference.

However, the phenomenon of intensity is not readily visible due to short wavelength of light; hence it came to notice after a long time.

Thomas young was the first person to demonstrate this phenomenon in his experiment.

CONDITIONS FOR CONSTRUCTIVE AND DESTRUCTIVE INTERFERENCE

We have assumed that light waves reaching S₁ and S₂ are already in phase.

Beyond S₁, S₂ waves travel as if they have originated together from there it self.

At a point P on a screen, Let us find the intensity.

Let a₁ and a₂ be the amplitudes of light waves arriving from S₁ and S₂ respectively. The waves travel different path to reach point P. Due to difference in path travelled by two light waves, there exists a path difference between them that eventually gives rise to phase difference.

CONSTRUCTIVE INTERFERENCE:

Position of dark fringe

FEW IMPORTANT POINTS:

Interference is not observed using two independent sources since they fail to be coherent. Two sources are said to be coherent if they have a constant phase difference between them.

The waves must have same frequency.

The separation between the two slits must be small to make the fringes visible.

The amplitudes must be nearly equal to produce stark contrast.