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When two light waves from different coherent sources meet together, then the distribution of energy due to one wave is disturbed by the other. This modification in the distribution of light energy due to super- position of two light waves is called "Interference of light".

CONDITIONS FOR INTERFERENCE


The two sources of light should emit continuous waves of same wavelength and same time period i.e. the source

should have phase coherence.
The two sources of light should be very close to each other.
The waves emitted by two sources should either have zero phase difference or no phase difference.

COHERENT SOURCES


Those sources of light which emit light waves continuously of same wavelength, and time period, frequency and amplitude and have zero phase difference or constant phase difference are coherent sources.

TYPES OF interference

The phenomenon of redistribution of energy in the region of superposition of waves is known as Interference.



There are two types of interference.

  •  Constructive interference. 
  •  Destructive interference.


CONSTRUCTIVE INTERFERENCE


When two light waves superpose with each other in such away that the crest of one wave falls on the crest of the second wave, and trough of one wave falls on the trough of the second wave, then the resultant wave has larger amplitude and it is called constructive interference.

It occurs at any location along the medium where the two interfering waves have a displacement in the same direction. For example, if at a given instant in time and location along the medium, the crest of one wave meets the crest of a second wave, they will interfere in such a manner as to produce a "super-crest." Similarly, the interference of a trough and a trough interfere constructively to produce a "super-trough."

Constructive Interference : If phase difference between two waves are zero than the interference wave must have Phase difference 2nπ where n belongs to whole number and path difference nλ where n is a whole number


Destructive interference

occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction. For example, the interference of a crest with a trough is an example of destructive interference. Destructive interference has the tendency to decrease the resulting amount of displacement of the medium.

Destructive Interference : If phase difference between two waves are zero than the interference wave must have Phase difference (2n-1)π where n belongs to whole number and path difference (n+1/2)λ where n is a whole number


Thomas Young showed that an interference pattern results when light from two sources meets up while traveling through the same medium.




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