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Difference between constructive and destructive intereference

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 What is interference?

Interference of light is defined as:”When two or more light waves having same frequency,same wavelength and same amplitude meet together in a medium at a point,they cancel or enhance the effect of each other at that point.This phenomenon is called interference of light waves.”

Types of interference:

There are two types of interference of light.

  • Constructive interference
  • Destructive interference

Constructive interference of light:constructive interference

When two light waves superpose with each other in such a way 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 intensity.Such type of interference is called constructive interference.

Effects of constructive interference:

  • In constructive interference,two waves of light reinforce each other.
  • In constructive interference,a bright fringe is obtained on the screen.

Destructive interference of light:destructive interference

When two light waves superpose with each other in such a way that the crest of one wave coincides the trough of the second wave,then the amplitude and intensity of resultant wave become zero.Such type of interference is called constructive interference.

Effects of destructive interference:

  • In destructive interference,two waves cancel the effects of each other.
  • Due to destructive interference a dark fringe is obtained on the screen.

Conditions for interference:

In order to observe the interference of light waves,following conditions must be met.

  • The two light sources must be coherent,that is ,they must maintain a constant phase with respect to each other.
  • The two light sources should be monochromatic,that is,of single wavelength.
  • The principle of superposition must apply.

 

  • Constructive and destructive interference with animation :

Young’s Double slit experiment:

Interference of light can be described by Young’s double slit experiment.Young’s experiment provided the first conclusive proof of wave nature of light.

Consider plane waves incident on two slits separated by distance d as shown on figure.The light waves from these two slits combine at a point P on the screen which is at a distance D from the two slits.youngs double slit experiment

Superposition of these waves produces a series of bright and dark fringes which are observed on the screen.If the waves arrive at the screen in such a way that at some points crests fall on crests and troughs fall on troughs then a bright fringe is seen on the screen due to constructive interference.

There are some points on the screen where crests meet troughs then a dark fringe is seen on the screen due to destructive interference.The bright fringe is called maxima and dark fringe is called minima.

young slits

Young’s experiment equation:

derivation of youngs experiment

young'syoung's experiment

Watch also video about young double slit experiment:

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