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explain young's double slit experiment?

In 1801, Thomas Young demonstrated the interference of light experimentally. His experiment gave a deadly blow to the corpuscular theory of light as the experiment led to the conclusion that light has wave nature.

A source of monochromatic light S is used for illuminating two narrow slits S₁ and S₂. The two slits lie very close to each other and at equal distances from the source S. See the figure below: The wave-fronts from the slits S₁ and S₂ spread out in all directions and in the region they superpose on the screen, alternate bright and dark fringes are observed. At the centre O of the screen, the intensity o light is maximum and its is called central maximum.

Explanation: According to Huygens' principle, the two slits S₁ and S₂ send out waves of equal wavelengths and of same amplitude in all directions. As the paths S₁O and S₂O are equal, the light waves from the two slits S₁ and S₂ reach point O in phase that is, either crest of one wave falls on the crest of the other wave or trough of one falls on the trough of the other. Hence, intensity of light at point O becomes maximum. It is called Central maximum.

As we move above the centre O  of the screen (or below the point O), we come across points D₁, B₁, D₂, B₂,.....

At points D₁, D₂,........, the crest of one wave falls on the trough of the other and vice-versa. The resultant amplitude and hence the resultant intensity of light becomes minimum at these points due to destructive interference of light. These points correspond to positions of dark fringes on the screen.

At points B₁, B₂, the crest of one wave falls on the crest of the other or trough of one on the trough of the other. Due to this, the resultant amplitude and hence resultant intensity of light at these points becomes maximum due to constructive interference of light. These points correspond to the position of bright fringes on the screen.

Further, in Young's double slit experiments, the following facts can also be verified:

1. The interference pattern disappears, if one of the two slits is closed. It shows that interference pattern is due to superposition of waves from the two slits.
2. If instead of two slits illuminated with a single source, two independent sources are used, the interference pattern obtained is not a permanent one and the positions of maximum and minimum intensity do not remain fixed. It shows that for producing a sustained interference pattern, two sources should be obtained from a single source. Such sources are called coherent sources.