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Thermodynamics

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Interference of light

The phenomenon of interference of light has proved the validity of the wave theory of light. Thomas Young successfully demonstrated his experiment on interference of light in 1802. When two or more wave trains act simultaneously on any particle in a medium, the displacement of the particle at any instant is due to the superposition of all the wave trains. Also after the superposition, at the region of cross over, the wave trains emerge as if they have not interfered at all. Each wave train retains its individual characteristics. Each wave trains behaves as if others are absent. This principle was explained by Huygens in 1678.

The phenomenon of interference of light is due to the superposition of two wave trains within the region of cross over. Let us consider the waves produced on the surface of water. In fig. points A and B are the two sources which produce waves of equal surface of water which are circular in shape. At any instant, the particle will be under the action of the displacement due to both the waves. The points shown by circles in the diagram will have minimum displacement because the crest of one wave falls on the trough of the crosses in the diagram will have maximum displacements because, either the crest of one will combine with the crest of the other or the trough of one will combine with the trough of the other. In such a case, the amplitude (I ∝ A2) the intensity at these points is four times the intensity due to one wave. It should be remembered that there is no loss of energy due to interference. The energy is only transferred from the points of minimum displacement to the points of maximum displacement.