YOUNG’S DOUBLE SLIT EXPERIMENT:We have earlier discussed the phenomenon of interference of waves, therefore, it is natural to expect that light also exhibits the phenomenon of interference. But interference of light is not so evident as the interference of sound waves. This is due to the very small wavelength of light waves as compared to the wavelength of sound waves.In 1801, Thomas young demonstrated successfully the interference of light by performing a double slit experiment.Young allowed sunlight to pass through a hole and then at some distance through two pin holes S1 and S2. When light was received on a screen, an uneven distribution of light intensity was observed on the screen. A number of alternate dark and bright spots were seen on the screen. The phenomenon of interference can be understood on the basis of Huygens principle. The secondary wavelets coming from S1 and S2 have equal amplitude, velocity and wavelength. In figure, the continuous circular arcs represent the regions of wave crest. The dotted circular arcs represent regions of wave trough. At points marked by small circles, the trough of one wave is superposed over the trough of another wave or the crest of one wave falls over the crest of another wave. According to the principle of super position, the amplitude and hence the intensity of the resultant wave is maximum at these points. Similarly, the intensity is minimum at points marked by a “cross”. Thus, the resultant intensity is maximum along the lines connecting the “crosses”. On the screen, we get a number of alternate dark and bright regions of equal width. This is called interference pattern.Analytical treatment of interference.

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