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uperposition theory in Young double slit experiment, Condition of Maxima and Minima and calculation of Fringe width.

In 1801 Thomas Young provided the first experimental evidence for the wave theory of light from double slit interference experiment. Young allowed the sunlight to pass a through a pin hole So on first screen followed by S1 and S2 at some distance on second screen as shown in figure. On the screen he observed few coloured bright and dark bands. To increase the brightness of bands the pin holes S1 and S2 are replaced by narrow slits and to increase the number of fringes sunlight is replaced by monochromatic source. Finally the interference pattern consists of equally spaced bright and dark fringes are obtained.

Explanation of Experiment
Spherical wave came out as the sunlight passes through the pin hole So, (as per Huygen’s wave theory), the radii of these waves increases as they move away from So, when these spherical waves reaches the second screen again spherical waves came out from S1 and S2, these waves move away from S1 and S2 and hence they superimpose on each other. At the point where wave crest (or trough) of one wave fall on the wave crest (trough) of other, the resultant amplitude is maximum (maximum intensity, as I = A2) and where the wave crest of one fall on the wave trough of other, the resultant intensity is minimum. In this way large number of dark and bright bands are formed on screen.



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