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EXPERIMENT DIFFRACTION OF LIGHT Introduction: When light or any other type of wave passes through an opening or goes by the edge of an obstacle, the wave extends (bends) into the region not directly exposed to the wave front. This phenomenon is called diffraction . An explanation of this effect was first proposed by Huygens (1629 – 1695), a contemporary of Newton, in the light of a wave theory of light. Huygens’ principle states that every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets. So, when a wave front arrives at an opening, each point along the part of the wave front that extends across the opening sends out wavelets, all in phase, which spread out in all directions. The wave theory of light was not always widely accepted. In the early 1800’s, the great scientist Poisson (1781 – 1840), a wave-theory detractor, pointed out what he thought was an obvious flaw with the theory, namely that it predicted a bright spot would occur at the center of the shadow created by a circular object! The experiment was performed and the spot was found! It is observed that a wave passing through a slit does not illuminate the region beyond the slit uniformly, but creates bands of high and low intensity. This phenomenon is known as interference , and occurs when two or more waves arrive at the same point simultaneously. The resulting light intensity at that point will depend on the relative phase of the component waves. (For example, waves which arrive in phase add to give a large intensity; waves which arrive out of phase cancel.) In the Huygens model, the large number of wavelets produced by each of the sources interfere with one another and result in an interference pattern. In this experiment, a diffraction pattern is created by allowing the light emitted by a diode laser to fall onto an aperture. The entire pattern can be observed by placing a piece of paper (or some other white screen) on the far side of the aperture, or can be examined point by point by having the light pass through a collimator onto a light sensor . The light sensor is mounted on a carriage which slides sideways (i.e. perpendicular to the optics bench) using a thumbwheel. There is a two-position switch on the top of the sensor which controls the gain. (If the signal is too weak, turn it up, and if it is too strong, turn it down.) The aperture discs consist of the Single Slit Disc and the Multiple Slit Disc and should be placed in a holder about 3 cm in front of the laser. They carry many slit systems of different sizes and shapes. Each disc is mounted on a frame in such a way that any of the slits can be rotated in front of the laser. The frame can be rotated in its mount slightly for better alignment. The entire mount can be removed from the optical bench for better viewing. A collimator disc in front of the detector is used to keep out stray light and to better define the position of the incoming beam. The disc has many
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