When a wave runs into a barrier that only blocks part of the wave, the shape of the wave seems to bend around the edge of the barrier. This bending, called diffraction, depends on both the properties of the wave and the size of the barrier. Diffraction can be demonstrated with a ripple tank, a shallow tank produces standing waves. A strobe light and projector are used so that the wave patterns can be easily seen by students.
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When a single barrier is placed in the centre of the ripple tank it will block the centre of the wave, but the wave on either side of the barrier will bend inward. Since the wave is bending inward from both sides, an interference pattern forms. Students should record the wave pattern that they see and measure the angle of incidence and the angle of refraction. The difference between these two angles tells you how much the water wave was bent going around the edge of the barrier.
Place a barrier in the ripple tank that has a single small slit in it. Most of the wave will be reflected when it hits the barrier. The portion of the wave that gets through the slit will act as a point source, producing a circular wave pattern on the other side. The smaller the slit, the more the outgoing waves will resemble a semicircle.
Place a barrier in the ripple tank that blocks off all of the wave except for a somewhat large opening. The portion of the wave that passes through the opening will be diffracted slightly around the edges of the opening, fanning outwards. This experiment should be done in conjunction with the single slit experiment.
These experiments also provide an important demonstration of the notion of scale, a concept that comes up repeatedly in physics. The notion of a large or small opening is relative to the wavelength of the waves being observed, around one centimetre in most ripple tanks. While a one inch opening would be large in this demonstration, a one inch opening would act as a small slit if it were hit by a typical ocean wave.
Place a barrier in the ripple tank with two small slits in it. Two semicircular wave patterns will form centred at each of the slits. The two semicircular waves will form an interference pattern. The distance between the slit can be varied to form different interference patterns. This experiment should be performed after the single slit experiment so that students already understand why the semicircular waves are being generated.
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