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Experiment7

# Experiment7 - Experiment#7 Diffraction and Interference 1...

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Experiment #7 Diffraction and Interference 1 Lab report for experiments 7 and 8 You will write one, formal, report for experiments 8 and 9 combined. It will differ from the reports for the previous experiments in that it will include the essential components of the type of report that is submitted to management or to a Journal for publication. A general outline of what should be included is provided in the page “Introduction to the course” on this web site. 2 Interference and diffraction In experiment 6 you measured the velocity of sound by finding the frequency of standing waves. Standing waves are a consequence of the phenomenon of constructive interference. In this experiment you will use another consequence of interference; the creation of a diffraction pattern due to the constructive and destructive interference of two or more waves of the same wavelength. You will measure the diffraction pattern due to the interference of identical waves emanating from two closely spaced sources. The two sources are obtained by placing a single source behind a barrier with two slots cut into it. The waves from the two slots behave as if they are two identical sources. Figure 1. Illustration of the waves from a plane wave incident on two slits in a barrier.

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The existence of a null or, more realistically a minimum, in the diffraction pattern is due to destructive interference between waves from the two apertures. Consider two narrow slits in a barrier with a plane wave incident on its back side. If the width of the slits, a, is very small compared to the wavelength, you will learn in experiment 8 that the wave will radiate uniformly in all directions from the slit. F or points of observation very far from the screen, strong intensity maxima will occur when the waves from both slits arrive in phase, so that they will add. This occurs when the differences in distance between the two slits and the point of observation are an integer number of wavelengths; λ θ n d = sin 2.1 Acoustic waves All wave phenomena display the properties of interference and diffraction. In this experiment you will measure the phenomenon using sound waves. Sound waves are obvious in our everyday experience and these phenomena have a strong influence on what we hear. They also play an important role in the design of acoustic devices such as speakers. A disadvantage of using sound waves for this experiment is that they reflect off of virtually all surfaces, including you. As a consequence you will see some interference between the waves you are interested in measuring and waves reflected off of many other surfaces. That makes this experiment more like a real experiment in which you are looking for one effect in the presence of many others. You will need to develop your own measuring technique and exercise some judgment in the way you obtain and analyze your data.
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