1067_Physics ProblemsTechnical Physics

1067_Physics ProblemsTechnical Physics - 408 Diffraction...

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408 Diffraction Patterns and Polarization Q38.4 An AM radio wave has wavelength on the order of 31 0 11 0 300 8 61 × × ms s m ~ . This is large compared to the width of the mouth of a tunnel, so the AM radio waves can reflect from the surrounding ground as if the hole were not there. (In the same way, a metal screen forming the dish of a radio telescope can reflect radio waves as if it were solid, and a hole-riddled screen in the door of a microwave oven keeps the microwaves inside.) The wave does not “see” the hole. Very little of the radio wave energy enters the tunnel, and the AM radio signal fades. An FM radio wave has wavelength a hundred times smaller, on the order of a few meters. This is smaller than the size of the tunnel opening, so the wave can readily enter the opening. (On the other hand, the long wavelength of AM radio waves lets them diffract more around obstacles. Long-wavelength waves can change direction more in passing hills or large buildings, so in some experiments FM fades more than AM.) Q38.5 The intensity of the light coming through the slit decreases, as you would expect. The central
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