408 Diffraction Patterns and PolarizationQ38.4An AM radio wave has wavelength on the order of 310110300861××−mssm~. This is large compared tothe width of the mouth of a tunnel, so the AM radio waves can reflect from the surrounding groundas if the hole were not there. (In the same way, a metal screen forming the dish of a radio telescopecan reflect radio waves as if it were solid, and a hole-riddled screen in the door of a microwave ovenkeeps the microwaves inside.) The wave does not “see” the hole. Very little of the radio wave energyenters the tunnel, and the AM radio signal fades. An FM radio wave has wavelength a hundredtimes smaller, on the order of a few meters. This is smaller than the size of the tunnel opening, sothe wave can readily enter the opening. (On the other hand, the long wavelength of AM radiowaves lets them diffract more around obstacles. Long-wavelength waves can change direction morein passing hills or large buildings, so in some experiments FM fades more than AM.)Q38.5The intensity of the light coming through the slit decreases, as you would expect. The central
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