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Physics/Tough Physics question


Hi! I am studying some of my brothers physics questions and I need help finding the answer to this one:

You need to use your cell phone, which broadcasts an 800 MHz signal, but you’re behind two massive, radio-wave-absorbing buildings that have a 20 m gap between them. What is the width of the electromagnetic “beam” that emerges 50 m on the other side of the buildings?


When a wave passes through a narrow slit the angle at which the first node occurs can be predicted from:
Where: N=1 for the first node, lambda=wavelength of the wave, d=width of the slit and theta=angle at which the signal node emerges.
In this case the wavelength lambda is equal to the speed of the wave divided by the frequency of the signal:
Therefore the angle theta of the emerging beam that corresponds to the first node will be:
At a distance of L=50 m from the gap the beam will have expanded to a width W of:
Therefore, x, the distance between the center of the beam and the location of the first antinode will be:
Finally, the total width X of the beam at a distance L=50m from the gap will be twice this distance x:


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James J. Kovalcin


I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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