Radar-based speed detection works by sending an EM wave out from a source and me
ID: 1339526 • Letter: R
Question
Radar-based speed detection works by sending an EM wave out from a source and measuring the Doppler shift of the reflected wave. Specifically, it adds the reflected wave to a fraction of the outgoing wave and measures the "beating" between their slightly different frequencies. Wikipedia tells me that most police radar units operate in the "K band" (18-27 GHz) or "K_a band" (27-40 GHz). Suppose that a police officer is using a radar unit emitting a frequency of exactly 22.435 GHz, aimed at a car coming toward her with a speed of 68 miles per hour. If the driver of the car has a radar detector that precisely measures the frequency of the EM wave hitting his car, does he measure a frequency higher or lower than 22.435 GHz? What is the difference in frequency compared to the 22.435 GHz that was originally emitted? The reflected radar signal is, effectively, emitted by the car at the shifted wavelength. When that reflected beam returns to the police officer, what frequency difference does the radar gun receive compared to the original 22.435 GHz ?Explanation / Answer
when the car is coming in the opposite direction, n(apparent)=n(1+v/C)
hence frequency increases. he measure frequency more than 22.435 GHz.
n(apparent)=n(1+v/C)=22.435GHz(1+103.435 /3X108)
since 68mph=103.435kmph
calculating we will get the answer.
b) when the reflected wave reaches the police car they receive a frequency=22.435GHz(1+103.435 /3X108) after that difference between original sound and reflected sound will give us the answer.
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