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Im not a hundred percent sure on this, but I think radar guns operate under the principle of the doppler effect. A radar gun shoots beams of light waves at a moving target. When it hits the target, it reflects back with a new wavelength that depends on the speed of the moving vehicle. If the moving vehicle is moving at a high speed, the wavelength reflected will be much longer than if it were moving at slower speeds. The gun then, detects the reflected light and interprets the wavelength (taking into account the speed of the police cruiser) and determines the speed. I think this is right. I don't think radar guns operate by measuring time differences from the light exiting the gun and reentering it.
Oops, I just realized that you were talking about radar detectors and not radar guns. Well, just take into account what I previously said about how the cops' radar operates and add to that the fact that these radar guns emit certain wavengths of light with certain frequencies. A radar detector then is programmed to detect these certain frequencies (I think its like KA band, K-band, or something and few more bands) and make a noise when it detects these. If you get an older detector it will only detect some of the older bands not used often by the police.
I have a radar detector and it works great. I have never been pulled over since getting it and most of the time it warns me of a cop at least minute before I see him. However, there are occassions where a cop drives by without the dectector going off. Maybe the cop had his radar off, or maybe he had a different operating frequency? Overall, radar detectors are pretty effective at least in my experience.
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