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A lot of people on the Radar Detector & Laser Jammer Forum are trying to paint their cars in the deepest black to avoid getting a speed ticket by making their vehicle stealthier for incoming radar waves. Black objects absorbing visible light without emitting or reflecting any would be a violation of the 2nd law of thermodynamics so I have learned that black objects actually emit a lot of radiation in the Infrared to Radiowave frequencies to prevent an increase in temperature.

Now since these are the frequencies that are used by the police to estimate a car's speed, does that mean that painting your car black will actually make it easier for radars to pick up? They don't reflect as much light but they emit more.

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Night bombers in WW2 were painted black underneath, but it didn't shield them from radar. The reason for the black underside was to make them less visible to searchlights and night fighters, but they could still be picked up on radar, and often were. There are coatings which absorb radar and increase stealth in modern ships and planes, but I don't think they are black in colour. Not the ones I've seen, anyway. Black cars today are not as common as they used to be, but there are still some about. Many have been clocked on police radar, which has no difficulty with them, so to have your nice, scarlet sports car sprayed black would be a lot of trouble and expense for nothing. You might also bear in mind that speed limits are there for a reason and have saved many lives, quite possibly your own.

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    $\begingroup$ That last sentence should be in bold, IMO. $\endgroup$ – StephenG Jul 6 at 20:13
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The theory is that because carbon has a comparable resistance to the impedance of free space, it can absorb the energy. That is, it is black at that frequency. But black paint does not noticeably absorb police radar because it isn't nearly thick enough (the thickness needs to be at least a sizable fraction of the wavelength--which is at least a centimeter or so), and because there isn't enough carbon to allow the particles to contact each other, anyway, so it's still simply a dielectric and is thus clear to the radar, letting the energy reflect off the metal of the car. It certainly shouldn't make it easier to be picked up. By the way, a thick layer of carbon turns the RF energy into heat because the RF induces currents in it, which lose their energy to the finite resistance.

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