Doppler in one line
A signal at frequency f bouncing off a target moving at velocity v returns at f × (1 + 2v/c). At 24 GHz (K-band) and 100 km/h, the shift is about 4,400 Hz — tiny, but easy to measure with a mixer.
K-band, Ka-band, Ku-band
Older guns use X-band (10 GHz) — long range but easily detected. K-band (24 GHz) is a sweet spot. Ka-band (33–36 GHz) gives a narrower beam and is harder to detect with consumer detectors. Photo-radar vans usually run Ka.
LIDAR (laser speed guns)
A 905 nm pulsed laser ranges the target several hundred times per second. Distance vs. time gives speed without Doppler. Pencil-thin beam — almost impossible to detect with a radar detector because the wavelength is in the infrared, not microwave.
What detectors actually detect
Consumer detectors hear the gun's emission, not the return. Against modern instant-on Ka or pulsed LIDAR, you get warned after the cop already has your speed locked. Jammers are a different (illegal in most jurisdictions) story.