What does the onboard computer in a radar gun listen for to compute vehicle speed?

The correct choice focuses on the concept of "beats" between outgoing and incoming signals, which is fundamental to how radar technology operates. When a radar gun emits a radio wave towards a moving vehicle, that wave reflects off the vehicle and returns to the radar gun. If the vehicle is moving, the frequency of the returning signal is altered due to the Doppler effect, which changes the frequency of the wave depending on whether the object is moving towards or away from the source.

The computer processes the difference, or "beat," between the frequency of the outgoing signal and the frequency of the returning signal. This difference in frequency is directly related to the speed of the vehicle, allowing the radar gun to compute how fast the vehicle is traveling. This technique is not just effective but is the basis for how radar guns provide accurate speed readings.

Frequencies, electromagnetic waves, and light signals, while related to the discussion of radar technology, do not capture the specific mechanism used in speed detection, which relies on the comparison of outgoing and incoming signal frequencies and their resulting beat frequency.