Vehicles approaching the unit ________ the waves.

When vehicles approach a unit equipped with radar or lidar, the waves emitted by the unit are affected by the motion of the vehicle. This phenomenon is often described as the Doppler effect, where the frequency of waves changes due to the relative movement between the source and the observer. As a vehicle gets closer to the radar or lidar unit, it effectively compresses the waves, leading to a shorter wavelength and higher frequency of the returning signals.

This compression of the waves is significant in determining the speed of the approaching vehicle. The radar or lidar system interprets the change in frequency of the returned waves to calculate how fast the vehicle is closing in on the unit. Therefore, the choice of compressing the waves accurately reflects the physical interaction between the moving vehicle and the emitted waves from the detection system.