A similar concept to sonar is the doppler effect, in which the frequency of light or sound seems greater when the source is apporaching the sensor than when it's moving away from it. An example would be how an ambulance seems to grow louder as it approaches and then instantly becomes much quieter once it passes. An application of the doppler effect is by police officers with their speed detectors. Bats also use the doppler effect to determine whether they will colide with an object or not.