Final answer:
A Doppler speed log measures a ship's speed through water using the Doppler effect. It does not directly indicate speed over ground. The principle is similar to that used by law enforcement radar to measure vehicle speed via frequency changes.
Step-by-step explanation:
A Doppler speed log does not actually measure speed over ground; rather, it measures the speed through water. It is an instrument used on ships to measure the speed of the vessel relative to the water around it, utilizing the Doppler effect of sound waves reflected off the seabed or water particles. The concept it relies on is similar to the Doppler effect observed with sound and light waves, where the observed frequency changes based on relative movement between the source and observer. This principle is widely used in various applications, including radar devices used by law enforcement to measure the speed of vehicles as they move towards or away from the radar source.
An example to illustrate the Doppler effect can be seen when a train with a 150-Hz horn moves at a certain speed and observers perceive a change in the frequency of the horn's sound due to the train's motion relative to the observers.