What is the Doppler effect?

The Doppler effect refers to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This phenomenon is commonly experienced in everyday life; for example, when a siren from an emergency vehicle passes by, the sound appears to get higher in pitch as it approaches (due to the compression of sound waves) and lower as it moves away (due to the stretching of sound waves).

This principle applies not only to sound waves but also to electromagnetic waves, such as light. In the context of radar, the Doppler effect is crucial for determining the speed of a moving object by analyzing the frequency shift between the emitted radar waves and the reflected waves from the object.

The other choices do not accurately define the Doppler effect. One option discusses variations in speed due to road conditions, which is unrelated to wave frequency changes. Another option focuses on vehicle engine noise, which may change with speed, but does not encompass the concept of wave frequency and motion. Lastly, the alteration of vehicle speed due to wind resistance pertains to physical forces acting on a car rather than any wave phenomena. Thus, focusing on the motion of the observer in relation to waves is what defines the Doppler effect.