Final answer:
The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It can be experienced with an ambulance siren changing pitch as it passes by, or with the change in color of light from stars moving towards or away from us.
Step-by-step explanation:
The Doppler effect is a shift in the observed frequency of a sound due to motion of either the source or the observer. If an ambulance, with its siren on, drives towards you and then passes you, the siren sounds higher in pitch as it approaches and lower in pitch as it moves away. This occurs because the sound waves are compressed as the ambulance comes closer, raising the frequency, and then stretched as the ambulance moves away, lowering the frequency.
Similarly, with stars, the Doppler effect is observed with light. A star moving toward us will have its light shifted to the blue end of the spectrum, whereas a star moving away from us will have its light shifted to the red end, known as a redshift. This phenomenon is one of the methods astronomers use to determine the motion and distance of stars.
In everyday life, the Doppler effect can help us determine, while driving, if an emergency vehicle is approaching. As the pitch of the siren changes, we can tell if it's getting closer or if it has passed us, prompting us to pull over and let it through if necessary.