Final answer:
The Doppler effect is the change in frequency observed when there is relative motion between a sound source and an observer, also called a Doppler shift. so, option 1 is the correct answer.
Step-by-step explanation:
The term used to describe a change in the perception of frequency due to the movement of either the source or the observer is known as the Doppler effect. This effect involves an alteration in the observed frequency of a sound when there is relative motion between the source and the observer. A common example of the Doppler effect can be observed when a train passes by a stationary warning bell; the bell's frequency will appear to shift from high to low as the train moves past. This phenomenon is named after Christian Johann Doppler, who conducted experiments on the subject. The actual change in frequency due to this relative motion is referred to as a Doppler shift.
This effect is easily noticed when a stationary observer hears a passing sound source, such as a siren on a moving ambulance or a train passing by a stationary warning bell. As the source and observer move closer, the frequency of the sound increases, resulting in a higher pitch. Conversely, as they move farther apart, the frequency decreases, causing a lower pitch. The Doppler effect is an important concept in physics that helps us understand how sound waves behave in relation to motion.