In the early 19th century christian doppler, an austrian physicist, proposed a theory regarding the properties of a moving source of sound. this is now known as doppler’s law or the doppler effect. what does this law state? a) the loudness of a moving source of sound is independent of the observer. b) the frequency of a moving source of sound is independent of the observer. c) the loudness of a moving source of sound is greater as it approaches an observer. d) the frequency of a moving source of sound is greater as it approaches an observer.

 D) The frequency of a moving source of sound is greater as it approaches an observer.

Explanation:

Doppler effect occurs when there is relative motion between source of sound and observer. The apparent frequency observed by observed is different from the actual frequency of the sound emitted from the source.

When the source and observer move closer, the apparent frequency is greater and when moving away, it is less than the actual frequency. Thus, the correct option is D.

The loudness is dependent on the amplitude of the sound which remains same.

causes

fossil fuels burn

cities become more industrialized

human population grows worldwide