A source of sound produces the same frequency underwater as it does in air. This source has the same velocity in air as it does underwater. The observer of the sound is stationary, both in air and underwater. Is the Doppler Effect greater in air or underwater when the source (a) approaches and (b) moves away from the observer

Doppler effect is greater when the source approaches the observer.

Explanation:

Doppler effect always deal with observer relation with frequency of wave.  Successive wave crest emitted from closer position towards observer than previous wave crest, since source of wave approaching towards observer. Since time between remittance of successive crest towards observer reduced, this leads to increase in frequency. So Doppler effect will be higher when source approaches towards observer and its reduced when source moves away from observer.

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