1. The problem statement, all variables and given/known data A stationary boat emits a sonar wave of frequency f to a school of fish which is moving a velocity u towards the boat. The sonar wave is then reflected by the school of fish. If v is the velocity of the sonar wave in water, the frequency of the wave reflected by the fish is 2. Relevant equations 3. The attempt at a solution My ans is same as the working (first part) . I cant understand with the 2nd part . in my opinion, the frequency reflected by the fish is same as the frequency received by the fish . my ans is (( v+u)/ v ) x f . why ? is my concept wrong?