A source is approaching towards an observer with constant speed along the line joining them. After crossing the observer, source recedes from observer with same speed. Let `f` is apparent frequency heard by observer. Then, 1) f will keep on increasing during approaching 2) f will keep on decreasing during receding 3) f will remain constant during approaching 4) f will remain constant during receding

Options

A. `f` will keep on increasing during approaching
B. `f` will keep on decreasing during receding
C. `f` will remain constant during apporaching
D. `f` will remain constant during receding

Answer

C, D

Explanation

During approach,
`f = f_(o) ((nu)/(nu - nu_(s)))`
`gt f_(o)` but `f` is constant.
During receding.
`f = f_(o) ((nu)/(nu - nu_(s)))`
`lt f_(o)` but `f` is again constant.