# A source is approaching towards an observer with constant speed along the line joining them. After crossing the observer, source recedes from observer

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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,
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

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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.