Question

A railway engine whistling at a constant frequency moves with a constant speed aixi it goes past a stationary observer standing beside the railway track. Then the frequency of (n') of the sound heard by the observer with respect to time (t) can be best represented by which of the following curve?

Options

• 

• 

• 

• 

Answer 1

or an observer at rest and source velocity constant, apparent frequency heard,

`"n'" = "n" ("v"/"v" pm "v"_s)`

When source approaches,

`"n'" = "n" ("v"/"v" - "v"_"s") = "constant" xx "n"`

Here, constant > 1, hence apparent frequency heard is constant and higher than n. When source moves away,

`"n'" = "n" ("v"/"v" +"v"_"s") = "constant" xx "n"`

Here, constant < l , hence apparent frequency heard is constant and lesser than n. The graph (D) represents the situation best.