A source of sound emits sound waves at frequency . It is moving towards an observer with fixed speed If the observer were to move towards the source with speed , one of the following two graphs (A and B) will give the correct variation of the frequency f heard by the observer as is changed.The variation of f with is given correctly by : Option 1) graph A with slope  =   Option 2) graph A with slope  =     Option 3) graph B with slope  =    Option 4)   graph B with slope  =

As we learnt in

Frequency of sound when observer is stationary and source is moving towards observer -

- wherein

speed of sound

speed of source

original frequency

apparent frequency

Graph A with slope

Correct option is 1.

Option 1)

graph A with slope  =

This is the correct option.

Option 2)

graph A with slope  =

This is an incorrect option.

Option 3)

graph B with slope  =

This is an incorrect option.

Option 4)

graph B with slope  =

This is an incorrect option.