CGPET 2019 Solved Paper

Given,
The apparent frequency equal to half of the original frequency v′=

1

2

v0
Now, the apparent frequency of observer is given as
∴v′=

v

v+vs

.v0
Here, v′= apparent frequency of observer,
v= speed of the sound,
vs= speed of the source of sound and
v0= original frequency.
∴v′=

1

2

v0
Then,

1

2

v0=

v

v+vS

v0 or

1

2

=

v

v+vs

or v+vs=2v
or vs=2v−v
or vs=v
Therefore, speed of source of sound (vs) more should be equal to the speed of sound (v), so that stationary observer finds the apparent frequency equal to half of the original frequency.