Question

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 increase during approaching
• B. f will decrease during receding
• C. f will remain constant during approaching
• D. f will remain constant during receding

Answer 1

Answer: (A), (B)

The correct options are
A f will increase during approaching
B f will decrease during receding

The apparent frequency $f^{\prime }$ of sound when a source is approaching with a speed of $v_s$ , towards a stationary observer is given by ,

$f^{\prime }=f(\frac{v}{v-v_s})$ ,

where $v=$ speed of sound in air ,

it is clear from the above relation that , $f^{\prime }>f$ , therefore frequency increases , when source approaches the observer .

The apparent frequency $f^{\prime }$ of sound when a source is receding with a speed of $v_s$ , from a stationary observer is given by ,

$f^{\prime }=f(\frac{v}{v+v_s})$ ,

where $v=$ speed of sound in air ,

it is clear from the above relation that , $f^{\prime }<f$ , therefore frequency decreases , when source recedes from the observer .