Question

A police car siren moving toward the stationary observer will have a _______ pitch than when it is travelling away from the same observer.

• A. higher
• B. lower
• C. louder
• D. softer
• E. faster

Answer 1

The correct option is A higher

Let the original frequency of light source be $\nu$ and speed of sound source be $v_{source}$.

Speed of sound in air $=v_{sound}$

Using Doppler effect in case when the source of sound moves away from the stationary observer :

Apparent frequency heard by the observer ${\nu }^{\prime }=\nu [\frac{v_{sound}}{v_{sound}+v_{source}}]$

Using Doppler effect in case when the source of sound moves towards the stationary observer :

Apparent frequency heard by the observer ${\nu }^{\prime \prime }=\nu [\frac{v_{sound}}{v_{sound}-v_{source}}]$

$⟹$ ${\nu }^{\prime \prime }>{\nu }^{\prime }$

Thus the siren moving towards the observer will have a higher pitch (frequency) than when it is travelling away from the same observer.