Question

A vehicle with a blowing siren approaches and recedes away from an observer at rest. Then, the apparent frequency heard by the observer will

• A. first increase then decrease
• B. first decrease then increase
• C. continuously decrease
• D. continuously increase

Answer 1

The correct option is A first increase then decrease

Doppler's effect is observed when the radial distance between source and observer changes. When there is no change in radial distance then there is no Doppler's effect.
Also, when separation increases then apparent frequency heard by the observer decreases and when separation decreases then apparent frequency increases.
Here, as the vehicle approaches the observer, its separation decreases, so frequency will increase and when recedes way its separation increases hence frequency will decrease.

Hence $\left(A\right)$ is the correct answer.