Question

Assertation(A): When a source moves towards a stationary observer, the wavelength of the sound received by the observer decreases.

Reason(R): Doppler effect is applicable only when the velocities of source of the sound and observer are less than the velocity of sound.

• A. A and R are true and R is the correct explanation o A.
• B. A and R are true, but R is not the correct explanation of A.
• C. A is true but R is false.
• D. Both A and R are false.

Answer 1

The correct option is B

A and R are true, but R is not the correct explanation of A.

Explanation for assertion:

1. We know that, $wavelength\left(\lambda \right)=\frac{velocity\left(v\right)}{frequency\left(f\right)}$
2. Wavelength and frequency are inversely proportional.
3. So, we can say that when the source of sound moves towards a stationary observer, frequency increases. So, the wavelength of the sound received by the observer decreases.
4. Hence, assertation is true.

Explanation for the reason:

Doppler effect:

1. Doppler effect is defined as the increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.
2. Doppler effect is applicable only when the velocities of the source of the sound and observer are less than the velocity of sound.
3. Hence, the reason is true.

So, A and R are true, but R is not the correct explanation of A.

The correct option is option (B).