What physical change occurs when source of sound waves is at rest and the listener moves?

Explanation

To solve the question regarding the physical change that occurs when the source of sound waves is at rest and the listener moves, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Initial Condition**:
- When both the source of sound and the listener are at rest, the listener receives a certain number of wavefronts per second, which we denote as \( X \). This means the frequency of the sound heard by the listener is \( f = X \) Hertz.

2. **Calculating the Time Period**:
- The time period \( T \) of the sound waves is the reciprocal of the frequency. Therefore, \( T = \frac{1}{f} = \frac{1}{X} \) seconds.

3. **Introducing Motion of the Listener**:
- Now, consider the case where the listener is moving with a velocity \( V \) meters per second. In one second, the listener travels a distance \( V \) meters.

4. **Determining the New Position**:
- As the listener moves, they will encounter more wavefronts than when they were at rest. The new position of the listener after one second is denoted as \( L' \).

5. **Counting the Wavefronts**:
- Since the listener is moving towards the source of sound, they will receive additional wavefronts in the time it takes to travel the distance \( V \). Therefore, the number of wavefronts received in one second becomes \( Y \), where \( Y > X \).

6. **Calculating the New Frequency**:
- The new frequency \( f' \) of the sound waves heard by the listener is now \( f' = Y \) Hertz, which is greater than the original frequency \( f \).

7. **Conclusion**:
- The physical change that occurs is an increase in frequency as the listener moves towards the source of sound. This phenomenon is known as the Doppler Effect.

### Final Answer:
The physical change that occurs when the source of sound waves is at rest and the listener moves is an **increase in frequency** of the sound waves heard by the listener.

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