in which of the following case frequency of sound heard by the observer increases?

Options

A. when source are moving away from the stationary observer
B. when observer are moving away from the stationary source
C. when both observer and source are moving away from each other
D. when both observer and source are moving towards each other

Answer

To determine in which case the frequency of sound heard by the observer increases, we need to analyze the scenarios provided.

### Step-by-Step Solution:

1. **Understanding the Doppler Effect**: The frequency of sound heard by an observer changes depending on the relative motion between the source of sound and the observer. If they are moving towards each other, the frequency increases; if they are moving apart, the frequency decreases.

2. **Case Analysis**:
- **Option 1**: Source moving away from a stationary observer.
- In this case, the observer will hear a lower frequency because the source is moving away. Thus, this option is incorrect.

- **Option 2**: Observer moving away from a stationary source.
- Here, the observer is moving away from the source. The frequency heard by the observer will decrease as the distance increases. Thus, this option is also incorrect.

- **Option 3**: Both source and observer moving away from each other.
- In this scenario, both the source and observer are moving away from each other, which means the frequency will decrease. So, this option is incorrect as well.

- **Option 4**: Both observer and source moving towards each other.
- In this case, since both the source and observer are moving towards each other, the frequency of sound heard by the observer will increase. This is because the distance between them decreases, causing the sound waves to compress, leading to a higher frequency.

3. **Conclusion**: The correct answer is **Option 4**, where both the observer and the source are moving towards each other, resulting in an increase in the frequency of sound heard by the observer.