A listener is at rest with respect to the source of sound. A wind starts blowing along the line joining the source and the observer. Which of the following quantities do not change?

Options

A. Frequency
B. Velocity of sound
C. wavelength
D. time period

Answer

To solve the question, we need to analyze the situation where a listener is at rest with respect to the source of sound, and wind starts blowing along the line joining the source and the observer. We need to determine which quantities do not change in this scenario.

### Step-by-Step Solution:

1. **Identify the Situation**:
- The listener is stationary relative to the sound source.
- Wind is blowing along the line joining the source and the observer.

2. **Understand the Effect of Wind**:
- Wind can affect the propagation of sound because it moves the air particles. However, since the listener is at rest with respect to the source, we need to consider how this affects sound properties.

3. **Frequency**:
- The frequency of the sound is determined by the source and does not change unless there is relative motion between the source and the observer. Since the listener is at rest with respect to the source, the frequency remains constant.

4. **Time Period**:
- The time period (T) of the sound wave is inversely related to frequency (T = 1/f). Since the frequency is constant, the time period also remains constant.

5. **Velocity of Sound**:
- The velocity of sound in air is affected by the medium's properties (temperature, pressure, etc.) and the wind. When wind blows, it can change the effective velocity of sound as it carries the sound waves along with it. Therefore, the velocity of sound changes.

6. **Wavelength**:
- The relationship between velocity (v), frequency (f), and wavelength (λ) is given by the equation: v = f * λ. Since we have established that the frequency remains constant and the velocity changes due to the wind, the wavelength must also change to maintain the equation.

7. **Conclusion**:
- The quantities that do not change are:
- Frequency (remains constant)
- Time Period (remains constant)
- The quantities that change are:
- Velocity of Sound (changes)
- Wavelength (changes)

### Final Answer:
The quantities that do not change are:
- Frequency
- Time Period