A source and an observer are at rest `w.r.t` ground. Which of the following quantities will remain same, if wind blows from source to oberserver ?

Options

A. Frequency
B. Speed of sound
C. wavelength
D. Time period

Explanation

To solve the problem, we need to analyze the situation where a source and an observer are at rest with respect to the ground, while wind blows from the source to the observer. We will evaluate how this affects various quantities: frequency, speed of sound, wavelength, and time period.

### Step-by-Step Solution:

1. **Identify the Situation**:
- The source (S) and observer (O) are both at rest relative to the ground.
- Wind is blowing from the source to the observer.

2. **Understand the Effect of Wind**:
- The wind affects the propagation of sound waves but does not change the frequency emitted by the source or the frequency perceived by the observer if both are stationary relative to the ground.

3. **Frequency**:
- The frequency of the sound produced by the source remains constant because the source is not moving.
- Therefore, the frequency does not change due to the wind.
- **Conclusion**: Frequency remains the same.

4. **Speed of Sound**:
- The speed of sound in air is affected by the medium (air) and its properties (temperature, pressure, etc.), but the wind does not change the inherent speed of sound in the medium.
- However, the effective speed of sound relative to the observer can change due to the wind, as it adds to the speed of sound in the direction of the observer.
- **Conclusion**: Speed of sound does not remain the same.

5. **Wavelength**:
- Wavelength is related to frequency and speed of sound by the equation:
\[ \text{Wavelength} = \frac{\text{Speed of Sound}}{\text{Frequency}} \]
- Since the frequency is constant but the speed of sound is affected by the wind, the wavelength will also change.
- **Conclusion**: Wavelength does not remain the same.

6. **Time Period**:
- The time period (T) is the reciprocal of frequency:
\[ T = \frac{1}{f} \]
- Since the frequency remains constant, the time period will also remain constant.
- **Conclusion**: Time period remains the same.

### Final Answer:
The quantities that remain the same when wind blows from the source to the observer are:
- Frequency
- Time Period