An observer is moving away from a source at rest. The pitch of the note heard by the observer is less because

Options

A. the pitch of the source decreases
B. the velocity of sound in air increases
C. wave length of the wave becomes will
D. wavelength of the wave remains unchanged but observer receives less number of waves

Answer

To solve the problem, we need to understand the Doppler effect, which describes how the frequency of a wave changes for an observer moving relative to the source of the wave.

### Step-by-Step Solution:

1. **Understanding the Scenario**:
- We have a sound source that is at rest (frequency \( f_0 \)).
- An observer is moving away from this source.

2. **Doppler Effect Formula**:
- The formula for the observed frequency \( f' \) when the observer is moving away from a stationary source is given by:
\[ f' = f_0 \frac{v - v_o}{v} \]
- Where:
- \( f' \) = observed frequency
- \( f_0 \) = source frequency
- \( v \) = speed of sound in air
- \( v_o \) = speed of the observer (which is positive when moving away)

3. **Analyzing the Formula**:
- In this case, since the observer is moving away from the source, \( v_o \) is positive.
- As the observer moves away, the term \( v - v_o \) becomes smaller than \( v \), which results in \( f' < f_0 \).

4. **Conclusion on Pitch**:
- The pitch of the sound is directly related to the frequency. A lower frequency means a lower pitch.
- Therefore, the pitch of the note heard by the observer is less than the actual pitch emitted by the source.

5. **Wavelength Consideration**:
- The wavelength of the sound remains the same as it is determined by the source. However, since the observer is moving away, they receive fewer waves per unit time, leading to a lower frequency.

### Final Answer:
The pitch of the note heard by the observer is less because the observer is moving away from the source, resulting in a lower observed frequency due to the Doppler effect.