A source of sound moves towards an observe.

Options

A. The frequency of the source is increased
B. The velocity of sound in the medium is increased
C. The wavelength of sound in the medium towards the observer is decreased
D. The amplitude of vibration of the particle is increased

Explanation

To solve the problem regarding a sound source moving towards an observer, we need to analyze the effects of this motion on frequency, wavelength, and amplitude of the sound waves. Let's break it down step by step.

### Step-by-Step Solution:

1. **Understanding the Scenario**:
- We have a sound source that is moving towards an observer. This motion will affect the sound waves reaching the observer.

2. **Effect on Frequency**:
- When the sound source moves towards the observer, the sound waves in front of the source get compressed.
- This compression leads to an increase in the frequency of the sound waves that the observer perceives.
- Thus, the frequency of the sound increases as the source approaches the observer.

3. **Doppler's Effect**:
- The phenomenon described above is known as the Doppler Effect. It explains how the frequency of a wave changes for an observer moving relative to the source of the wave.

4. **Effect on Velocity of Sound**:
- The velocity of sound in a medium is determined by the properties of that medium (like temperature and pressure) and is not affected by the motion of the source or the observer.
- Therefore, the velocity of sound in the medium remains constant regardless of the movement of the sound source.

5. **Effect on Wavelength**:
- Since the frequency increases (as established in step 2), and knowing the relationship \( v = f \lambda \) (where \( v \) is the speed of sound, \( f \) is frequency, and \( \lambda \) is wavelength), we can infer that if frequency increases, the wavelength must decrease.
- Thus, the wavelength of the sound waves reaching the observer decreases as the source approaches.

6. **Effect on Amplitude**:
- The amplitude of the sound waves is related to the energy of the sound source. The motion of the source towards the observer does not inherently change the amplitude of the sound waves.
- Therefore, the amplitude remains unchanged unless there is a change in the energy output of the sound source.

### Conclusion:
- The correct answers to the question are:
- **Option A**: The frequency of sound increases.
- **Option C**: The wavelength of sound decreases.
- **Option B** (velocity of sound in the medium increases) and **Option D** (amplitude of vibration increases) are incorrect.