What does not change when sound enters from one medium to another?

Options

A. Wavelength
B. Speed
C. Frequency
D. None of these

Answer

To determine what does not change when sound enters from one medium to another, we can analyze the properties of sound waves and how they interact with different media.

### Step-by-Step Solution:

1. **Understanding the Properties of Sound**:
- Sound waves have three key properties: speed (v), wavelength (λ), and frequency (f).

2. **The Relationship Between Speed, Wavelength, and Frequency**:
- The relationship among these properties is given by the equation:
\[ v = f \cdot \lambda \]
- This equation indicates that the speed of sound in a medium is equal to the product of its frequency and wavelength.

3. **Effect of Medium on Speed and Wavelength**:
- When sound travels from one medium to another (for example, from air to water), both the speed of sound and the wavelength change. This is because both speed and wavelength are dependent on the properties of the medium.

4. **Frequency and Its Dependence**:
- Unlike speed and wavelength, the frequency of a sound wave is determined by the source of the sound and does not change when the sound enters a different medium.
- This is because the frequency is related to how often the source vibrates, which remains constant regardless of the medium through which the sound travels.

5. **Conclusion**:
- Therefore, when sound enters from one medium to another, the quantity that does not change is the **frequency** of the sound wave.

### Final Answer:
The quantity that does not change when sound enters from one medium to another is **frequency**.

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