When sound wave is refracted from air to water, which of the following will remain unchanged?

Options

A. Wavelength
B. Wave number
C. wave velocity
D. Frequency

Explanation

To solve the question regarding the refraction of sound waves from air to water, we need to analyze the properties of sound waves in different media.

### Step-by-Step Solution:

1. **Understanding Wave Properties**:
- Sound waves have three main properties: velocity (v), frequency (f), and wavelength (λ). The relationship between these properties is given by the equation:
\[ v = f \cdot \lambda \]

2. **Refraction of Sound Waves**:
- When sound waves travel from one medium (air) to another (water), they undergo refraction. This means that their speed and wavelength will change due to the different densities and elastic properties of the two media.

3. **Change in Velocity**:
- The speed of sound is different in air and water. In general, sound travels faster in water than in air. Therefore, when sound refracts from air to water, the velocity of sound changes.

4. **Change in Wavelength**:
- Since the speed of sound changes and the frequency remains constant, the wavelength must also change. This can be understood from the equation \( v = f \cdot \lambda \). If \( v \) changes and \( f \) remains constant, \( \lambda \) must change to maintain the equation.

5. **Frequency Remains Unchanged**:
- The frequency of a wave is determined by the source of the sound and does not change when the wave moves from one medium to another. The particles in the new medium (water) will vibrate at the same frequency as they did in the original medium (air).

6. **Conclusion**:
- Therefore, when sound waves are refracted from air to water, the property that remains unchanged is the frequency (f).

### Final Answer:
The frequency of the sound wave remains unchanged when it is refracted from air to water.