When a sound wave passes from air into water, which of the following properties of the wave remains unchanged?

Introduction / Context:
Sound is a mechanical wave that requires a medium to propagate. When sound travels from one medium to another, such as from air to water, some of its properties change while others remain the same. Examinations frequently test whether you understand which characteristics depend on the medium and which are determined by the source of the sound. This question focuses on how frequency, wavelength, speed, and amplitude behave at a boundary between media.

Given Data / Assumptions:

• The sound originates from a source in air.
• The sound wave passes from air into water.
• The medium changes from air (less dense) to water (denser).
• We neglect complex effects like strong reflection and consider the general principle for transmitted sound.

Concept / Approach:
For a wave, the fundamental relation is v = f * λ, where v is wave velocity, f is frequency, and λ is wavelength. Frequency is determined by the source of the sound and does not change when the wave enters a new medium. However, the velocity of sound depends strongly on the properties of the medium, such as density and elasticity, so it changes when moving from air to water. Since v changes while f remains constant, the wavelength adjusts accordingly to satisfy v = f * λ. Amplitude can also change due to partial reflection and transmission at the interface. Therefore, frequency is the property that remains unchanged.

Step-by-Step Solution:
Step 1: Recall the relation v = f * λ for any wave. Step 2: Identify that the source of the sound is in air, which sets the frequency of vibration. Step 3: When the wave enters water, the medium changes, so the speed of sound changes because it depends on the medium's elasticity and density. Step 4: Since v changes and f is fixed by the source, wavelength λ must change to satisfy v = f * λ. Step 5: Amplitude can also be altered at the boundary due to partial reflection and absorption. Step 6: Therefore, the only quantity that stays constant during transmission from air to water is the frequency.

Verification / Alternative check:
Practical examples support this conclusion. When you hear a sound underwater that originated in air, the pitch of the sound (which depends on frequency) is perceived the same, although the loudness and quality may change. Pitch is tied directly to frequency. Textbook explanations of wave refraction also stress that frequency does not change at a boundary, whereas speed and wavelength do. This is analogous to light waves, where frequency remains constant when passing from air into glass, but speed and wavelength change because of the different optical properties.

Why Other Options Are Wrong:
Velocity: The velocity of sound clearly changes when the medium changes, because water transmits sound faster than air due to its physical properties.
Wavelength: Since v = f * λ and v changes while f remains constant, wavelength must adjust, so it does not remain the same.
Amplitude: Amplitude can be reduced or otherwise altered at the boundary due to reflection and absorption effects, so it is not constant either.

Common Pitfalls:
Students sometimes think that wavelength remains constant and frequency changes, but it is actually the reverse. This confusion often comes from focusing only on the formula without considering the role of the source. Remember that the medium controls speed, the source controls frequency, and the wavelength adjusts to maintain the wave relation. Keeping this idea clear helps avoid many conceptual errors in wave problems, for both sound and light.

Final Answer:
When sound travels from air to water, the property that remains unchanged is its Frequency.