When a sound wave passes from air into water its

Analysis

This question tests your understanding of wave properties as they travel through different media. To solve this, we must look at how the physical properties of a wave (speed, frequency, and wavelength) behave when moving from a less dense medium (air) to a denser medium (water).

Concept Explanation

The relationship between the speed ($v$), frequency ($f$), and wavelength ($\lambda$) of a wave is given by the formula:

1. Frequency ($f$): This is determined entirely by the source of the sound. When a wave moves from one medium to another, the frequency remains constant.
2. Speed ($v$): Sound travels faster in denser media because the particles are closer together, allowing the vibration to be transmitted more efficiently. Therefore, sound travels faster in water than in air.
3. Wavelength ($\lambda$): Since $v = f \times \lambda$ and $f$ is constant, if the speed ($v$) increases, the wavelength ($\lambda$) must also increase to maintain the equality.

Step-by-Step Solution

1. Identify the constant: As the sound wave enters the water, its frequency ($f$) does not change.
2. Determine the speed change: Sound travels significantly faster in water (approx. 1,480 m/s) than in air (approx. 340 m/s). Thus, the speed ($v$) increases.
3. Determine the wavelength change: Using the formula $\lambda = v / f$, since $v$ increases and $f$ is constant, $\lambda$ must increase.
4. Conclusion: Both speed and wavelength increase, while frequency remains the same.

Identifying the Answer

Comparing our conclusion to the options provided:

• A is incorrect because frequency does not change.
• B is correct because it states that speed and wavelength increase while frequency remains constant.
• C is incorrect because speed actually increases.
• D is incorrect because wavelength does not decrease.

Exam Tip: Always remember the "Source Rule": The frequency of a wave is a "DNA" trait—it is fixed by the source and never changes regardless of the medium. If you know the frequency is constant, you can immediately eliminate any option that suggests it changes!