Wave Rule #2 states what about frequency when a wave passes into a new medium?

• A. The speed remains the same.
• B. The wavelength remains the same.
• C. The frequency stays the same.
• D. The amplitude changes.

Answer: (C)

The frequency stays the same when a wave passes into a new medium. The rate at which the wave crests arrive is set by the source and doesn’t change at the boundary. What changes is the speed of propagation in the new medium. Since speed changes but frequency remains fixed, the wavelength must adjust to satisfy v = f λ. If the new medium speeds up the wave, the wavelength increases; if it slows it down, the wavelength decreases. Amplitude, however, can change at the boundary because some energy is reflected and the transmitted wave can have a different amplitude depending on how well the media match.