Practice question · Multiple choice
The relation v = fλ says a wave's speed is frequency times wavelength. When a sound passes from air into water its speed rises sharply. Which of the other two changes, and why?
Hints
- At the boundary, what is driving the water molecules? At what rate?
- If frequency is fixed and v goes up, what must lambda do?
Show the answer
A. The wavelength, since frequency is set by the source.
Why
At the boundary the air molecules push the water molecules at exactly the rate they are oscillating, they cannot push at any other, so frequency carries across unchanged and λ must grow with v. Sound is about four times faster in water, so its wavelength is four times longer. The same reasoning explains refraction, and why colour does not change underwater.
Practise Waves
The app has 5 more questions on this lesson, and keeps your place in the course. Mathematics I is free to start.
More questions on Waves
- A wave carries both energy and matter across the room.
- Sort each wave by the direction in which the medium oscillates relative to the direction of travel.
- An ambulance siren drops in pitch as it passes. Nothing about the siren changed. What did?
- Complete the statement about what a wave carries.
- A high-frequency wave has a SHORT period, not a long one. Why must the two run opposite ways?