Wave Properties
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 5 practice questions.
A wave carries energy from place to place without carrying the material along. Every wave obeys v = fλ. The medium sets the speed; the source sets the frequency; the wavelength follows from the two.
1. Describing a wave
2. Kinds of waves
Transverse: the medium moves perpendicular to the wave’s travel (a wave on a string, light).
Longitudinal: the medium moves back and forth along the travel direction, making compressions and rarefactions (sound).
At a fixed end, a pulse reflects inverted. At a free end, it reflects upright.
A bigger amplitude carries more energy, but it doesn’t change the speed.
3. Example, then try it
A 256 Hz tuning fork sounds in air, where sound travels at 340 m/s. Find the wavelength.
λ = v/f = 340 ÷ 256 ≈ 1.3 m.
A snapshot of a wave on a 1.8 m string shows exactly 3 full wavelengths. The wave’s frequency is 50 Hz. Find the wavelength, speed and period. Then the frequency is doubled at the same tension. What changes?
Wavelength: 1.8 m ÷ 3 = 0.60 m.
Speed: v = fλ = (50)(0.60) = 30 m/s. Period: T = 1/50 = 0.020 s.
Double the frequency: same string, same tension, so v stays 30 m/s, and λ = 30 ÷ 100 = 0.30 m, half as long.
Answer: λ = 0.60 m, v = 30 m/s, T = 0.020 s; doubling f halves λ.
4. Common mistakes
Sound passes from air into water, where it travels faster. Which quantity stays the same?
The source sets the frequency, so it doesn’t change. The speed goes up, so the wavelength gets longer: λ = v/f.
Ready to practice?
5 questions on Wave Properties, each with a hint and an explanation for every wrong answer.