All waves obey the wave equation: wave speed = frequency × wavelength, v = f λ. Speed is in m/s, frequency in hertz and wavelength in metres. The period is the time for one complete wave: T = 1 ÷ f. When a wave passes from one medium into another, its frequency stays the same but its speed and wavelength change.
The speed of sound in air can be measured by timing an echo, or by two people with a clap and a stopwatch over a long distance. Ripples on water can be measured with a ripple tank: a strobe light freezes the pattern, a ruler measures the wavelength, and frequency is set by the vibrating bar. In transverse waves the oscillations are perpendicular to the direction of energy transfer; in longitudinal waves they are parallel.
Worked example
A wave has frequency 50 Hz and wavelength 4 m. What is its speed?
- v = f λ.
- v = 50 × 4.
Answer:v = 200 m/s.
Key words
- wave speed
- v = f λ
- period
- time for one wave: T = 1 ÷ f
- ripple tank
- shallow tank for studying water waves
- oscillation
- a repeated back-and-forth motion
New questions every time.
NGSS-referenced: HS-PS4-1