Wave Speed

Waves carry energy from one place to another without carrying matter along with them. A floating leaf on water bobs up and down while the wave moves across the pond. Wave speed tells us how fast the disturbance travels.

Key ideas

• Wave speed (v): how fast a wave crest or compression moves, measured in metres per second (m/s).
• Wavelength (λ): distance between two matching points on a wave (crest to crest), measured in metres (m).
• Frequency (f): number of waves passing a point each second, measured in hertz (Hz).

Equations you need

Average speed over a distance:

$$v = \frac{s}{t}$$

Link between speed, frequency and wavelength:

$$v = f\,\lambda$$

So if the medium stays the same, increasing $f$ makes $\lambda$ decrease, because $v$ stays (about) constant.

What affects wave speed?

• Wave speed depends on the medium (what the wave travels through). In deeper water, water waves often move faster. Sound travels about 340 m/s in air at room temperature, faster in liquids and solids.
• All electromagnetic waves (light, radio, microwaves) travel at about $3.0 \times 10^{8}\,\text{m/s}$ in a vacuum.
• Changing amplitude does not change speed. In one medium, changing frequency changes wavelength, not speed.

Measuring wave speed

• Time one crest moving a known distance: use $v=\tfrac{s}{t}$.
• Find $f$ with a timer and count waves, measure $\lambda$ with a ruler, then use $v=f\lambda$ (e.g. in a ripple tank).

Common misconceptions

• “Higher frequency means faster wave.” In one medium, $v$ is fixed; higher $f$ gives shorter $\lambda$, not higher $v$.
• “Particles travel with the wave.” The medium’s particles oscillate; the pattern moves.