The wave equation
- Wave speed is the speed at which energy is transferred (or the wave moves) through the medium.
- v = f λ: wave speed (m/s) = frequency (Hz) × wavelength (m).
- Rearranging: f = v ÷ λ and λ = v ÷ f.
- All electromagnetic waves travel at 3 × 10⁸ m/s through a vacuum.
Measuring the speed of sound
- Two people stand a measured distance apart, for example 400 m. One makes a loud sound with a visible action (clapping cymbals); the other starts a stopwatch on seeing it and stops it on hearing it.
- Speed = distance ÷ time. The speed of sound in air is about 330 m/s.
- An echo method can also be used: the sound travels to a wall and back, so the distance is doubled.
Required practical: ripple tank
- Use a ripple tank with a vibrating bar to make waves, and a lamp above to show the waves on a screen below.
- Measure the wavelength by measuring across several waves (say 10) and dividing by the number of waves. Use a strobe or photograph to freeze the waves.
- Count the waves passing a point in 10 seconds to find the frequency, then calculate the speed with v = f λ.
Required practical: waves on a string
- Attach a string to a vibration generator and pass it over a pulley with a hanging mass to keep it tight.
- Adjust the frequency until a clear standing wave forms. Measure the length of several half-wavelengths to find the wavelength.
- Read the frequency from the signal generator and calculate the speed with v = f λ.
Key terms
- Wave speed
- The speed at which a wave transfers energy through a medium.
- Wave equation
- v = f λ.
- Echo
- A reflected sound wave.
- Ripple tank
- A shallow tank of water used to study water waves.
- Signal generator
- A device that sets the frequency of a vibration generator or loudspeaker.
- Stroboscope
- A flashing light that makes moving waves appear to stand still.