Section 1 of 6The wave equation
| Quantity | Symbol | Unit |
|---|---|---|
| Wave speed | v | metres per second (m/s) |
| Frequency | f | hertz (Hz) |
| Wavelength | λ | metres (m) |
| Relationship | v = f × λ | Speed = frequency × wavelength |
| Speed from distance and time | v = d ÷ t | Distance in m, time in s |
Section 2 of 6Ripple tank method
1
Set up the tank with a shallow layer of water and a motor-driven dipper that makes regular waves.
2
Light the tank from above so that the wave crests project onto a sheet of paper or screen below.
3
Use a stroboscope or camera to appear to freeze the waves, then measure the distance across several wavelengths with a ruler.
4
Divide by the number of waves to get one wavelength. Frequency is read from the motor setting or by counting waves passing a point in a given time.
5
Calculate speed using v = f × λ.
Section 3 of 6Other methods
Measuring wave speed
Waves on a string
- Waves on a string Attach a string to a vibration generator at a known frequency and tune it until a clear standing wave forms.
- Measure the wavelength, then use v = f × λ.
Sound in air: echo
- Sound in air: echo Clap near a large wall and time the echo.
- Speed = 2 × distance to wall ÷ time.
- Repeat to reduce random error.
Sound in air: two microphones
- Sound in air: two microphones Place two microphones a known distance apart connected to an oscilloscope or data logger.
- The time delay between signals gives the speed.
Typical values
- Typical values Sound in air is about 330 m/s. Water ripples move at less than 1 m/s.
Section 4 of 6Improving accuracy
Measure many wavelengths
- Measure many wavelengths Measuring across ten waves and dividing by ten reduces the percentage uncertainty in one wavelength.
Repeat readings
- Repeat readings Take several readings and calculate a mean to reduce the effect of random errors.
Longer distances and times
- Longer distances and times In echo experiments a larger distance gives a bigger time interval, so reaction time matters less.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Wave speed | The distance travelled by a wave each second. |
| Frequency | The number of waves passing a point each second, measured in hertz. |
| Wavelength | The distance from one point on a wave to the same point on the next wave. |
| Ripple tank | A shallow tray of water used to study the behaviour of water waves. |
| Stroboscope | A flashing light used to make a moving pattern appear stationary. |
Section 6 of 6Exam technique
1
Measuring the width of the tank as the wavelength. Measure from crest to crest.
2
Using frequency in kHz or wavelength in cm without converting to Hz and m.
3
In the echo method, forgetting that the sound travels to the wall and back, so the distance is doubled.
4
Believing that changing the frequency of a wave in a given medium changes its speed. The wavelength changes instead.