Properties of waves
Waves transfer energy but not matter. Waves may be transverse (eg water wave) or longitudinal (eg sound wave). Wave motion can be described using the terms amplitude, wavelength, frequency and period.
Specified practical
Investigation of water waves - measure the wavelength and speed of water waves
A ripple tank can be used to measure the wavelength of waves on the water's surface. A ripple tank is a transparent shallow tray of water with a light shining down through it onto a white card below to more easily see the motion of the ripples created on the water's surface. Ripples can be made by hand but to generate regular ripples it is better to use a motor.
A simple tray of water can be used to measure the speed of waves travelling in water. One end of the tray is lifted and dropped gently to produce the waves.
Aim of the experiment
Method 1: To measure the wavelength of waves in a ripple tank.
Method 2: To measure the speed of waves in a tray of water.
Method 1
- Set up the ripple tank as shown in the diagram with about 5 cm depth of water.
- Adjust the height of the wooden rod (plane wave dipper) so that it just touches the surface of the water.
- Switch on the lamp and motor and adjust until low frequency waves can be clearly observed.
- Freeze the wave pattern using a stroboscope.
- Measure 10 wavelengths on the card and find the mean value. Repeat for a greater depth of water.
Example results table
| Depth of water in cm | Length of 10 waves in cm | Wavelength in cm |
|---|---|---|
| ... | ... | ... |
Method 2
- Measure the length of the tray and record the result.
- Add water to the tray to give a depth of eg 0.5 cm and note the volume of water used.
- Lift one end of the tray by a few cm and gently drop onto the table.
- Start the timer when the wave in the tray hits one end of the tray.
- Record how long it takes for the waves to travel eg four lengths of the tray.
- Repeat steps 3 to 5 four more times.
- Repeat steps 2 to 6, adding more water each time to increase the depth by 0.5 cm up to eg 3 cm.
Example results table
This table measures the time taken for waves to travel three lengths of the tray in seconds.
| Depth of tray (cm) | Length of tray (cm) | Trial 1 | Trial 2 | Trial 3 | Trial 4 | Trial 5 | Mean | Mean (cm/s) |
|---|---|---|---|---|---|---|---|---|
| ... | ... | ... | ... | ... | ... | ... | ... | ... |
Risks/hazards
| Hazard | Consequence | Control measures |
|---|---|---|
| Electrical components near water | Shock, damage to components | Secure electrical components before adding water taking care not to splash |
| Wet floors are slippery | If water splashes on floor during the experiment people may slip and be injured | Do not overfill the trays, place them down |
Analysis
- Do the results of your experiments agree with your hypothesis?
- Suggest a conclusion. Think about the accuracy of the measurements you made and how you minimised the uncertainty in your measurements.