Waves

Properties of waves in liquids and solids.

During this experiment, students are required to make observations to assess the suitability of different apparatus used to measure the frequency, wavelength and speed of waves in a ripple tank, and in a solid by observing waves on a length of string. A video showing the experimental set up for the waves on a string practical can be found over on our YouTube Channel.

Equipment (per pair of participants)

• Safety glasses
• Metre ruler
• Vibration generator
• Power signal generator
• Connecting leads
• String

• Hanging slotted mass sets
(100g, 1kg)
• Wooden bridge
• Bench pulley
• Retort stand
• Large sheet of white paper

Equipment for the class to share

• Ripple tank

Method

Activity 1: Observing waves in a liquid.

Equipment • Ripple tank • Metre ruler • White paper

A number of bench top ripple tanks could be set up for a class to share. The ripple tanks should be pre-assembled by the Technical staff in readiness for the lesson. A depth of around 5mm of water should suffice and the rod-shaped dipper should be attached to the wave generator. A sheet of white paper should be attached to the place on the ripple tank that the waves are projected onto for tracing purposes (on some models of ripple tank, this will be underneath the tank, some will be on the screen at the front).
Switch on the ripple tank lamp and wave generator.
The speed of the motor should be adjusted so that low frequency waves are clearly seen in the water.
The height of the lamp can be adjusted to ensure the wave patterns are seen clearly on the viewing area.
A metre ruler is held at right angles to the waves.
Students should measure across as many waves as possible and an average is taken.
This measurement is then divided by the number of waves which gives them the wavelength.
The wavelength is recorded in a suitable table.
Students then need to count the number of waves which pass a certain point in the pattern over a chosen time (e.g. 10 seconds). They divide the number of waves counted by the chosen time period (10) to give the frequency of the waves in Hz. (e.g. 10 waves, counted in 10 seconds = 10 divided by 10 = 1Hz)
Students then calculate the wave speed by multiplying the frequency by the wavelength.

Activity 2: Observing waves in a solid.

Equipment • Vibration generator • Power signal generator • String • 100g masses and hanger • 10g masses and hanger • Wooden bridge • Pulley

The vibration generator is connected to the low impedance sockets on the power signal generator. A length of string should be attached to the armature of the vibration generator. The string is extended over a wooden bridge and over a pulley which is attached to the bench. Ensure that the mass is just heavy enough to keep the string tight.
A mass hanger with masses should be attached to a loop made in the end of the length of string.
Switch on the power signal generator and select Sine wave. Adjust the frequency range selector on the power signal generator to 10-110Hz. Adjust the frequency dial on the power signal generator so that as many half wavelengths are seen as possible (90Hz produced 3 half wavelengths in our trials using approximately 1M of string) and adjust the amplitude accordingly so clear wavelengths are seen.
Students then use the ruler to measure across as many half wavelengths as possible. They should then divide the total length of the string from the armature to the pulley by the number of half waves they have counted. This value, multiplied by two will give the wavelength.
The frequency of the waves is the frequency shown on the power signal generator.
Students can now calculate the speed of the waves by multiplying the frequency by the wavelength calculated.

Technician tips

The wooden bridge required could be produced using excess wood from the Design Technology department.
The number of ripple tanks, vibration generators and power signal generators required could be reduced, depending on your science department budget.
The two activities could be done as a circus.
If a power signal generator is unavailable to drive the vibration generator, a bench signal generator could be used alongside a power amplifier. Feeding the signal from the bench signal generator through a power amplifier will boost the signal enough to drive the vibration generator.