Using a stroboscope to 'freeze' continuous ripples

Demonstration

A stroboscope makes it easier to see patterns of wave behaviour with continuous ripples in a ripple tank, especially with ripples at higher frequencies.

Apparatus and Materials

• Hand stroboscope

• Ripple tank and accessories

• Motor mounted on beam, with beam support
• Paper, white

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Photo-induced epilepsy

In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments.

Read our standard health & safety guidance

You will also need an appropriate power supply for the motor (special, or 1.5 V cell with 12Wrheostat)

If there are not enough hand stroboscopes to go round, with a little practice students can wave a hand, with fingers spread, in front of their eyes to freeze the wave motion.

See also:

Vibrator to generate continuous waves

Procedure

1. Generate continuous circular ripples and then continuous straight-line ripples, possible with reflections at a straight barrier.
2. Ask students to use the stroboscopes to ‘freeze’ the motion of the waves.

Teaching Notes

• Students should already be familiar with hand stroboscopes. They may take a little time to develop skill in freezing the motion.
• Point out how patterns easily seen with the unaided eye at low frequencies become impossible to see as the frequency is increased. Stroboscopes make visible phenomena at higher frequencies.
• To make this exercise more interesting, you might ask students to observe:
• Whether wavelength changes when waves are reflected.
• Whether (and how) wavelength changes with frequency.
• What happens to frequency and wavelength as waves enter shallow water.
• See also:

  Vibrator to generate continuous waves

This experiment was safety-tested in February 2006