(ii) techniques and procedures used to demonstrate wave effects using a…

The idea

A ripple tank makes water waves visible so you can demonstrate reflection, refraction, diffraction and superposition. Set the tank level, add water about 5 mm deep, and drive a dipper bar with a signal generator so plane waves travel across the tank. Illuminate from above with a lamp; the wave crests focus light and cast bright and dark bands on the screen or bench below, letting you measure wavelength with a ruler. A straight barrier shows reflection, a change of water depth or a glass plate shows refraction, and a gap or obstacle shows diffraction spreading. Two dippers produce two sets of circular waves that overlap to show interference. Adjust the driving frequency to change wavelength and observe how the pattern changes.

What earns the marks

• Plane water waves are produced by a straight dipper bar driven at a set frequency by a signal generator.
• The tank is illuminated from above so crests and troughs cast bright and dark bands on the screen below.
• Wavelength is found by measuring the distance across several bright bands and dividing by the number of wave spacings.
• A straight barrier demonstrates reflection; a change of depth or a glass plate demonstrates refraction.
• A gap or obstacle demonstrates diffraction, with more spreading when the gap is comparable to the wavelength.
• Two dippers produce overlapping circular waves whose superposition demonstrates interference.

Common mistakes and how to put them right

• Thinking the bright bands are the water waves themselves.

  Put it right: They are images of crests formed by the lamp, so the pattern is a projection of the wave crests.

• Measuring a single band spacing and treating it as the wavelength without averaging over many bands, which increases the percentage uncertainty.

• Assuming diffraction only happens when the gap is much larger than the wavelength.

  Put it right: In fact spreading is most noticeable when the gap is similar to the wavelength.

Exam tips

• Link each wave effect to the specific tank arrangement that produces it, for example barrier for reflection and gap for diffraction.
• When describing measurement, state that you measure across several wavelengths and divide to reduce uncertainty.
• Use the words crest, trough, frequency and wavelength precisely; examiners look for correct terminology rather than vague descriptions.