Refraction of waves at curved boundaries
When carrying out experiments on refraction, using very shallow water, it will be found that a very slight trace of detergent in the water will improve the results by reducing surface tension effects. The wave-focusing action of a shallow lens-shaped patch of water may be studied by placing a bi-convex piece of perspex in the tank. About 2 mm depth of water above the perspex coupled with a full water depth of about 8 mm gives satisfactory results.
Fig. 26.8 illustrates the focusing effect for straight waves. Having tried straight waves, the water dropper should be used to start circular pulses at various distances along the axis of the water lens and diagrams drawn in the practical notebook. The positions of the images formed by the refracted pulses in these wave diagrams should be compared with the corresponding ray diagrams for light on page 264.