A ripple tank is a shallow glass tank of water used in schools to demonstrate the basic properties of waves. The tank is illuminated from above, so that the light shines through the water.
The ripples on the water show up as shadow on the screen underneath the tank.
All the basic properties of waves can be demonstrated.
A stroboscope, also known as a strobe, is an instrument used to make a moving object appear to be slow-moving, or stationary.
The water acts as a lens to produce a pattern of bright and dark regions on the screen (or a piece of white paper).
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