Peter Ballantyne, of Ardgowan School, asks :-

How can there be a reflection from water when it is clear?

John Campbell, a physicist at the University of Canterbury, responded.

Because light travels at a different speed when it is in water than when it is in air.

This seems a surprising answer but it is a general property of waves travelling from one material into another that some of the wave is reflected at the interface if the wave speed is different in the two materials. The amount of reflection depends on the two wave speeds.

We usually dont show people how to predict the amount of light reflected until they are taking physics at second year at university by which time they know enough mathematics to work out the properties of waves for themselves.

In water, light slows down to three-quarters of its speed in air. From this fact alone we can predict that 1/50th of the incident light intensity is reflected from an air-water surface. Similarly for window glass, in which the speed of light is two-thirds that in air, 1/25th of the light intensity is reflected.

If you want to make a clear material invisible then the above tells us that one way is to place it in another clear material for which the speed of light is the same for both. Light travels faster in water than almost every other transparent material so we cannot hide anything in water. However we can hide a chip out of a car windscreen by filling it with a transparent epoxy in which the speed of light is the same as that in the windscreen glass.