Josephine Brown, of Kirkwood Intermediate School, asks :-

Why is water clear?

Michael Staines, a physicist at Industrial Research Ltd, Lower Hutt, who has worked with sea ice in the Antarctic, responded.

Whether a material is transparent or coloured depends on the types of atoms in the material and how they bind together.

Light is a type of wave which exerts a force on an electrically charged object, such as an electron in an atom. Like a wave on the surface of the sea, it carries energy. In the case of light waves the amount of energy depends on the frequency - that's the number of wave crests passing per second if you could count them as the wave goes by. Red light, for example, has a lower frequency and lower energy than blue light.

We and all the things around us are composed of atoms which contain electrons. As it travels past, a light wave will exert forces on these charged particles, pushing them to and fro at the frequency of the wave.

So why don't the charged particles in matter absorb all the energy from any passing light waves? It turns out, that in much the same way as a playground swing will only get going if you give it a little push each time it comes back to you, atoms and molecules can only absorb energy at particular frequencies.

In the case of water, none of these absorption frequencies are in the range of visible light so water is clear. But if our eyes could see light of lower frequency, in the infrared, or higher frequency, in the ultraviolet, water would appear brilliantly coloured.