Malcolm Ives, of Nayland College, asks :-

Why is window glass clear?

Michael Staines, a physicist at Industrial Research Ltd, Lower Hutt, who studies the optical properties of materials, 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 window glass, none of these absorption frequencies are in the range of visible light so window glass is clear. But, as an example, if we added cobalt ions when making the glass it would appear blue. Also if our eyes could see light of lower frequency, in the infrared, or higher frequency, in the ultraviolet, window glass would appear brilliantly coloured.