Elliot Taylor, of Devon Intermediate School, asks :-

Why is glass clear when the sand it is made from is opaque?

John Campbell, a physicist at the University of Canterbury and who has grown quartz crystals in his laboratory, responded.

If you have access to a microscope use it to look at sand. You will see it is made up of small grains. These come from rocks grinding together as they wash down fast flowing rivers. Many of these grains are transparent little crystals of quartz. These are the part of sand used to make glass.

Quartz is just the elements silicon and oxygen (burnt silicon) combined in the ratio of two oxygen atoms for each silicon atom. The mineral quartz was formed when the Earth solidified and/or when hot water under high pressure came in contact with hot rocks. (I can make quite big single crystals in a month by reproducing these conditions in apparatus in my lab.) In nature these single crystals are sometimes metres long. (Ask your local museum if they have any large quartz crystals in their mineral collection.) Natural crystals a few centimetres long can be purchased from rock shops for a few dollars.

Some types of atoms absorb visible light and hence make things coloured. For example cobalt atoms make glass pink or blue, iron atoms make window glass slightly green (look at a piece through one edge.) For these atoms the oscillations of the light wave are just right for that atom, much like you can pump up a swing to large amplitudes by giving it a little energy each oscillation, ie by pushing in step with the swing. Large quartz crystals are completely transparent if they contain none of these colouring atoms.

Your grains of sand dont look like crystals because they are just chipped up large crystals. Glass can be made by melting quartz. This is expensive so is used mainly for oven-proof glass-ware. Usually quartz (white sand) is mixed with sodium and calcium salts to make glass which melts at a lower temperature and hence which is cheaper to make. This soda-lime glass is what is usually used for drinking glasses, window glass etc.

I should mention that quartz sand isn't transparent because light is reflected at each surface of each grain of quartz so little light gets through a sand layer more than a few grains thick. If you put the white sand into a glass of water it becomes more transparent. (If you chose the right liquid there will be no reflection of light from crystal surfaces and you can make the quartz crystal invisible. What a great way to hide something.)