Conon and Brennan of Dunedin asks :-

Why is water invisible?

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

Water isnt invisible but it is clear though slightly coloured.

It is clear because the atoms in it do not absorb or scatter much visible light. Raspberry drink, for example, is coloured because there is a little red food colouring in it. This contains some special molecules that absorb blue light leaving only red light to be transmitted and scattered.

Milk is white, ie not clear, because the little fat globules in the water scatter the white light strongly.

The water in the open sea or in a swimming pool looks blueish showing that water does absorb red light, though only weakly. Divers below 10 metres see mostly blue light. They get surprised when they turn on a torch and see the full range of bright colours that the fish and plants have. Through short distances, for example through a glass of water, the blue colour isn't apparent. Sellers of spring water usually put it in a bottle which is slightly tinted blue to make it look more pure.

Window glass is another example of something we regard as clear but which is actually coloured. If you get the chance to look at a pane of window glass away from its frame look at its thin edge. The glass is green. This is due to about one atom in a million in glass being an atom of iron. In order to send signals by light in glass fibre optic cables the glass must be highly purified. Then signals can be sent over distances of up to 100km. Now that is clear.

Water isn't invisible. We can easily see whether or not a glass has water in it. This is because of the reflections from the water surfaces. It is a general property of light that we get reflection from a surface between two materials in which light has different speeds. When light enters water from air its speed slows down by a quarter which means that one part in every fifty of the light intensity is reflected at the surface. That isn't much and if the surface is very still we usually dont notice this reflection. Then water seems invisible. You may notice that in the diving pool at the Olympic Games water is sprayed onto the surface to make the surface more visible to the divers.

We can make something that is clear appear invisible by immersing it in a liquid which has the same speed for light. For example, Pyrex glass in peanut oil is a close match.