Question: Why is water clear?

Kirsty Ross answered on 19 Jun 2012:

When we see colour, we are actually seeing the energy of the light that bounces of the object. Our eyes then interpret that energy as a colour. We can only see visible light, not UV or infrared, so can only see colours in the visible range. When you look at a yellow tennis ball, all other energy of light is absorbed and only yellow is reflected back. If you shone a blue light at a yellow tennis ball, it would actually appear black as all the blue light is absorbed and there is no yellow light to be bounced back to your eye.

Water is really interesting as it doesn’t absorb or reflect visible light very much at all (apart from absorbing tiny quantities of red light). Light passes through it pretty easily. That’s not to say that water is full of holes that light can pass through like cars through a tunnel. It is made from tightly packed molecules of H2O. These molecules contain electrons, which are responsible for causing colour (or no colour at all!). Electrons are tiny active particles that travel around the atoms that make up water and all substances. The electrons in water aren’t too fond of the energy of visible light. They prefer other types of energy like ultraviolet and infrared, which get absorbed. We can’t see those types of energy with out eyes. Since waters’ electrons aren’t that interested in the light or energy we can see, the light travels by them without much of it being absorbed and so water appears clear!

Large quantities of water do appear blue. To do this with the water in your glass, you can do an experiment at home.

If you want to make the water in your drinking glass appear blue, you can actually do it by adding something white: Milk! A few drops of milk in a glass of water will make it appear slightly blue. That’s because the white milk is helping to scatter light and the blue light is scattering more than other colors.