Anthea Dempsey, of Mokoia Intermediate School, asks :-

Kate Johnston, of Ardgowan School, asked:-

Why is it that when you get water out of the tap it is clear but when it is in a lake it is blue or green?

Bob Buckley, a physicist at Industrial Research Ltd, who has studied the optical properties of sea ice in the Antarctic, responded.

The light from the sun is white and is composed of all the colours, from red to blue. Water absorbs red light weakly but moreso than blue light. The amount of red light absorbed depends on the distance travelled by the light in the water. Tap water appears clear because the light has only traveled a few centimetres through water. However when we dive a few metres under the surface of a pool or lake, it is blue because the red light has been absorbed. The greater the depth of water the bluer the light.

Light that is reflected from lake water is reflected (or scattered ) off particles suspended in the water. Otherwise the water would appear black - no light would come into your eye. The number of particles in the water determines the distance the light travels into the water before it comes out again.

If the light has not travelled far into the water, because of large numbers of particles to scatter off of, then the lake will appear light green or even white. This is the case in the glacial lakes of the South Island which contain large amounts of finely ground rock suspended in the water. In that case very little red light was absorbed by the water before the light was scattered back out.

Thus the colour of lakes, from white to blue-green to blue, depends on the distance the light travels into the water which depends on the number of particles in the water - how clean it is. Now you know why two lakes near where you live are called the Blue and Green Lakes. Can you get a sample of each to check which has the most suspended particles?