How far down could you see in a completely clear lake?

Asked by: Lawrence Cuthbert, Jarrow

Even when there are no suspended particles to scatter the light, water will still absorb light, with red light being absorbed around 100 times more effectively than blue light. The intensity of blue light is reduced by 90 per cent over a distance of 230m, by 99 per cent over 460m, by 99.9 per cent over 690m, and so on.

The distance that you could actually see in perfectly clear water would depend on the intensity of the light source, whether the light has passed down through the water and then up again after reflection from the bottom (as with light from the Sun) or whether the source was on the bottom (a searchlight on a submarine, say). It would also depend on whether reflection from the surface was swamping the light from below. On a dark night, you might see a submarine searchlight 1km below the surface, but only if it was pointing in your direction, and then only just.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.