If you've ever seen a swimming pool being emptied, you will have noticed that as the volume of water decreases, so does the 'blueness' of the water. Why is this? Well, basically, water absorbs light. You probably already know that light at wavelengths that are visible to humans is made up of the seven colours of the spectrum, ie. Red, Orange, Yellow, Green, Blue, Indigo, Violet. Water absorbs light at shorter wavelengths first, so the red gets absorbed and is therefore not visible. The blue is at longer wavelengths and doesn't get absorbed and that's why you can see it. The substances that are suspended in the water also have an effect on the distribution of light in the water by scattering the light at different angles. When you're emptying the pool, you're decreasing the amount of water there is to 'look through', so the effects of the absorption of light become less-visible. The absorption of light is also what causes the oceans to appear blue (rather than the reflection of the sky, which is a popular misconception).
In : Did You Know...
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blue "light absorption" spectrum "emptying swimming pools"
