The foam created by dashing waves on seashores is milky white even though the water is clear and transparent. The froth of a glass of beer is white even though the beer itself is brown and clear. Why is it that the foam is white when the material that goes to make it isn’t ?The colour of a substance depends on its light-absorbing properties, When it absorbs all the colours, it looks black, and when it reflects all the colours it looks white. When a material allows all the colours to pass through without absorbing, it looks transparent, example, water or glass. Some substances allow only a particular colour to pass through while absorbing the rest. The colour intensity depends on the amount of material traversed by light. Thus even though a bulk of a material may have a certain colour, a thin section may be nearly colourless and transparent.
The foam of the sea waves (or the froth in a beer glass) is made of thin films of water (or beer) containing air and these thin films reflect the light at random without allowing them to penetrate deeper below. Since all the colours are reflected back, the foam looks white.
A rather similar situation exists in crushed powders of transparent substances. Whereas the bulk of the material may be transparent, the powdered material looks opaque and whitish. You may note this if you crush transparent sugar crystals into powder. Even crystals which have some colour, on being powdered look whitish. Here too, the fine powder scatters light randomly without allowing light to penetrate deeper to give the characteristic colour. It is said, even the lustrous diamonds, when crushed into fine powder looks like white chalk.
My other posts are :
