Why does milk look white?
The short answer
Milk looks white mainly because tiny particles inside it scatter light. These include clusters of milk protein called casein micelles and small droplets of fat called fat globules. Many colors of light reach our eyes together, so the milk looks white. It is not simply dyed with a white pigment.
How does scattered light look white?
White light from the Sun or a lamp contains many colors, including red and blue. As light travels through milk, it meets suspended particles and changes direction. This spreading of light is called scattering.
Scattered light reaches your eyes from many directions. When light of many colors arrives together, it looks white. Because the light is scattered again and again, little of it travels straight through a full glass, so milk also looks cloudy.
What are the particles in milk?
Casein is one of the main proteins in milk. Casein molecules join with minerals such as calcium phosphate to form microscopic clusters called casein micelles. Milk also contains tiny droplets of fat called fat globules. You cannot see these particles one by one, but there are many of them, and both types scatter light.
Why can skim milk look slightly blue?
Skim and low-fat milk contain less fat, so they have fewer fat globules. The remaining casein micelles scatter blue light more strongly than red light. This can give the milk a faint bluish cast, although it may be hard to notice and can vary with the product and lighting.
Milk fat can contain beta-carotene, a yellowish pigment that comes from a cow's feed. It can add a slight creamy or yellow tint to milk and cream.
Is this how the sea looks blue?
Both colors involve light, but their main causes differ. Milk looks white mainly because its particles scatter light. The sea looks blue mainly because water absorbs red light more strongly than blue, leaving more blue light to reach our eyes. Learn more in Why does the sea look blue?