Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
7. Color
Why are so many powders white?
and …
Why is milk white?
and …
Why are clouds white?
All of these things are white because the light that hits them is scattered by all the tiny particles or droplets in them. When light hits a tiny particle or droplet of water or fat, it bounces off. All colors bounce off equally. When we see something that emits or reflects all colors at the same time, it appears white.
The fat droplets that make milk white come in a variety of sizes but group into three main diameters: 120 nanometers, 400 nanometers, and 1,500 nanometers. Visible light waves range from 400 nanometers to 800 nanometers, so the size of the fat droplets is just about the size of a wavelength of visible light.
The water droplets in clouds range from 6,000 to 14,000 nanometers. This is still much too small to see, especially when the clouds are a mile above you, so we see what looks like a solid object.
If particles are too small, a powder does not look white, especially when it is in a liquid. This is why the same chemical, titanium dioxide, can be a bright white in paint but invisible in sunscreen if the particles are smaller than a wavelength of visible light.