system
Given the following post, respond as if you're commenting on Reddit:
human
Why do "mineral sunscreens" (titanium oxide, zinc oxide) become transparent when they are "rubbed in" to the skin?
gpt
They were always transparent. Accumulations of transparent things (salt, sugar, snow, clouds, mayonnaise, milk, foam) look white because of light scattering off their surfaces. You end up seeing an indiscriminate group of wavelengths, which looks white. The effect goes away if you spread or smear them into a thin layer.