Specular Reflection

Light meeting skin does one of two things. Some reflects straight off the outermost film at the mirror angle and never enters. That is the specular part, and it carries no information about the skin. It is the colour of whatever is lighting the room. The rest goes in, scatters among the cells, and comes back out carrying the skin's own colour. That is the diffuse part, and it is what makes skin look like skin rather than like paint.

A perfectly flat surface returns the specular component as a single bright patch. Skin is not flat: its fine relief breaks that patch into thousands of small highlights, and a field of small highlights is what the eye reads as healthy sheen. Sebum, sweat, a heavy oil or an occlusive film level the relief and merge the highlights back into one large one. A single large highlight is what we call shine.

So the difference between glow and oily is not how much light is reflected but at what scale. Mattifying powders scatter the specular light instead of letting it reflect coherently; a product sold for strobing does the opposite deliberately, with the same physics.

It is also why before-and-after photographs are so easy to steer. Move the lamp, or add a polarising filter that removes the specular component altogether, and the same face can be made to look wet or matte, textured or smooth, with nothing having changed.