This term can refer to the surface finish of a material. If it’s not specular (see below), it’s probably diffuse. Diffuse means a rougher surface than, say, a mirror and a rough surface means that light coming it to hit that surface will encounter lots of different incident angles. So then, when it bounces off the surface, it will be leaving at all sorts of angles.

If you think about a small, red laser beam hitting a specular mirror surface, that beam is still going to be a small dot of light when it leaves. On the other hand, if your small, red laser beam hits a diffuse surface like sand paper, that small red dot is going to spread out into a bigger blob. The light rays going onto the sandpaper would be closely similar, and the light rays leaving the sandpaper would be shooting in all sorts of directions.

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