Diffuse and Specular Reflection

Every surface reflects light by the same rule: angle of incidence equals angle of reflection, measured from a line perpendicular to the surface at the point of contact, called the normal. A mirror is flat and smooth at the scale of a light wave, so parallel rays hitting it stay parallel after they bounce. Your eye catches an organized beam and reconstructs a sharp image. That is specular reflection.

Paper, skin, asphalt, and most everyday objects look rough under magnification, covered in microscopic bumps and facets tilted in every direction. Each tiny facet still obeys the law of reflection perfectly, local angle in equals local angle out. But because neighboring facets point every which way, parallel incoming rays leave in a spray of different directions. No organized image forms; instead you get a surface that looks evenly lit from any angle. That is diffuse reflection, and it is why you can read this page in any seat in a room but a mirror only shows you an image from specific angles.

This distinction sets up everything else in geometric optics: ray tracing and image formation only work cleanly for specular surfaces, which is why mirrors and lenses are built smooth. It also explains everyday optical surprises, like why a glossy magazine page glares under a lamp while a matte page does not, and why polishing or wetting a rough surface can suddenly make it act like a mirror.