When a wave hits a boundary, the angle of incidence equals the angle of reflection — measured from the normal.
A light ray hits a mirror 30° from the surface. What angle does the reflected ray make with the surface?
Snell's law sin θᵢ = n · sin θₜ — light entering a denser medium bends toward the normal and slows down; some always reflects too. (Going the other way, past a critical angle it can't escape at all — total internal reflection, the trick behind fibre optics.)
Angles measured from the normal — the line perpendicular to the surface at the point of contact. The incoming ray, outgoing ray, and normal all lie in one plane.
Two kinds of reflection
- Specular — smooth surface (mirror, still water): parallel rays stay parallel, you get a clear image.
- Diffuse — rough surface (paper, a wall): rays scatter every way, which is how you see ordinary objects from any angle.
- Same law applies to each tiny facet — rough just means the normals point all over.
Always measure angles from the normal, not from the surface itself. Mixing the two is the classic ray-optics slip-up.