Rays, the Laws of Reflection and Plane Mirrors

• Incident ray: the ray of light arriving at a surface.
• Reflected ray: the ray leaving the surface after bouncing.
• Point of incidence: where the incident ray hits the surface.
• Normal: an imaginary line drawn perpendicular (at $90^{\circ }$) to the surface at the point of incidence. Draw it dashed.
• Angle of incidence $\angle i$: the angle between the incident ray and the normal.
• Angle of reflection $\angle r$: the angle between the reflected ray and the normal.

1. The angle of incidence equals the angle of reflection: $\angle i=\angle r$.
2. The incident ray, the reflected ray and the normal all lie in the same plane (you can draw all three on one flat sheet of paper).

Diffuse reflection still obeys the law of reflection at every single point. The surface is just bumpy on a tiny scale, so the normal points in a different direction at each point and the rays scatter. That is what lets you read this page from any angle, and why a wet road (smoothed by a film of water) suddenly reflects headlights like a mirror.

To locate an image, draw at least two rays from the same point on the object, reflect each one using $\angle i=\angle r$, and extend the reflected rays backward (dashed) behind the mirror. Where the extensions meet is the image of that point. Because the reflected rays never actually pass through that point, the image is virtual: it cannot be caught on a screen. A real image is one where light rays actually meet, and it can be projected onto a screen.