i About this experiment — click to learn the physics ▼
What you're looking at
A beam of light strikes a mirror and bounces off. At the point it hits, we draw the normal — an imaginary line perpendicular to the surface. The whole of reflection is captured by comparing the incoming and outgoing rays to that normal.
The law of reflection
For any mirror, flat or curved, light obeys one simple rule:
The incident ray, the reflected ray, and the normal all lie in the same plane, and the angle the light comes in at equals the angle it leaves at. Drag the source on the Flat mirror and watch the two angles stay locked together.
Curved mirrors
A curved mirror is just a flat mirror at every tiny point — the normal there points along the radius, from the surface to the mirror's centre of curvature. Apply the same law at each point and something useful happens to a parallel beam (like light from a distant object):
- A concave mirror bends all the rays inward to a single focal point, a distance f = R/2 in front of it — this is how reflecting telescopes and satellite dishes gather light.
- A convex mirror spreads the rays outward, so they appear to come from a virtual focus f = R/2 behind it — giving the wide field of view of a car's wing mirror.
Things to try
On the flat mirror, slide the angle to graze along the surface (near 90°) and to hit head-on (0°). Switch to concave and change R — the focal point moves to exactly half the radius. Compare with convex, where the focus sits behind the mirror (dashed).