3,000 light rays bouncing inside an ellipse
Light from one point, bounced off a curved mirror. Where the paths bunch up, bright curves appear.
angle in = angle out3,000 rays of light leave one point inside an elliptical mirror, spread evenly in every direction. Each ray travels in a straight line and bounces off the wall with angle in = angle out.
Where the bright curves come from
Most of the picture is a blur of crossing lines. But in some places the rays crowd together, and the glow is much brighter. Those bright curves are caustics: places where many neighbouring rays run almost side by side.
A hidden rule of the ellipse
An ellipse has two special points, its foci. A ray that passes through one focus always bounces through the other.
Our starting point is not a focus, and something subtler happens: every ray, bounce after bounce, keeps touching one curve that shares the ellipse's foci, either a smaller ellipse or a hyperbola. Different rays touch different curves, and where many of these curves pile up, the caustics appear.
This makes the ellipse a calm table for light. Rays never go wild; each one keeps to its own lane forever.
Compare the heart (No. 011) and the stadium (No. 015). Change the shape of the mirror and that calm order can break.
