Our heart video looks designed, but nobody drew the heart. It appears by itself, out of a lamp, a round mirror and one rule: light leaves a mirror at the same angle it arrives. Pick a number of rays below and watch it form.
Why a circle of light draws a heart
Published 2 October 2026
The setup
A circular mirror, seen from above. The lamp sits on the mirror itself, at the bottom, and sends rays into the circle in a wide fan, from straight up to almost along the wall. Each ray travels in a straight line until it meets the mirror, then reflects and carries on.
Where the heart comes from
Each ray on its own is just a straight line. But neighbouring rays leave the lamp at slightly different angles, reflect at slightly different points, and after the first reflection they cross each other again and again along the same curve. Where many rays cross, the light piles up, and that bright edge is a curve called a cardioid, from the Greek word for heart. A bright curve like this, drawn by rays bunching together, is called a caustic.
You have probably seen a caustic already: the bright curve inside a cup of coffee or a ring lying in sunlight. Sunlight comes from far away, so the curve in the cup is a close relative with two lobes, called a nephroid. With the lamp sitting right on the rim, as in our video, the curve is exactly the cardioid.
Why 10 rays aren't enough
With 10 rays you see 10 lines and no heart. With 100, the outline starts to show. With 1000, the lines blur into one smooth glowing curve. The curve was there all along; it only needs enough rays to become visible. That's why our ray videos go from 10 to 100 to 1000 rays: the picture appears as the melody gets denser.
The first heart didn't look like one
The first version of the video had the lamp at the top. The physics was correct, and the heart came out upside down. On top of that, the rays that kept bouncing after the first reflection drew a busy flower over it. Shown the result without any explanation, the first question was simply: where's the heart?
It was a fair question. A shape that exists in the maths is not the same as a shape people recognize in half a second on a phone. We moved the lamp to the bottom, which turns the heart the right way up, and made the mirror imperfect on purpose: each reflection keeps only 35% of the light. The first reflection, the one that draws the heart, stays bright, and the later ones fade away. The demo above uses the same settings.
Hear it
In the video every reflection plays one note of Greensleeves, so the heart fills in as the melody speeds up. You can watch it on the payantha channel, or read how a payantha Short is made.