In our ellipse video a lamp fires rays in every direction, and a moment later they all collapse onto one point at the same instant. It looks like an editing trick. It's a property of the ellipse that geometers have known for more than two thousand years, and you can watch it happen below.
The ellipse trick: every ray arrives at once
Published 2 October 2026
Two special points
An ellipse has two special points inside it, called its foci. In the demo the lamp sits on the lower one, and the small dot near the top marks the other.
The ellipse is defined by those two points: it's every point whose distances to the two foci add up to the same total. That's how you can draw one by hand. Tie the ends of a piece of string to two pins, pull the string tight with a pencil, and move the pencil all the way round.
Why every ray arrives together
Now follow one ray from the lower focus. It goes straight to some point on the mirror and reflects. An elliptical mirror has a second property: at every point, the wall makes the same angle with the lines to both foci, so a ray coming from one focus always reflects straight toward the other.
So every ray travels focus, mirror, focus. By the definition above, that total distance is the same wherever the ray hits the wall. In the demo the ellipse is 400 pixels tall, and every ray has travelled exactly 400 pixels when it reaches the top focus, whichever way it left. Same distance, same speed, same moment. Watch the counter under the demo when the rays meet.
Then they come back
Keep watching: the rays spread out again, reflect a second time, and meet again on the lamp, all at once, after another 400 pixels. In a perfect mirror this would go on forever. In the demo each reflection keeps 60% of the light, so every return is a little dimmer.
The same trick with sound
It works for sound too. In rooms with elliptical walls or ceilings, a whisper made at one focus can be heard clearly at the other, across the room, because every echo off the walls arrives there at the same time. Places like this are known as whispering galleries.
In the video
In the video every reflection plays a note of The Blue Danube, and the rounds grow from 10 rays to 100 to 1000. Because the rays leave in every direction, they hit the wall at different moments, so the notes spread out. Then they all meet at the far focus together, and the round of 1000 rays ends the video in an explosion. Read more in how a payantha Short is made, or see the heart that a circular mirror draws in why a circle of light draws a heart.