Last time we used wild properties of ellipses to build some really easy—and some really devilish—golf courses. Specifically, I claimed that every ellipse has two magical points F_{1} and F_{2} (called foci) such that a ray from F_{1} always bounces off the ellipse and lands precisely at F_{2}, and furthermore, this path always has the same length. Why does this happen? And how do we find these foci?

Let’s focus(!) on the last question first. Recall that an ellipse is a stretched circle. In other words, an ellipse is what forms when you slice a tall, circular tube (cylinder) along a slant:

Take a sphere that snugly fits inside the tube, and drop it down until it touches the ellipse-slice at a single point F_{1}. Do the same with a sphere underneath, touching the slice at F_{2}. These points turn out to be the foci of the ellipse. Let’s see why.

We can use this tubular setup to answer one mystery from earlier: for any point X on the ellipse, the sum of distances of XF_{1} and XF_{2} is always the same! The proof lies in the following animation. Segments XF_{1} and XA have the same length because they’re both tangent to the upper sphere from X, and similarly, XF_{2}=XB. So the sum XF_{1}+XF_{2} is just the length of segment AB, the height between the spheres’ equators.

This has two neat consequences. First, it provides an elementary method for drawing ellipses (in real life!): all you need are two push pins and a loop of string, as illustrated below. The string ensures that the sum XF_{1}+XF_{2} stays fixed while you trace the pen around, as long as you’re careful to keep the string taut throughout.

Second, what happens if we slice a cone instead of a cylinder? Perhaps surprisingly, we still get an ellipse! Indeed, as above, we can create a sphere on either side of the slice that snugly fits against the slice and the walls of the cone (the so-called Dandelin spheres), and exactly the same proof shows that XF_{1}+XF_{2} stays constant as X moves around the edge.

But wait, there’s still an unanswered question! We’ve seen that the path F_{1}X+XF_{2} has a fixed length, but why does light bounce off an ellipse along such a path? This is what we really cared about for mini-golf! Come back next time for the answer, and in the meantime, have a great 2 weeks.

Michael,
I’m glad, and thank you for your very nice comments in the last few weeks! =D

Josh,
Thank you too! I think at least part of the reason is that, in high school, conics are often taught just when analytical/coordinate geometry is being introduced. This reinforces valuable algebraic techniques, but at the expense of some pretty geometry.

Zach — this is WONDERFUL. It’s hard to feel jaded about mathematical beauty when you run across a completely elementary argument that you somehow missed over several decades of rigorous mathematical schooling.

Michael,

I’m glad, and thank you for your very nice comments in the last few weeks! =D

Josh,

Thank you too! I think at least part of the reason is that, in high school, conics are often taught just when analytical/coordinate geometry is being introduced. This reinforces valuable algebraic techniques, but at the expense of some pretty geometry.

I LOVE this!

Zach — this is WONDERFUL. It’s hard to feel jaded about mathematical beauty when you run across a completely elementary argument that you somehow missed over several decades of rigorous mathematical schooling.