I've just created interactive demo showing this parabola property.
Of course, I created the demo using processing.org
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhUSUaaFta7ftf4UXe1Xz7mMA_qfhqePLOvt8gbC90XUY_4J_XM8fRhvJVT2ho6Zzg-02rRopqDwdcgGVVYFzbwBbpnn98ChupxCiKqpDAhu3wfZctKCoBOy1OZj7vN33YgDNsdQg/s400/parabola.jpg)
Demo simulates light photons physics assuming photons to be rigid particles. The particles bounce off parabola. When you move mouse, you change parabola focus.
Demo is located here.