The Parabola

Focus, directrix and dishes

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• A parabola: points equally far from a focus and a directrix.
• Focus $(0, p)$ and directrix $y = -p$ give $y = \dfrac{x^2}{4p}$.
• Parallel rays reflect through the focus, and rays from the focus leave parallel.

Steps

1. Focus and directrix
2. Where to put the receiver
3. Headlights
4. The latus rectum
5. Find the directrix
6. Dish depth