Torch on the Wall

aim the torch so its light lands on the chalk.

aim a torch at a dark wall so the edge of its light fits four chalk drawings: a circle, an ellipse, a parabola and a hyperbola, one cone of light cut four ways

A torch throws a cone of light, and the wall slices it. Pointed straight, the slice is a circle; tilted, an ellipse; at the one tilt where the top edge of the beam runs parallel to the wall, a parabola that never closes; past it, a hyperbola. Apollonius of Perga named the three, c. 200 BC. A TV satellite's orbit is a circle, a planet's an ellipse; a satellite dish is a parabola, and a lampshade throws a hyperbola on the wall.

Concepts: light, geometry, projection.

An interactive you play with one thumb, on Wundr.