The graph of an hyperbola from hinged mirrors
Tape 2 mirrors together as in the diagram below. Put a small object (like a white Cuisenaire rod) between the mirrors. Change the angle between the mirrors and keep track of the number of images you see.
If one does a light ray diagram of the mirrors at say 90°, the images (3 + the original= 4) all lie on one circle whose center is at the hinge, with a radius from the hinge to the small object.
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