A. Draw four rays parallel to the optical axis of your mirror. Two above the optic axis and two below it. b. Trace the rays all the way to the surface of the spherical cap.
c. Draw a normal to the surface of the mirror at each of the four points where the rays touch the mirror.

   f = q

Explanation:

In the attachment we can see a diagram of the parallel rays.

The dotted line represents the normal to the mirror surface

These rays when reflected using the constructor equation

where p and q are the distance to the object and the image respectively.

Since the rays are parallel P = inf

          1 / f = 1 / inf + 1 / q

          f = q

this means that all the rays focus on one focal point.

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