In the plane coordinate rectangular coordinate system, a ray is drawn from a fixed point on the bisector of a curve to this curve, and the tangent at the intersection point is reflected by a mirror. The light is parallel to the bisector, and the curve satisfying this condition is the convex lens surface curve.

This curve is a paraboloid.

But in fact, parabolic optical elements are difficult to process, so most convex-concave lenses are replaced by spherical surfaces, because in some parts of the curve, parabola and circle can overlap well.

It should be in the physics textbook of high school. If the landlord is interested, you can have a look.