PHYS102: Introduction to Electromagnetism

The formulas in this section are similar to those for thin lenses; they are also based on an approximation. For mirrors, the approximation is that the mirror diameter that captures the light is much smaller than the radius that characterizes the curvature of the mirror. This means we assume the mirror is nearly flat, but not quite.

When a mirror is flat like this, it is impossible to distinguish whether its shape is actually part of a sphere or part of a different curved form, such as a parabolic cross section. To draw ray diagrams, it is actually more convenient to assume we have a mirror of parabolic shape, because these mirrors reflect all rays that come in parallel to the optical axis back into a single focal point.

In an approximate way, we can apply what we learn about parabolic mirrors in the next video to all weakly curved mirrors because they are indistinguishable in shape from a parabolic cross section.