# Why does a spherical lens/mirror have spherical aberration.

I know that a spherical lens does indeed have spherical aberration, and I know that this is caused by the marginal and axel rays of light converging at different points. My question is why? What is it about the lens that makes the rays incedent on the edges of the lens focus at a closer point? Just curious.
Thanks!

## The Attempt at a Solution

Related Introductory Physics Homework Help News on Phys.org
vela
Staff Emeritus
For a spherical mirror, for instance, rays from infinity intersect the axis as a distance $l=\frac{r}{2}\cos\theta$ from the center of the sphere, where r is the radius of the sphere and θ is the angle of incidence to the mirror surface. Then θ is small, you can approximate cos θ to be equal to 1. This implies then that the rays focus at a distance r/2 in front of the mirror. The rays for which the approximation cos θ≈1 isn't good result in spherical aberration.