spherical abberation
Summary
Spherical aberration is image blur from lenses or mirrors with spherical surfaces: rays far from the optical axis focus at a different point than rays near it, so a point source becomes a smeared spot. It can be reduced by limiting light to rays near the axis, such as with a smaller aperture.
Detail
Meaning: Spherical aberration is a defect of lenses and mirrors with spherical surfaces. Rays far from the optical axis do not meet at the same point as rays near it, so a point object forms a blurred spot. For a concave spherical mirror, outer rays cross the axis closer to the mirror than paraxial rays; for a simple converging lens, edge rays bend more and focus nearer the lens. The cause is the spherical surface itself, which bends outer rays too strongly; the thin-lens and mirror equations ignore this because they assume paraxial (small-angle) rays.
Equation: 1/o + 1/i = 1/f, where o is object distance (m), i is image distance (m), f is focal length (m); for a spherical mirror f = r/2, with r the radius of curvature (m). These assume paraxial rays.
On the MCAT: Passages ask how to sharpen an image. Shrinking the aperture blocks outer rays and lessens the blur, at the cost of a dimmer image. A parabolic mirror focuses parallel on-axis rays to one point.
Common confusion: Chromatic aberration is different: it comes from dispersion (refractive index depends on wavelength). Spherical aberration occurs even with single-wavelength light.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.