What are spherical and chromatic aberrations? Explain how each can be reduced, with diagrams.

1. 1 · Define aberration. A defect in which a lens or mirror does not form a sharp point image of a point object.
2. 2 · Explain spherical aberration. Marginal rays and paraxial rays focus at different points, because the surface is spherical.
3. 3 · Draw spherical aberration. [Convex lens; paraxial rays meeting at Fp; marginal rays meeting at Fm, nearer the lens]
4. 4 · Reduce it.
   • Use a stop to pass only paraxial rays (the image is dimmer)
   • Use a plano-convex lens, or a combination of lenses
   • Use parabolic mirrors
5. 5 · Explain chromatic aberration. Because of dispersion, violet focuses nearer the lens than red: [convex lens with Fv near and Fr farther away]
6. 6 · Reduce it. Use an achromatic doublet of crown and flint glass with ω₁/f₁ + ω₂/f₂ = 0.
7. 7 · Close in one line. So, good optical instruments combine lenses and stops to remove both defects.

Scoring tip: Name both kinds of rays (marginal and paraxial) and both foci (Fv and Fr) on your diagrams.