Spherical and chromatic aberration (qualitative)
Real lenses blur images. Spherical aberration comes from the curved shape; chromatic aberration comes from dispersion, so colours focus at different points.
What an aberration is
A perfect lens would bring all the rays from one point of an object to one point of the image. Real lenses and mirrors do not quite do this. Hold a lens in sunlight and look at the bright spot on a paper: it is never a perfect point. Such defects are called aberrations. They blur the images in cameras, telescopes and microscopes. They depend on the shape and quality of the lens or mirror and on the kind of light used. The two main ones are spherical aberration and chromatic aberration. Your NIOS book prints them at the end of lesson 20, but the NIOS curriculum lists them under this lesson, so they can be asked in the theory exam.
Spherical aberration vs chromatic aberration
A mirror reflects every colour by the same law, so mirrors have no chromatic aberration.
How each aberration is reduced
| Aberration | Ways to reduce it |
|---|---|
| Spherical | Use a stop, a ring that lets in only paraxial rays; the image gets dimmer. Use a plano-convex lens with its curved side facing the parallel light. Use a combination of convex and concave lenses. Use parabolic mirrors instead of spherical ones. |
| Chromatic | Use an achromatic doublet: a convex crown glass lens in contact with a concave flint glass lens. Choose them so that ω₁/f₁ + ω₂/f₂ = 0. Then red and violet come to the same focus. |
How to draw the two aberrations
- Spherical: draw a convex lens with its principal axis. Draw two parallel rays near the axis meeting the axis at a point; label it Fp (paraxial focus).
- Draw two parallel rays near the edges of the lens meeting the axis closer to the lens; label this point Fm (marginal focus).
- Chromatic: draw another convex lens with a parallel beam of white light falling on it.
- Draw violet rays meeting the axis close to the lens at Fv, and red rays meeting it farther away at Fr. Label the colours.
Through a cheap magnifying glass the edges of printed letters look coloured: that is chromatic aberration. Good camera and mobile phone lenses use several lens elements of different materials to remove both aberrations.
2 marks: “What is chromatic aberration? How can it be removed?” The failure of a lens to focus all colours at one point, because μ depends on wavelength; violet focuses nearer than red. It is removed with an achromatic doublet: a convex crown glass lens and a concave flint glass lens in contact, with ω₁/f₁ + ω₂/f₂ = 0.
2 marks: “Give two ways of reducing spherical aberration in a lens.” Use a stop so that only paraxial rays pass; use a plano-convex lens with its curved side facing the incident parallel light; use a suitable combination of convex and concave lenses (any two).
1 marks: “Why is a concave mirror free from chromatic aberration?” Reflection does not depend on wavelength, so all colours are reflected to the same focus.
Spherical goes with Shape; Chromatic goes with Colour. Mirrors have no colour trouble.