When a beam of white light is allowed to pass through convex lens parallel to principal axis, the different colours of light converge at different point on the principle axis after refraction. This is called :

Solution

Physics sir · 3 steps

Different colours, different focal points: that colour-wise spreading of the focus is chromatic aberration.

1. Refractive index varies. The refractive index of glass depends on wavelength. Violet light has a larger μ than red light, so violet bends more.

   1/f = (μ - 1)(1/R₁ - 1/R₂)

2. Focal length varies. By the lens maker's formula, a larger μ gives a smaller f. So f_V < f_R: violet focuses closest to the lens and red focuses farthest.
3. So. White light gives a separate focus for each colour along the principal axis, not a single sharp point. This defect is chromatic aberration, option (d).

The idea to keep. If the trouble depends on colour (wavelength), it is chromatic aberration. If it depends on where the ray hits the lens (edge or centre), it is spherical aberration.

Answer from the NTA final answer key. Solution by PrepWiser.