Chromatic aberration in lenses can be corrected by using a combination of

Step 1: Analyze the question We are asked how chromatic aberration is corrected in lenses.

Step 2: Explain the concept Chromatic aberration happens because a lens refracts different colours of light by different amounts. Violet light bends more than red light, so each colour comes to focus at a different point. The result is a blurred image with coloured fringes.

Spherical aberration is a different defect. It is caused by the curved surface of the lens, and it is reduced by covering the rims (using a stop or opaque material) or by using narrow beams.

Step 3: Work through the options

• A. Diverging and converging lenses: Correct. A converging (convex) lens made of crown glass is combined with a diverging (concave) lens made of flint glass. The two lenses have different dispersive powers. The diverging lens cancels the colour spreading produced by the converging lens, while the combination still converges light overall. This is called an achromatic doublet.
• B. Opaque material covering the rims: This reduces spherical aberration, not chromatic aberration.
• C. Wide parallel beams: Wide beams make spherical aberration worse, and they do not fix colour dispersion.
• D. A thin convex lens: A single lens always disperses light, so it cannot correct chromatic aberration by itself.

Step 4: Answer

Exam tip: Link the words chromatic = colour = two lenses (achromatic doublet), and spherical = shape = cover the rims or use narrow beams. WASSCE often tests these two defects against each other.