Question

The focal length of a converging lens are fv and fr for violet and red light respectively.
(a) fv > fr
(b) fv = fr
(c) fv < fr
(d) Any of the three is possible depending on the value of the average refractive index μ.

Answer 1

The correct answer is option c, which is fv < fr.

Explanation:

• To the refractive index, the focal length is inversely proportional. To λ2, the refractive index is inversely proportional. So, when rest of the parameters are kept same, we can define that f∝1λ2.

• Therefore, λr<λv.

• Therefore, the focal length of the converging lens for violet (fv) is always lesser than the focal length of converging lens for red light (fr).