Arrange the following in increasing order of focal length of a given lens.
(A) fV- focal length for violet colour
(B) fB- focal length for blue colour
(C) fY- focal length for yellow colour
(D) fR- focal length for red colour
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B), (C), (D)
The focal length of a lens depends on the refractive index through the lens maker’s formula. Because of dispersion, the refractive index of glass is largest for violet and smallest for red.
\( \displaystyle \frac{1}{f} = (n - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \Rightarrow f \propto \frac{1}{n - 1} \)
\( n_V \gt n_B \gt n_Y \gt n_R \)
\( f_V \lt f_B \lt f_Y \lt f_R \)
So the increasing order of focal length is (A), (B), (C), (D).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), but it is still in the CUET 2026 syllabus, so do not skip it.
NCERT (Pre-rationalised edition, 2022-23):
Dispersion by a Prism – …
