The focal length of a converging lens is measured for violet, green, red and yellow colours. Arrange the focal length for different colours in increasing order.
(A) \( f_v \)
(B) \( f_g \)
(C) \( f_r \)
(D) \( f_y \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B), (D), (C)
By the lens maker's formula, a larger refractive index gives a shorter focal length for the same lens.
\( \displaystyle \frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
Because of dispersion, the refractive index of glass is largest for violet and decreases towards red.
\( \mu_v \gt \mu_g \gt \mu_y \gt \mu_r \)
\( (A)\ f_v \lt (B)\ f_g \lt (D)\ f_y \lt (C)\ f_r \)
The order is A, B, D, C.
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 – “… Dispersion takes place because the refractive index of medium for different wavelengths (colours) is different. For example, the bending of red component of white light is least while it is most for the violet.”