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:

(A), (B), (D), (C)

The correct answer is Option (2) → (A), (B), (D), (C)

For a converging lens, due to dispersion, the focal length depends on the refractive index: $f = \frac{R}{n-1}$. Higher refractive index gives shorter focal length.

Refractive index order for visible spectrum: $n_v > n_b > n_g > n_y > n_r$

Thus, focal lengths: $f_v < f_g < f_y < f_r$

Answer: $f_v < f_g < f_y < f_r$