A lens shown below is made up of two different materials. A point object is placed on the principal axis of the lens. The number of images formed will be:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2
The focal length of a lens depends on the refractive index of its material, through the lens maker’s formula:
\( \displaystyle \frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
The lens in the figure is built from two materials, of refractive indices \( \mu_1 \) and \( \mu_2 \). The light that passes through the \( \mu_1 \) portions of the lens is focused with focal length \( f_1 \), and the light that passes through the \( \mu_2 \) portions with a different focal length \( f_2 \). Each material therefore forms its own image of the object, at a different position. The number of images is the number of different materials, which is 2.