Question

A point object $O$ is placed on the principal axis of a convex lens of focal length $f=20 \, cm$ at a distance of $40 \, cm$ to the left of it. The diameter of the lens is $10 \, cm$ . An eye is placed $60 \, cm$ to the right of the lens and a distance $h$ below the principal axis. The maximum value of $h$ to see the image is. (assume paraxial approximation to be valid here)

• A. 0
• B. $2.5 \, cm$
• C. $5 \, cm$
• D. $10 \, cm$

Answer 1

Answer: (B)

$\frac{1}{\text{v}} = \frac{1}{f } + \frac{1}{\text{u}}$
$= \frac{1}{2 0} - \frac{1}{4 0} = \frac{1}{4 0}$
v = 40 cm

To locate field of view ray passing through upper end should be used. At location of eye, its height should not be below 20 tan $\theta $ from axis to see the image i.e., $2 0 \left(\frac{5}{4 0}\right) = \text{2.5} cm$