Question

A plano concave lens is placed on a paper on which a flower is drawn. How far above its actual position does the flower appear to be ?

• A. $10\text{cm}$
• B. $15\text{cm}$
• C. $50\text{cm}$
• D. None

Answer 1

The correct option is A $10\text{cm}$

This problem can be drawn as follows
Here we will consider the image is loacted on negative axis and the radius of curvature is on positive axis.
$u=-20\text{cm}R=+20\text{cm}$
$n_1=\frac{3}{2}{n}_2=1$
from $\frac{n_2}{v}-\frac{n_1}{u}=\frac{(n_2-n_1)}{R}$
$=\frac{1}{v}+\frac{3}{2\times 20}=\frac{1-\frac{3}{2}}{20};v=-10\text{cm}$
The flower appears to be $20-10=10\text{cm}$ above from initial position