Question

Rays from a lens are converging toward a point $P$, as shows in figure. How much thick glass plate having refractive index $1.6$ must be located between the lens and point $P$, so that the image will be formed at $P^{\prime }$?

• A. $0.8\text{cm}$
• B. $1.6\text{cm}$
• C. $5\text{cm}$
• D. $2.4\text{cm}$

Answer 1

The correct option is A $0.8\text{cm}$

Due to insertion of glass plate/slab, the image will get shifted by amount $s$ in the direction of the incident ray on the slab.
Normal shift due to refraction from slab,
$P{P}^{\prime }=s=0.3\text{cm}$

Since the surrounding medium is air, using formula for normal shift,
$\Rightarrow t(1-\frac{1}{\mu })=0.3$
$t(1-\frac{1}{1.6})=0.3$
$t\left(\frac{0.6}{1.6}\right)=0.3$
$\therefore t=0.8\text{cm}$

$\begin{array}{c}\text{Why this question?}\\ \text{Always remember that inserting a}\\ \text{glass plate will always shift the}\\ \text{image position, whether it is a mirror or lens.}\end{array}$