Question

An object 2.4 m in front of a lens forms a sharp image on a film 12 cm behind the lens. A glass plate 1 cm thick, of refractive index 1.50 is interposed between lens and films with its plane faces parallel to film. At what distance (from lens) should object shifted to be in sharp focus on film?

• A. 7.2 m
• B. 2.4 m
• C. 3.2 m
• D. 5.6 m

Answer 1

The correct option is D 5.6 m

Case $1:-$

$u=-240cm$ and $v=12cm$

using lens formula$:$

$\frac{1}{f}=\frac{7}{80}$

Case$2:-$

$v=12-\frac{1}{3}=\frac{35}{3}$ $($normal shift=1-(2/3)=1/3$)$

$f=\frac{7}{80}$

$u=5.6m$

Hence,

option $\left(D\right)$ is correct answer.