Question

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

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

Answer 1

The correct option is D $5.6m$

Case-1: $u=-240cm$, $v=12cm$, by Lens formula

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

Case-2: $v=12-\frac{1}{3}=\frac{35}{3}$

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

$\Rightarrow \frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\Rightarrow \frac{80}{7}=\frac{3}{35}-\frac{1}{u}$

$\Rightarrow u=5.6$