Question

An object of height $4$ cm is placed at a distance of $15cm$ in front of a concave lens of power, $-10$ dioptres. Find the size of the image.

• A. $1.6cm$
• B. $1.2cm$
• C. $1.4cm$
• D. $0.8cm$

Answer 1

The correct option is A $1.6cm$

$h_0=4cm$

$u=-15cm$

$P=-10D\Rightarrow f=\frac{-1}{10}m$

We know: $\frac{1}{f}=\frac{1}{V}-\frac{1}{u}$

So, $-10=\frac{1}{V}+\frac{1}{0.15}$

or, $\frac{1}{V}=-\frac{10}{1}-\frac{1}{0.15}=-\frac{1.5-1}{0.15}$

or, $\frac{1}{V}=\frac{-2.5}{0.15}$

or, $V=\frac{-0.15}{2.5}m$

So, $m=\frac{v}{u}=\frac{-0.15}{2.5\times -15}$ $=\frac{1}{250}m=0.4cm$

or, $\frac{h_i}{4}=0.4cm$

or, $h_1=1.6cm$