Question

A far sighted person can see object beyond $71cm$ clearly if separation between glasses and eye lens is $2cm$, then find focal length of glass?

• A. $23cm$
• B. $69cm$
• C. $18.4cm$
• D. $30cm$

Answer 1

The correct option is B $69cm$

$\begin{array}{c}\text{v=71cm =0.71m; v=image distance}\\ \text{Distance between glass and eye =2cm=0.02m}\\ \text{Now,}\\ \text{Distance between image and eye =0.71-0.02=0.69m}\\ \text{Lets focal length be f}\\ \text{v=-0.69(because left side is fo principle axis)}\end{array}$

$u=\infty$

$\begin{array}{c}\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ \frac{1}{-0.69}-0=\frac{1}{f}\\ f=-0.69m\\ f=0.69m\\ \text{concave mirror}\end{array}$