Question

A biconvex symmetrical lens made of glass of refractive index $1.56$ has radii of curvature of magnitude $20 \, cm$ . Find the position of the image formed if an object is placed at a distance of $10 \, cm$ from this lens.

• A. $22.86$ same side of the object.
• B. $22.86$ opposite side of the object.
• C. $44.89$ same side of the object.
• D. $44.89$ opposite side of the object.

Answer 1

Answer: (A)

Here, $R_{1}=20 \, cm, \, R_{2}=-20 \, cm, \, u=-10 \, cm$
and $\mu =1.56$
Using lens maker's formula,
$\frac{1}{f}=\left(1.56 - 1\right)\left(\frac{1}{20} + \frac{1}{20}\right)\Rightarrow f=\frac{20}{0.56 \times 2}=17.86 \, cm$
Now, from lens equations,
$v=\frac{u f}{u + f}=\frac{- 10 \times 17.86}{- 10 \times 17.86}=2.86 \, cm$
Since $v$ is negative, the image will be formed on the same side as that of object.