Question

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

• 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)

$22.86$ same side of the object

Answer 2

Here, $R_1 = 20 \,cm, R_2 = - 20 \,cm, u = -10\, cm$ and $\mu = 1.56$ Using lens make'rs formula, $\frac{1}{f} = \left(1.56 -1\right)\left(\frac{1}{20}+\frac{1}{20}\right)$ $\Rightarrow f = \frac{20}{0.56\times2} = 17.86 \,cm$ Now, from lens equation, $v = \frac{uf}{u+f} = \frac{-10\times 17.86}{-10+17.86} = -22.86 \,cm$ Since $v$ is negative, the image will be formed on the same side as that of object.