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. 21.73 same side of the object
• B. 21.73 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)

21.73 same side of the object

Here, $R_1=20cm,R_2=-20cm,u=-10cmand\mu =1.56$
Using lens marker's formula,
$\frac{1}{f}=(1.56-1)[\frac{1}{20}+\frac{1}{20}]\Rightarrow f=\frac{20}{0.56\times 2}=17.86cm$
Now, from lens equation,
$v=\frac{uf}{u+f}=\frac{-10\times 17.86}{-10+17.86}=-21.73cm$
Since v is negative, the image will be formed on the same side as that of object.