Question

The plane faces of two identical plano-convex lenses each having a focal length of $50cm$ are placed against each other to form a usual biconvex lens. The distance from this lens combination at which an object must be placed to obtain a real, inverted image which has the same size as the object is:

• A. 50 cm
• B. 25 cm
• C. 100 cm
• D. 40 cm
• E. 125cm

Answer 1

Answer: (A)

The focal length of the combination is $\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}$
Given, $f_1=50cm,f_2=50cm$
$\therefore$ $\frac{1}{F}=\frac{1}{50}+\frac{1}{50}=\frac{2}{50}$
$\Rightarrow$ $F=\frac{50}{2}=25cm$
Object when placed at centre of curvature forms a real, inverted image of same size as object $=(2\times 25=50cm)$ .