Question

Two thin convex lenses of focal length 10 cm each are taken, they are placed on a common axis, so that the centre of one of them . coincides with the focus of the other. An object is placed at a distance twice the focal length from the left-hand lens. The image is formed at a distance of

• A. 2 cm
• B. 10 cm
• C. 20 cm
• D. 5 cm

Answer 1

Answer: (D)

5 cm

For the second lens, the first image will behave as a virtual object hence using lens formula we have
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
putting u = f we have
$\frac{1}{v}=\frac{1}{f}+\frac{1}{f}=\frac{2}{f}\Rightarrow v=\frac{f}{2}$
Given, f = 10 cm
$v=\frac{10}{2}=5cm$