Question

A beam of parallel rays is brought to focus by a plano-convex lens. A thin concave lens of the same focal length is joined to the first lens. The effect of this is :

• A. the focus shifts to infinity.
• B. the focal point shifts towards the lens by a small distance.
• C. the focal point shifts away from the lens by a small distance.
• D. the focus remains undisturbed.

Answer 1

The correct option is A the focus shifts to infinity.

$\frac{1}{f_{combination}}=\frac{1}{f_1}+\frac{1}{f_2}$

$=\frac{1}{f}+(-\frac{1}{f})=0$

$⟹f_{combination}=\infty$