Question

Abeam of a parallel rays is brought to a focus by convex lens. If a thin concave lens of equal focal length is joined to the convex lens, the focus will

• A. Be shifted to infinity
• B. Be shifted by a small distance
• C. Remain undisturbed
• D. None of the above

Answer 1

The correct option is A Be shifted to infinity

Abeam of a parallel rays is brought to a focus by convex lens. Now, when thin concave lens of equal focal length is joined to first lens, then combined focal length be

$\frac{1}{F}=\frac{1}{F_1}+\frac{1}{F_2}=\frac{1}{f}-\frac{1}{f}=0[\because F_1=f,F_2=-f]⟹F=\infty$

Thus, the image can be focused on infinity or focus shifts to infinity.