Question

A convex lens of focal length 1.0 m and a concave lens of focal length 0.25 m are 0.75 m apart. A parallel beam of light is incident in the convex lens. The beam emerging after refraction from both lenses is :

• A. Parallel to principal axis
• B. Convergence
• C. Divergence
• D. None of these.

Answer 1

Answer: (B)

Convergence

Power of the combination of lens $P=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1{f}_2}$

Given : $f_1=1$ m, $f_2=-0.25$ m and $d=0.75$ m

So, $P=\frac{1}{1}+\frac{1}{-0.25}-\frac{0.75}{\left(1\right)(-0.25)}$

$P=1-4+3=-3+3=0$

Thus the above combination acts as plane surface and so, the emerging beam is parallel to the principal axis.