Question

A beam of parallel rays is brought to focus by a plano-convex lens. Then, a 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

Answer: (A)

The focal length of plano-convex lens,
$F_1=f$
Now when concave lens of same focal length is joined to first lens, then combined focal length
$\frac{1}{F\prime }=\frac{1}{F_1}+\frac{1}{F_2}$
$=\frac{1}{f}-\frac{1}{f}(\because F_1=f,F_2=-f)$
$=0$
$F^{\prime }=\infty$
Thus, the image can be focused on infinity ( $\infty$ ) or focus shifts to infinity.