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
$\frac{1}{F ′}=\frac{1}{F_{1}}+\frac{1}{F_{2}}$
$=\frac{1}{f}-\frac{1}{f} \, \, \left(\right.\because F_{1}=f, \, F_{2}=-f\left.\right)$
$=0$
$F'= \, \infty$
Thus, the image can be focused on infinity ( $\infty$ ) or focus shifts to infinity.