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

Answer 1

The correct option is B

The focus shifts to infinity.

Step 1: Given Data

Let the focal length of the plano-convex lens be $f_1$.

Let the focal length of the concave lens be $f_2$.

Given that $f_1=f_2=f$

Step 2: Formula Used

We know that the resultant focal length is given as,

$\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}$

Step 3: Estimate the Focal Length

We know that for a concave lens the focal length is negative.

Upon substituting the values we get,

$\frac{1}{F}=\frac{1}{f}-\frac{1}{f}$

$\Rightarrow \frac{1}{F}=0$

$\Rightarrow F=\infty$

Hence, the correct answer is option (B).