Question

Parallel rays of light are focussed by a thin convex lens. A thin concave lens of same focal length is then joined to the convex lens and the result is that :

• A. the focal point shifts away from the lens by as mall distance
• B. the focal point shifts towards the lens by as mall distance
• C. the focal point of lens does not shift at all
• D. the focal point shifts to infinity

Answer 1

The correct option is D the focal point shifts to infinity

Parallel rays on convex lens $(u=\infty )$

So, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\Rightarrow \frac{1}{v_1}=\frac{1}{f}+\frac{1}{u}$

$\Rightarrow \frac{1}{v_1}=\frac{1}{f}$

$\Rightarrow v_1=f$

Now for concave lens $v_1$ becomes $u$

So, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\Rightarrow \frac{1}{v}=\frac{-1}{f}+\frac{1}{v_1}$

(because concave lens has negative focal length)

$\Rightarrow \frac{1}{v}=\frac{-1}{f}+\frac{1}{f}$ (because focal length of both lenses are same )

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

Or $v=\infty$

So, image is formed at infinity or focal point of combination shifts to infinity.