Question

The combination of convex lens and concave lens each of focal length $10$ cm when combines, behave as:
A. Convex lens
B. Concave lens
C. As a slab of glass
D. As convex mirror

Answer 1

Hint:In order to solve this question, we will use the definition of concave lens and convex lens Concave lens is that lens which has thickness around edges while Convex lenses are those lenses having center thicker than edge. Light falls on the Convex lens and gets focused on a point on the other side of the lens. That point is known as the focal point. Concave lens are diverging lens they diverges light such that it appears to originate from focal point of lens

Complete step by step answer:
Concave lenses have negative focal length whereas convex lenses have positive focal length where focal length is distance between center and focal point in lens. We have given that,
${f_1} = 10\,cm$ (focal length of convex lens)
$\Rightarrow {f_2} = - 10\,cm$ (focal length of concave lens)
formula for finding net focal length in combination of two lenses given by
$\dfrac{1}{f} = \dfrac{1}{{{f_1}}} + \dfrac{1}{{{f_2}}}$
where $f$ is the net focal length of the combination.
So putting formulae in expression we found
$\dfrac{1}{f} = \dfrac{1}{{10}} - \dfrac{1}{{10}}$
$\Rightarrow f = \dfrac{1}{0}$
$\therefore f = \infty $
Since $f = \infty $ only for slabs of glass. So the combination behave as slab of glass having infinite focal length and it will not converge or diverge any light falling on it

Hence, the correct option is C.

Note: It should be remembered that, both concave and convex lenses have opposite direction of focal length so their value must be put with sign in formula. Also do remember that concave lens and convex lens are different types then convex and concave mirror.Convex mirror diverges light whereas convex lens converges it and concave mirror converges light whereas concave lens diverges it.