Question

(b) A student claims he can pass a beam parallel to the principal axis through a combination of two
lenses each of equal focal length and the beam would emerge parallel to the principal axis. What
would be the separation between the lenses? Draw a ray diagram depicting the situation. ​

Answer 1

The student would use a convex lens for the purpose, because it's converging lens. A concave lens being diverging lens, won't produce a parallel ray by combination.

Let the focal length of the two lenses be 'f'

Then, the separation between the lenses would be '2f'

Answer : 2f

Explanation

(Refer attachment for figure)

First of all, the beam parallel to principal axis would pass through the first lens ($\sf L_1$) and then would pass through its focus ($\sf F_1$). (We know that ray parallel to principal axis passes through its focus after passing through the convex lens)

Now, the rays passing through focus of first lens would hit the second convex lens ($\sf L_2$having same focal length. If we put the lens $\sf L_2$ in such a way that it seems like the rays are coming from it's focus ($\sf F_2$, the beam would emerge parallel after hitting the second lens. (We know that rays coming from focus emerge parallel to the principal axis if they hit a comvex lens)

Hence, according to ray diagram, we get that the lens should be placed at a separation of sum of their focal lengths, which here, is 2f.