Refraction by Spherical Surfaces: Practice Problem & Solution

Solution Explained:

To solve this problem, we apply the core principles of Refraction by Spherical Surfaces. Understanding the underlying formula is key to arriving at the correct answer below:

For the emergent beam to be parallel, the image formed by the convex lens must lie exactly at the focal point of the concave lens. The convex lens focuses the parallel beam at $+20 cm$. The concave lens requires a virtual object at its focus ($+5 cm$ behind it) to collimate the rays. Thus, $d + 5 = 20$, which gives $d = 15 cm$.