A convex mirror of focal length F₁ is placed at a distance d from a convex lens of focal length F₂. A beam of light rays coming from infinity falling on convex lens-convex mirror combination and returns black to infinity. Distance d is

(F₂ - 2F₁)

The situation can be presented in a ray diagram as shown

From the figure d + 2F₁ = f₂

$\Rightarrow$ d = f₂ - 2F₁