A convex lens of focal length 20 cm and a concave lens of focal length 10 cm are placed 10 cm apart with their principal axes coinciding. A beam of light travelling parallel to the principal axis and having a beam diameter 5.0 mm, is incident on the combination. Show that the emergent beam is parallel to the incident one. Find the beam diameter of the emergent beam.

Explanation

Image by convex lens is formed at focus of concave lens Hence emergent beam is parallel to incident beam. Similar triangles `ABE` and `CDE`
`(AB)/(20)=(CD)/(10)implies(5)/(20)=(d)/(10)`
`d=2.5mm`
`(AB)/(10)=(CD)/(20)implies(5)/(10)=(d)/(20)`
`d=10mm`