Question

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.

Answer 1

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`