Question

The distance between a convex lens and a plane mirror is $10cm$. The parallel rays incident on the convex lens, after reflection from the mirror, form image at the optical centre of the lens. Draw ray-diagram and find out the focal length of the lens.

Answer 1

• The parallel rays incident on convex lens coveergae at focus F of lens .
  
• At mirror,after reflection forms an image at optical center of lens C.
  
• Thus image of virtual point object F is formes at c.
  

$f=CF$

$=CM+MF=10+10=20cm$

The focal length is 20 cm.