Question

Two lenses of focal lengths 20 cm and – 40 cm are held in contact. The image of an object at infinity will be formed by the combination at
(a) 10 cm
(b) 20 cm
(c) 40 cm
(d) infinity

Answer 1

Answer:

The image is formed at the focus, f = 10 cm

Explanation:

f1 = f2 = 10 cm

Focal length of lenses in combination : 1 /f = 1/f1 + 1 /f2

1 /f = 1 /20 + 1 /20

     = 2 /20

f = 10 cm

u = infinity

Lens formulae : 1/f = 1 /v - 1 /u

1 /v = 1 /f + 1/u          ( 1/∞ = 0 )

      = 1 /10 + 0

      = 1 /10

v = 10 cm