Question

A point object is placed at 20 cm from a thin plano-convex lens of focal length 15 cm whose plane surface is silvered. Locate the position and nature of the final image.

Answer 1

Given: u = - 20 cm

P = P_L + P_M + P_L

`= 1/"f"_"L" + (- 1)/(("f"_"m")_(->∞)) + 1/"f"_"L"`

P = `2/"f"_"L"`

`(- 1)/"f" = 2/"f"_"L"`

f = `- 15/2`

Mirror formula,

`1/"f" = 1/"v" + 1/"u"`

`- 2/15 = 1/"v" - 1/20`

`1/"v" = (-2)/15  + 1/20`

`= (- 8 + 3)/60`

`= (-5)/60`

`1/"v" = 1/(-12)`

v = - 12 cm