Question

An object is placed at a distance of 20 cm in front of a concave lens of focal length 20 cm.

1. Find the position of the image, and
2. the magnification of the image.

Answer 1

1. Object distance, u = −20 cm
   Focal length, f = −20 cm (concave lens)
   lens formula is,
   `1/v - 1/u = 1/f`
   ∴ `1/v = 1/f + 1/u`
   ∴ `1/v = 1/-20 + 1/-20 = -2/20`
   ∴ v = −10 cm
   Hence, the image is 10 cm in front of the lens on the same side as the object.
2. Magnification m = `v/u`
   m = `(-10)/-20`
   = `1/2`
   = 0.5 cm
   As m = +ve indicates, the virtual erect image is half the size of the object.