Question

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

(a) the position of image, and

(b) the magnification of image.

[ 2 Marks]

Answer 1

Object distance ,u=-20 cm
Focal length ,f=-20 cm (concave lens)

(a)
Lens formula
1/f=1/v-1/u
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
[ 1 Mark]
(b)
For a lens, magnification is
m=v/u
m=-10/-20
m=+0.5
[ 1 Mark]