Question

An object 20 centimetre tall is placed on the principal axis of a convex lens is 30cm image is formed at a distance of 10 cm from the lens calculate focal length of the lens

Answer 1

height of object , h=20cm

height of image ,h'=30 cm

image distance ,v=10cm

magnification m=v/u=h'/h

10/u=30/20

u=10*2/3

u=6.66666666667

then lens formula =1/f=1/v-1/u

1/f=1/10-1/6.66

f=-0.050cm

Answer 2

m= -v/u = h1/h

-10/u = 30/20

-10 x 2/3 = u

-20/3 = u

using lens formula;

1/f = 1/v - 1/u

   = 1/10 + 3/20

   = 2 + 3/ 20

   = 5/20

1/f = 1/4

therefore, f = +4