Question

The image obtained with a convex lens is erect and its length is 4 times the length of the object. If the focal length of the lens is 20 cm. (i) calculate the object and image distances. (ii) Find the size of image .I had it on my test for class XII.

Answer 1

m = v/u = +4 = h'/h

   v = 4 u

1/f = 1/v - 1/u
1/20 = 1/4u  - 1/u = -3/4u
4u/3  = -20 cm        =>  u = -15 cm
 so v = - 60 cm        so this is a virtual image.

image size = 4 times object size..

Answer 2

M=+4

f=20cm

1/v-1/u=1/f

1/4u-1/u=1/f

U-4u/4u^2=1/f

-3u/4u^2=1/f

-3uf=4u^2

f=4u^2/-3u

20=4u^2/-3u

-60=4u

-60/4=u

u=-15

v=4u

v=4(-15)

v=-60cm