Question

An object 4cm high is placed at a distance 40cm from a converging lens of focal length 20cm.calculate the size of image formed

Answer 1

Answer:h= 4 cm

u = -10 cm

f = 20 cm.

According to lens formula,,

1/v - 1/u = 1/f

1/v = 1/f + 1/u

1/v = 1/20 - 1/10

1/v = -1/20

v = -20

We have, m = v/u

                   = -20/-10 = 2

Also, m= h'/h  ( where h = height of object and h'= height of image)

2=h'/4

h' = 4X2 = 8cm.

Therefore, the image is real and inverted

Answer 2

Explanation:

U = -40 cm

f= - 20 cm

V = ?

1/f =1/v - 1/u putting the values we get

v= - 13.33 cm

m = v/u

m= hi/ho

putting values v/u = hi/ho

-13.3/-40 = hi/4

hi = 1.3 cm