Question

a filament of lamp is 80 cm from a screen and a converging lens forms an image of it on a screen magnified 3 times . find the distance of the lens from the filament and the focal length

Answer 1

Answer:

v+(-u) = 80cm

v-u=80cm ----------(1)

m=-3 (image obtained is real since it is formed on a screen)

m=v/u=-3

v=-3u

substituting it in eq(1)

-3u -u = 80cm

-4u +80

u=80/-4

u= -20 cm

The distance of the lens from the filament is -20 cm

v= -3u = -20 x -3

v = 60cm

lens formula:

1/v-1/u=1/f

1/60 - 1/-20 = 1/f

1/60 - 1/20 = 1/f

1+3/60 = 1/f = 4/60

1/15 = 1/f

f = 15cm