Question

The filament of a lamp is 80 cm from a screen and a converging lens from an image of it on a screen, magnified three times and the distance of the lens from the filament and the focal length of the law
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Answer 1

Explanation:

Here, filament of the lamp acts as an object

v = image Distance

u = object Distance

According to question

v+u = 80 ( taking magnitude only ) ....(1).

and magnification m = v/u = 3

or , v= 3u

or , v-3u = 0 ....(2)

solving (1) and (2) , we get ...

u= 20cm

For the focal length of the lens , we have :

Distance of lens from the filament , u = -20cm

v = 3×u = 3 × 20 = 60cm

Applying lens formula , we get

1 / v - 1/ u = 1 / f

1/ 60 + 1 / 20 = 1 / f

1+3 / 60 = 1 / f

1/f = 4 / 60

f = 15cm

so,the focal length = 15cm

Hope it helps you