Question

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

Answer 1

Here Filament of a lamp acts as an object.
v =image distance from lens
and u = object distance
According to the question,
v + u = 80 ( taking only magnitude )....(i)
and magnification m = v/u = 3
or v = 3u
or v-3u=0---(ii)
Solving (i) and (ii),we get
u = 20 cm.
For the focal length of the lens we have
u = -20 cm , v =3xu=3 x 20= 60 cm
Using lens formula 1/v - 1/u = 1/f
1/60+1/20=1/f
So f=15 cm

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