Question

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

Answer 1

magnification=distance of image/distance of object
3=u/80
u=80×3=240
1/f=1/v+1/u
1/f=1/80+1/240
3+1/240=4/240=1/60=1/f
then f=60cm

Answer 2

according to question
v-u=80cm..-----i
now it's given that image formed is 3 times magnified.
so h2/h1=-3x/x
h2/h1=-3
now
v/u=-3
v=-3u
now putting v=-3u in first equation, we get
-3u-u=80
-4u=80
u=-20
so v=-3u =-3*-20=60
now by using lens formula we get
1/v-1/u=1/f
1/60+1/20=1/f
20+60/1200=1/f
f=1200/80=+15cm
so focal length will be 15cm.
the distance of lens from the filament =u
so u =20cm. (we have get above)
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