A convex lens of power +4D is placed at a distance of 40cm from a screen.At what distance from the lens should a candle placed, so that the image is formed on the screen?
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Given power of. convex ok lens={1/f}=4
hence focal length (f)=0.25 meter
f=0.25 * 100 = 25cm
now image distance (v)=40cm
(screen is at 40 cm)
hence using lens formula .(1/f)=(1/v)-(1/u)
where u =object distance
hence putting all value
(1/25)= (1/40) - (1/u)
hence( 1/u) = (1/40) - (1/25)
(1/u). =(5-8) /200
u(object distance) = (-200/3) cm
where -ve sign indicates the object is left side of the lens..