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Question
A lens of focal length 20 cm is used to produce a ten times magnified image of a film slide on a screen. How far must the slide be placed from the lens?
Solution
Given:
m = -10 (negative sign implies real image)
f = 20 cm
Object distance = -u
Lens formula is given by:
`1/f=1/v-1/u`
Magnification in case of convex lens is given by:
`m=v/-u`
`v=mu`
`v=(-10)xx(-u)=10u`
Putting these values in lens formula, we get:
`1/f=1/v-1/u`
`1/20=1/(10u)+1/u`
`1/20=1+10/(10u)`
`1/20=11/(10u)`
`10u=220`
`u=220/10=22` cm
Thus, object distance = -u = -22 cm
So, the slide is placed at a distance 22 cm from the poles of the lens.
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