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Question
An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed.
Calculate focal length of the lens.
Solution
Magnification (m) = 3
Object distance (u) = ?
Image distance (v) = ?
Focal length (f) = ?
Distance between image and object (v + u) = 80 cm
v = 80-u
We know that:
`m=v/u`
`or 3=(80-u)/u`
`or 3u=80-u`
`or 4u=80`
`or u=20cm`
thus, v=80-u
=80-20=60 cm
Putting these values in lens formula, we get:
`1/v-1/u=1/f`
`1/60-1/-20=1/f`
`(1+3)/60=1/f`
`f=60/4=15 cm`
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