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.
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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`
Is there an error in this question or solution?
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