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Question
An illuminated object lies at a distance 1.0 m from a screen. A convex lens is used to form the image of the object on a screen placed at a distance of 75 cm from the lens. Find:
- the focal length of the lens, and
- the magnification.
Solution
Distance of object from screen = 1.0 m = 100 cm
v = 75 cm,
u = 100 – 75 = 25 cm = -25 cm
(i) f = ?
`1/"f" = 1/"v" -1/"u"`
= `1/75 - (1/-25)`
= `1/75 + 1/25`
`1/"f" = (1 + 3)/75 = 4/75`
f = `75/4`
f = 18.75
(ii) Magnification (m) = `v/u = 75/-25 = -3`
Magnification (m) = 3
The -ve symbol indicates that the created image is actual and inverted.
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