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Question
An object is placed at a distance of 20 cm in front of a concave lens of focal length 20 cm.
- Find the position of the image, and
- the magnification of the image.
Solution
- Object distance, u = -20 cm
Focal length, f = -20 cm (concave lens)
lens formula is,
`1/"v" - 1/"u" = 1/"f"`
`therefore 1/"v" = 1/"f" + 1/"u"`
`therefore 1/"v" = 1/-20 + 1/-20 = -2/20`
∴ v = -10 cm
Hence, the image is 10 cm in front of the lens on the same side as the object. - Magnification m = `v/u`
m = `(-10)/-20`
= `1/2`
= 0.5 cm
As m = +ve indicates, the virtual erect image is half the size of the object.
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