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A concave lens has focal length of 20 cm...

A concave lens has focal length of 20 cm . At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens ? Calculate the size of the image formed.

Text Solution

Verified by Experts

Given
Focal length , f=-20 cm (concave lens)
`h_(o)=5 cm`
Height of the object , `h_(o)=5 cm`
Image distance `v=-15 cm`
`1/f=1/v -1/u , 1/v -1/f`
`=1/u`
`1/u =1/(-15)+(1)/((20))`
`=(-20+15)/(300) =(-5)/(300)`
`u=-60 cm`
Magnification , m `=(h_(i))/(h_(o))=v/u`
`h_(i)=v/u *h_(o)`
Size of the image , `h_1 =(-15)/(-60) xx5`
`=5/4 =1/25 cm `
Image is diminished & virtual
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