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Question : A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that is forms an image at 15 cm from the lens ? Also calculate the size of the image formed . 4

Answer»

SOLUTION :f= - 20 cm , `h_(0)` = 5 cm , v = - 15 cm
Using. `(1)/(f) = (1)/(v) - (1)/(U)` , we GET
`(1)/(u) = (1)/(v) - (1)/(f) = (1)/(-15) - (1)/((-20))`
`(1)/(u) = (-20 + 15)/(300) = - (5)/(300)`
u = - 600 cm .
Since,m = `(h_(1))/(h_(0)) = (v)/(u)` , we get
h `= (v)/(u).h_(0) = ((-15))/((-60)) xx 5 `
`= (5)(4) = 1.25` cm


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