1.

The focal length of a convex lens is 18 cm and the size of the image is a quarter of the object. The object is situated at a distance of

Answer»

90 cm
54 cm
22.5 cm
60 cm

Solution :`because` Focal length of CONVEX lens f = + 18 cm and `m = (h.)/(h) = (v)/(u) = (1)/(4)`, hence `v = (u)/(4)`. Moreover, diminished image means that the image is a real image.
Using lens formula `(1)/(v) - (1)/(u) = (1)/(f)`, we have
`(1)/(+(u//4)) - (1)/((-u)) = (1)/((+18)) rArr (4)/(u) + (1)/(u) = (5)/(u) = (1)/(18) rArr u = 90` cm


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