1.

An object is placed at a distance of 40 cm from a convex lens and its real and inverted image of same size is formed on a screen placed on other side of lens. The focal length of lens is

Answer»

`+40` cm
`+20` cm
`-20` cm
`+80` cm

SOLUTION :As u = -40 cm and v = +40 cm, hence
`(1)/(f) = (1)/(v) - (1)/(u) = (1)/((+40)) - (1)/((-40)) = (1)/(40) + (1)/(40) = (1)/(20) rArr f = +20 cm`


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