The plane faces of two identical planoconvex lenses each having focal length of `40 cm` are pressed against each other to form a usual convex lens. The distance from this lens, at which an object must be placed to obtain a real, inverted image with magnification one is

Options

A. 80 cm
B. 40 cm
C. 20 cm
D. 162 cm

Answer

B

Explanation

`(1)/(f_(eq)) = (1)/(f_(1)) + (1)/(f_(2)) `
`rArr (1)/(f_(eq)) = (1)/(40) + (1)/(40) rArr " " f_(eq) = 20 ` cm
At centre of curvature m = 1
`therefore " " u = R = 2f_(eq) = 40 ` cm.