Question

The plane faces of two identical plano-convex lenses, each having focal length of 40 cm, are placed 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 -1' is
(1) 80 cm
(2) 40 cm
(3) 20 cm
(4) 160 cm
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Answer 1

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Two Plano convex lenses of focal length f on combining give convex lens of focal length f/2 = 40/2 =20 cm obtain real image the object must be place at a 2F = 2 x 20 = 40.

Answer 2

Answer:

2)20

Explanation:

The formula for focal length of two thin lenses combination is

1/F=1/f1+1/f2

1/F=1/40+1/40

   =2/40

    =1/20

Hence F=20 cm