First slide

The lens

Question

A plano-convex lens of refractive index 1.5 and radius of curvature 30 cm is silvered at the curved surface. Now this lens has been used to form the image of an object. At what distance from this lens an object be placed in order to have a real image of the size of the object

Moderate

A

20 cm
B

30 cm
C

60 cm
D

80 cm

Solution

Focal length of the system (concave mirror)
$F = \frac{R}{2 μ} = \frac{30}{2 \times 1 .5} = 10 cm$
In order to have a real image of the same size of the object, object must be placed at centre of curvature u = (2f)=20cm.

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