(a) Where should an object be kept on the principal axis of a convex lens of focal length 20 cm in order to get an image, which is double the size of the object?
(b) Name the part of the electromagnetic spectrum which is
(i) Suitable for radar systems used in aircraft navigation.
(ii) Produced by bombarding a metal target with high electrons.

Explanation

Use the formula `m=f/(u+f)` then calculate the value of u from the given conditions.