Question

How will you decide whether a given piece of glass is a concave lens, convex lens, or a plane glass plate?

Answer 1

Hold the given piece of glass over some printed number:
(i) If the letter appears magnified, the given piece is a convex lens.
(ii) If the letters appear diminished, the given piece is a concave lens.
(iii) If the letters appear to be of the same size, then it is a plane glass mirror.