Amplitude and intensity

QUESTION: Amplitude of the electric field of an electromagnetic wave is increased , from E to 4E.
What is change in the intensity of the wave?
(a) intensity is unchanged by the increase E
(b) intensity is increased by a factor of 16
(c) intensity is increased by a factor of 4
(d) intensity is decreased by a factor of 4
(e) intensity is decreased by a factor of 16

Answer

The intensity of a wave is proportional to its amplitude squared.

Therefore if the amplitude is increased by a factor of four (E to 4E) then the intensity will be increased by a factor of four squared – i.e. by 16 times.

The correct answer is B.