Light that is shone onto a metal surface may result in the emission of electrons from the surface. Three statements regarding the emission of the electrons are the

I. number of electrons emitted per unit time depends on the intensity of the incident light
II. energy of the electrons depends on the frequency of the incident light
III. emission of the electrons takes place instantaneously.

Which of the above statements can only be explained by assuming light consists of photons?

A. II only

B. III only

C. II and III only

D. I, II and III

C

This question did involve careful reading. The statistics would suggest that many candidates rushed the question and guessed the answer. The question was not asking which of the statements was true – but rather which of them showed that light consists of photons. Clearly I will be true if light is either a wave or photonic, so D, the most popular answer, must be incorrect.