Effects of frequency on rate of emission of photoelectrons

Light of constant intensity is incident on a metal surface, causing electrons to be emitted. State and explain why the rate of emission of electrons changes as the frequency of the incident light is increased.

My answer: As frequency increases, energy of photons also increases, so more energy is available per electron, so more electrons can escape and rate of emission increases

Book’s answer: at higher frequency, fewer photons (per second) for same intensity, so rate of emission decreases. (Allow argument based on photoelectric efficiency)

I totally don’t understand what the book is saying. Why fewer photons at higher frequency? And what is photoelectric efficiency?

Someone please explain to me, thanks.


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