Question

In the photoelectric effect, does brightness or frequency determine the Kinetic energy of the ejected electrons? Which determines the number of the ejected electrons?

Answer 1

Brightness also refers to intensity, or # of photons, what determines the kinetic energy of the ejected electrons is frequencey, as each photon releases 1 electron, if the photon has enough energy to overcome the work function, the remaining energy is then transformed into the electrons kinetic energy (conservation of energy). If it was more intense(brighter), more photons would hit the electrons, freeing them from the materials binding energy, also know as work function. This would lead to more electrons released, but not speed or Ek of electrons.
In short, frequencey affects kinetic energy, and brightness affects number of ejected electrons.