Answer
Frequency determines the KE of ejected electrons. Brightness determines how many electrons are ejected.
Work Step by Step
The ejection of an electron depends on the interaction of one photon with an electron.
For photons, E = hf, where E is the energy, h is Planck's constant, and f is the photon's vibrational frequency. High-frequency photons are more energetic and give an ejected electron more KE.
The number of ejected electrons depends on how many photons are hitting the surface, so that depends on the brightness of the light.
This is discussed on pages 585-587.
