Then what could be done to increase the energy of each incident light particle, so that the electrons emitted by collision had more kinetic energy?

The answer was that the speed of the outgoing electrons depended on the frequency of light used.

The higher the frequency, the faster the electrons moved (right).

The ejected electrons behaved as if the energy (E) of the incoming particles of light was directly proportional to the light frequency: E or E = h, in which h is a constant.