The two factors affecting maximum kinetic energy of photoelectron are the frequency of the incident radiation and the material on the surface As shown in the graph below, electron energy increases with frequency in a simple linear manner above the threshold. All three curves have the same slope (equal to Planck's Constant) which shows that the energy-frequency relation is constant for all materials. Below the threshold frequency photoemission does not occur. Each curve has a different intercept on the energy axis, which shows that threshold frequency is a function of material. Most elements have threshold frequencies in the ultraviolet, but a few dip down low enough to be visible with potassium's lying somewhere between yellow and green. The materials with the lowest values are all semiconductors with some even reaching down into the near infrared.