These are the experimental observations we can make of the photoelectric effect when illuminating a negatively charged metal plate with monochromatic light of various frequencies and intensities. For each, how they conflict with the wave model has been outlined.

Threshold Frequency

Photoemission only occurs when the frequency of the light illuminating the plate is above a certain threshold frequency, no matter the intensity. This directly conflicts with the wave model which suggests that the photoelectric effect should occur for any frequency.

The threshold frequency depends on what metal the plate is made from. This suggests that it may be somehow related to the work function.

Lack of Measurable Delay

There is no measurable time delay between the activation of the lamp and the first photoemission from the plate, provided that the light exceeds the threshold frequency. This does not depend on the intensity of the light. However, according to the wave model of light, the delay should increase as we lower the intensity.

Speed and Number of Photoelectrons

Whilst above the threshold frequency, increasing the frequency of the incident light leads to faster photoelectrons after emission. If we instead increase the intensity, it leads to an increased number of photoelectrons with the same speed being emitted per unit time. In the wave model, we expect the intensity to also affect the speed of the photoelectrons, and for the frequency to have no effect whatsoever.

When we increase the intensity, we are supplying more energy to the plate per second. The result of more photoelectrons with the same kinetic energy suggests that the distribution of energy may be more complicated than a continuous stream of energy.