When a gold-leaf electroscope is negatively charged the leaf and the rod repel each other. A zinc plate is placed on top of the electroscope, and u.v. light is shone on the zinc plate. The gold leaf falls. 
The incident radiation causes the electrons to be emitted from the zinc. These electrons are known as photoelectrons. Photoelectrons are emitted only when the incident radiation is above a threshold frequency. Above the threshold frequency, the emission of photoelectrons is instantaneous. Increasing the intensity of the radiation increases the number of photoelectrons emitted and does not increase the maximum kinetic energy of the photoelectrons. These observations cannot be explained using a wave model of light. Each electron on the surface of the metal interacts with one photon. There is a one-to-one interaction between an electron and a photon. Each photon transfers its energy to one electron. 
Each electron requires a certain amount of energy to escape from the metal. As the energy of a photon depends on its frequency (E = hf). If the frequency is too low no photoelectrons are emitted. As long as the incident photon has a frequency equal to or greater than threshold frequency there is no time delay. Depending on their position relative to the positive ions in the metal, electrons require different amounts of energy to free them. Increasing the intensity increases the number of photons per second. This in turns increases the number of photoelectrons emitted per second.
The incident radiation causes the electrons to be emitted from the zinc. These electrons are known as photoelectrons. Photoelectrons are emitted only when the incident radiation is above a threshold frequency. Above the threshold frequency, the emission of photoelectrons is instantaneous. Increasing the intensity of the radiation increases the number of photoelectrons emitted and does not increase the maximum kinetic energy of the photoelectrons. These observations cannot be explained using a wave model of light. Each electron on the surface of the metal interacts with one photon. There is a one-to-one interaction between an electron and a photon. Each photon transfers its energy to one electron. Each electron requires a certain amount of energy to escape from the metal. As the energy of a photon depends on its frequency (E = hf). If the frequency is too low no photoelectrons are emitted. As long as the incident photon has a frequency equal to or greater than threshold frequency there is no time delay. Depending on their position relative to the positive ions in the metal, electrons require different amounts of energy to free them. Increasing the intensity increases the number of photons per second. This in turns increases the number of photoelectrons emitted per second.