Describe the photoelectric effect.

If we give a peice of gold negative charge and shine long wave, low frequency, electro-magnetic (E-M) radiation on it (for example radio waves) absolutely nothing will happen.

If you start to shorten the wavelength and therefore increase the frequency of the E-M radiation a point will come where the gold will discharge.  This is called the threshold frequency.

This occurs because the raditation is causing negatively charged electrons to leave the gold thereby discharging it.

Let me explain, E-M radiation can manifest itself in the form of particles of light called photons, each photon is a discrete packet of energy. The energy of the photon is directly proportional to the frequency of the radiation according to the equation:

E = hf

where E is energy of the photon, f is frequency and h is a constant.

When the E-M radiation is shone onto the gold, the photons collide with the electrons in the metal and transfer their energy to the electrons. When the frequency of the wave is above the threshold frequency, the photons contain enough energy that when they collide with the electrons, the electron has enough energy to escape the attraction of the gold's nucleus leaving the gold without its negative charge. The energy barrier the electron must overcome to leave the gold is called the potential well.