The photoelectric effect is the ejection of
electrons from the surface of a metal plate when light falls on it.
Light is composed of particles called photons, the energy of which is given by
Planck’s relation
E = hf,
where f is the frequency of the light, and h is Planck's constant.
An atom in the metal can absorb either a whole photon or nothing. If the energy of the
photon is sufficient, the electron will be
dislodged from the atom of metal. If this photon does not carry the minimum amount of energy required to dislodge the electron from the atom, the photon will not be absorbed and the electron will stay in orbit, no matter how intense the light.
| The photoelectric effect occurs only with a radiation above a specific frequency (energy). For a given metal there is a threshold frequency below which no electrons are extracted. |
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The main application of the photoelectric effect is the process where light is transformed (via photovoltaic cell) to electric current. The given figure is a diagram of an apparatus in which the photoelectric effect can occur. An evacuated glass or quartz tube contains a metallic plate E connected to the negative terminal of the battery and another metallic plate C that is connected to the positive terminal of the battery. When the tube is kept in the dark, the ammeter reads zero, indicating no current in the circuit. However, when the plate E is illuminated by light having a wavelength shorter than some particular wavelength that depends on the metal used to make plate E, a current is detected by the ammeter, indicating a flow of charges across the gab between plates E and C. This current arises from photoelectrons emitted from the negative plate (the emitter) and collected at the positive plate (the collector).