Photoelectric effect is any effect that results in the transfer of energy from light to an electric current. Photoelectric effects are used to measure the intensity of light or to provide a supply of electricity. The three main types of photoelectric effect are the (1) photoemissive, (2) photoconductive, and (3) photovoltaic effects.
The photoemissive effect
occurs when a material emits (sends out) electrons as a result of light falling on it. This effect was used in some early photoelectric cells. These photoemissive cells consist of a glass tube containing a negatively charged electrode (electric terminal) called the cathode and a positively charged electrode called the anode (see Electrode). A light-sensitive coating on the cathode causes it to emit electrons when light falls on it. The electrons, which are negatively charged, are attracted to the positively charged anode.
The electron flow is an electric current proportional to the light intensity at the cathode. A light-intensity meter can be constructed by connecting the cell to a meter that measures current.
The photoconductive effect
occurs when the electric current flowing through a substance increases as a result of light falling on it. The flow of electrons forming the current will vary according to the intensity of the light. Photoelectric cells using this effect usually contain a piece of cadmium sulfide or selenium. A photoconductive cell connected to a battery and current meter is used in many cameras to measure light intensity.
The photovoltaic effect
occurs when light falling on the junction (boundary) between two substances causes a transfer of electrons from one side of the junction to the other. One material thus acquires an excess of electrons and becomes negatively charged. The other material, having a deficiency of electrons, becomes positively charged. As in a battery, this imbalance produces an electromotive force, which can be used to make a current flow around a circuit.
Photovoltaic cells are used in some photographic light meters and have the advantage of needing no supply of electricity. A simple light meter can be made by connecting a voltmeter to a photovoltaic cell to measure the strength of the electromotive force produced. Photovoltaic cells are used in calculators and in spacecraft as solar cells for producing energy.
See also Einstein, Albert (The photoelectric effect); Electric eye; Light (Photoelectric effect); Light meter; Photomultiplier tube.
