# Photo electric effect

The photoelectric effect

The photoelectric effect is the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light.A material that can exhibit The photoelectric effect is said to be photoemissive, and the ejected electrons are called photoelectrons. The process was discovered by Heinrich Hertz in 1887. Attempts to explain the effect by classical electromagnetic failed. In 1905,Albert Einstein presented an explanation based on the quantum concept of Max Planck.

Einstein described the photoelectric effect using a formula that relates the maximum kinetic energy (Kmax) of the photoelectrons to the frequency of the absorbed photons (ƒ) and the threshold frequency (ƒ0) of the photoemissive surface.

Kmax = h(f − f0)

Following Conclusions can be drawn for Photoelectric Effect:-

1) If the frequency vof the light is constant, the photoelectric current increases with increasing intensity of the light.

2) The photoelectrons are emitted within less than 10″9’sec after the surface is illuminated by the light. The emission is essentially instantaneous with illumination.

3) For a given photosensitive surface, the emission of the photoelectrons takes place only if the frequency of the light is equal to or greater than a certain minimum frequency v0, sometimes called the threshold frequency. The value of v0 is different for different materials.

4) The maximum kinetic energy, Kmax, of the photoelectrons is independent of the intensity I of the incident light.

5) The maximum kinetic energy, Kmax of the photoelectrons depends on the frequency of the incident light.

6) The relationship between Kmax and v is linear.

The photoelectric effect has many practical applications which include the photocell, photoconductive devices and solar cells. A photocell is usually a vacuum tube with two electrodes. One is a photosensitive cathode which emits electrons when exposed to light and the other is an anode which is maintained at a positive voltage with respect to the cathode. Thus when light shines on the cathode, electrons are attracted to the anode and an electron current flows in the tube from cathode to anode. The current can be used to operate a relay, which might turn a motor on to open a door or ring a bell in an alarm system.

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