Photo electric effect first discovered by Heinrich Hertz (1888).It was observed that when a metal pleat,such as zinc is exposed to ultra violate radiation became positively charged which says that some charge is missing from its surface which are nothing but electrons(P.Lenard 1899).This electron are known as photo electrons.
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| Experimental arrangement for photoelectric emission |
Ultra violate radiation(U.V) from a source enters the vacuum chamber through a quartz side window G and falls on the cathode plate c.Another plate p is kept at a positive potential with respect to c. p serves as a collector for electron that are emitted.The plate p is connected through a low current measuring device A and a commutator to a source of a variable potential.The potential difference between p and c can be measured by electrostatic voltmeter V.With the help of commutator, it is possible to reverse the polarity so that p can also be made negative with respect to c.
When light falls on the cathode c electrons comes out which is caught by anode p.And i vs v graph looks something like this
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| Variation of photoelectric current with V for different intensities of light. |
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The variation of photoelectric current for different wavelengths can be describe by the following graph.
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| variation of photoelectric current for different wavelength. |
We can equate the maximum kinetic energy of the electron to the energy required to overcome the effect of retarding potential.
m=mass of the electron; Vm=maximum velocity of an electron: e=charge of an electron V0=stopping potential.
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