Photo electric effect | Quantum mechanics | Physical basis of quantum mechanics

Photo electric effect

Photo electric effect

• When light or E.M. radiation of suitable frequency or wavelength falls on a metal surface, it emits electrons.
• The process of emission of electrons from a metal surface, when illuminated by light of suitable wavelength or frequency is photoelectric effect.
• The electrons so emitted are photoelectrons.

Experimental Arrangement 

• T = An evacuated glass tube with a quartz window W
• P = Photoelectrical sensitive plate
• C = Hollow cylinder which have a small hole to permit the incident light
• G = A sensitive galvanometer or ammeter
• P is connected with negative end of a battery with G
• C is connected with positive end of a battery

Working

• When light of suitable frequency falls on P from a source, the photoelectrons are emitted.
• Since collecting cathode C is connected to positive terminal of battery, so these electrons are attracted towards C.
• Thus photo electric current (I_C) flows from P to C, and this current can be detected or measured by G.
• I_C increases as the potential of C is more and more positive with respect to P.

Laws of photoelectric emission

• For every metal there is a particular frequency below which there is no photoelectric emission.
• The minimum frequency required for photoelectric emission is threshold frequency.
• As the intensity of incident light (I_P) increases, I_C also increases, i.e. I_C ∝ I_P.
• When the frequency of incident light is greater than threshold frequency.

• On increasing the positive potential of C, I_C increases and it become maximum at a particular value of potential.
• A particular positive potential of collecting cathode C at which the photoelectric current is maximum, is known as saturated current.
• On increasing the negative potential of C, I_C decreases and it become zero at a particular value of potential.
• The energy of the emitted photoelectrons is independent of intensity of incident light, and depends on the frequency of incident light and nature of the metal.
• Photoelectric emission is an instantaneous process.

Failure of the electromagnetic theory

• The calculation shows that the electron take 500 days to eject from sodium by using visible light of wavelength 4000Å, whereas experimentally there is no time delay.
• According to classical theory, light of greater intensity should impart large kinetic energy to the ejected electrons. But this does not happen.
• Classical theory shows that the velocity (energy) of the ejected electron should not depend on the frequency of the incident light, but in actual it depends.

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