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 (IC) flows from P to C, and this current can be detected or measured by G.
- IC 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 (IP) increases, IC also increases, i.e. IC ∝ IP.
- When the frequency of incident light is greater than threshold frequency.
- On increasing the positive potential of C, IC 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, IC 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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