Photo Electric Effect & Laws of Photo Electric Effect

 ♦ Learn about : Photo Electric Effect : LENARD’S EXPLANATION , EXPERIMENTAL ARRANGEMENT ♦

  • Hertz observed that when ultra violet rays are allowed to fall on negative plate of an electric discharge tube, then conduction takes place more easily. This shows that electrons are ejected from a metal surface when illuminated by light of suitable wavelength .
  • After some time, Hall wach confirmed above observation through his following convincing experiment : His apparatus consists of two zinc plates enclosed in an evacuated quartz tube.

photo-electric-effect

The plates are connected to a battery through a galvanometer (fig.). He noted following observations :

  • When U.V. rays are allowed to fall on the cathode, deflection is produced in the galvanometer i.e., a current flows in the circuit.
  • As soon as rays are stopped, the deflection in the galvanometer becomes zero or current stops.
  • If rays are made to fall on anode then either no current or a very small current flows in the circuit.

LENARD’S EXPLANATION :

(a) He told that when ultraviolet rays fall on cathode electrons are ejected from it which are attracted toward anode or positively charged plate. Hence, the circuit which was incomplete till now, due to air gap between two plates in the tube, gets completed due to flow of electrons and a current starts flowing in the circuit.

  • However, when rays fall on anode, the electrons are again emitted from the plate in the same way as earlier but because of being negatively charged do not reach the cathode i.e., circuit again remains incomplete and the current does not flow.

This phenomenon of emission of electrons from a metallic surface when illuminated by light of appropriate wavelength frequency is known as PHOTO ELECTRIC EFFECT. The electrons emitted in this process are called as photo-electrons and the current produced in the circuit is called as photo-electric current.

Note :-

  • Photo-electric effect is a general phenomenon exhibited by all substances when illuminated by radiations of suitable wavelength. This suitable wavelength is different for substances.
  • Certain alkali-metals e.g. Sodium, Potassium, Calcium etc., show the photo electric effect when visible light (l = 4000 Aº − 7500 Aº) falls on them.
EXPERIMENTAL ARRANGEMENT :

Study of the dependence of maximum kinetic energy and photo electric current.

LENARD’S EXPLANATION

♦ Learn about : Laws of Photo Electric Effect : Intensity effect , Frequency effect , Effect of nature of metals , Time-delay effect ♦

(a)Intensity effect : For a given metal, rate of emission of photo-electrons i.e., photo current is directly proportional to the intensity of incident radiation for a given light i.e., bright light always gives more photo-current than a dim one for a given frequency.

(b)Frequency effect : For a given metal, maximum kinetic energy of photo electrons varies linearly with the frequency of incident radiation and is independent of its intensity i.e., blue light will always give more energetic photoelectrons than red, whatever be its intensity.

(c) Effect of nature of metals : If light of different frequencies in turn is incident on a given metal, photo electric effect takes place only if the frequency of incident radiation is more ( or wavelength is less ) than a specific value V0 . This specific value of frequency (V0) is called threshold-frequency or cut-off frequency and depends only on the nature of metal.

So if from a certain metal, green light can emit photo electrons while yellow can not, blue light will emit photo electrons (as VB>VG) while red will not. For PEE ν ≥ ν­0   , λ ≤ λ­0

(d) Time-delay effect : Within the limits of experimental accuracy (about 10–9 s.), there is no time lag between incidence of radiation and emission of photo electrons i.e., as light is incident on the metal photo electrons are emitted.

(e) The rate at which the electrons are emitted from a photocathode is independent of its temperature. This shows that it is different from thermionic emission.

Note : –

  • In accordance with conservation of energy photo electric effect represents conversion of light energy reverse of what happens in an electric bulb.
  • Photo electric effect takes place only if the electron is loosely bound to the metal, i.e., photo-electric effect can never take place with completely free electron.
  • In photo-electric effect all the emitted photo electrons do not have same kinetic energy. The emitted photo electrons have kinetic energy in the range from 0 to a maximum value. Kmax depends both on frequency of incident radiation and nature of metal.
  • With emission of photo electrons the metal will become positively charged and if isolated, photoelectric effect will cease after some time when potential acquired by the metal V is such that eV = Kmax. in this situation any electron emitted by the metal will turn back to it by attractive force and will not escape.
  • The opposite of photo-electric effect i.e. emission of electromagnetic radiation. From a metal when electrons strikes it results in the production of X-rays and is called ‘inverse photo electric effect.
  • Apart from photo electric emission, electrons can also be emitted from a metal by heating (Thermionic – emission), by applying a strong electric field (field emission) or by bombarding it with electrons (secondary emission).
  • Irradiation of a substance by light under specific conditions instead of producing photoelectric effect may increase its electrical conductivity resulting in ‘photo conductive effect’ or may produce an EMF across it resulting in photo Voltaic effect.

     

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