Hertz and Lenard՚s Observations of the Photoelectric Effect (For CBSE, ICSE, IAS, NET, NRA 2022)

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Hertz and Lenard՚s Observations of the Photoelectric Effect

  • In 1887 when Heinrich Hertz was conducting experiments to prove Maxwell՚s electromagnetic theory of light, at that time he noticed a strange phenomenon.
  • He used a spark gap (two sharp electrodes placed at a small distance so that electric sparks can be generated) for detecting the presence of electromagnetic waves.
  • Then he placed it in a dark box and found that the spark length was reduced.
  • When hertz used a glass box, the spark length increased and when he replaced it by quartz box, the spark length increased further.
  • This was the first observation of the photo-electric effect.
  • After a year, Willhelm Hallwachs confirmed these results and showed that UV light on a Zinc plate connected to a battery generated a current (because of electron emission) .
  • J. J. Thompson found that the amount of current varied with the intensity and frequency of the radiation used, In 1898.
  • Lenard observed in 1902, that the kinetic energy of electrons emitted increased with the frequency of radiation used.
  • This could not be explained as Maxwell՚s electromagnetic theory predicted that the kinetic energy should be only dependent on light intensity
  • The resolution would only come a few years later by Einstein when he would provide an explanation to the photoelectric effect.

Experimental Set Up

Experimental Set Up
  • J. J. Thompson՚s set up to study this effect is of great importance.
  • It consists of two zinc plate electrodes placed on the opposite ends of an evacuated glass tube.
  • A small quartz window illuminates one of the electrodes that is made the cathode.
  • It is used because ordinary glass blocks Ultra-Violet light. A variable voltage is exerted across the two electrodes using a battery and potentiometer.
  • The current in the circuit can be recorded using an ammeter as the potential and light intensity is changed.

Observations

Effect on Photoelectric Current

Effect on Photoelectric Current
  • The photoelectric current (same as the rate of emission of electrons) is directly proportional to the intensity of light falling on the electrode.
  • With increasing intensity, the current is increasing.

Effect on Voltage

Effect on Voltage
  • The Maximum kinetic energy increases with increase in the frequency of light. With a higher frequency of light (ν) , the stopping potential becomes more negative which implies that the kinetic energy of electrons also increases.
  • Also, observe that as the voltage has decreased the current also decreases. But to obtain zero current, the voltage has to be reversed to a certain known as the stopping potential.
  • The voltage must be reversed to such an extent that the electrons cannot reach the anode. This is the maximum kinetic energy an emitted electron can achieve,

Maximum Kinetic energy,

e = charge of the electron

Effect on Light Frequency

Effect on Light Frequency
  • All frequencies of light, however, cannot cause a photoelectric current to develop.
  • Only light above a certain frequency can produce a photoelectric current.
  • This frequency is known as the threshold frequency.
  • This varies with the electrode material.
  • Also, the maximum kinetic energy of the electrons increases linearly with increasing light frequency.
  • If we extend the graph below the x-axis, the intercept on the Kinetic energy axis represents the minimum energy required for emission of the electron; this is known as the work function of the material.

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