Hertz and Lenard's observation of photoelectric effect

First Observation of photoelectric effect

   In 1887 when Heinrich Hertz was conducting experiments to prove Maxwell’s electromagnetic theory of light, that he noticed a strange phenomenon. Hertz used a spark gap (two sharp electrodes placed at a small distance so that electric sparks can be generated) to detect the presence of electromagnetic waves. To get a closer look, he placed it in a dark box and found that the spark length was reduced. When he used a glass box, the spark length increased and when he replaced it with a quartz box, the spark length increased further. Later it was explained that this occured because quartz does not block the high frequency UV rays which are highly energetic.This was the first observation of the photo-electric effect.


    A year later, 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). In 1898, J.J. Thompson found that the amount of current varied with the intensity and frequency of the radiation used.

    In 1902, Lenard observed that the kinetic energy of electrons emitted increased with the frequency of radiation used. This could not be explained as Maxwell’s electromagnetic theory (which Hertz proved correct) predicted that the kinetic energy should be only dependent on light intensity (not frequency).
The resolution would only come a few years later by Einstein when he would provide an explanation to the photoelectric effect.

Experimental Setup

J.J. Thompson’s set up (later improved by Lenard) to study photoelectric 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.and is used because it does not block

Ultra-Violet light (an ordinary glass blocks UV 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. The set up is shown below:

Observations:

1. The photoelectric current (same as the rate of emission of electrons) is directly proportional to the intensity of light falling on the electrode. It can be noted from the figure below that with increasing intensity the current is increasing, and with the voltage decreasing the current decreases. But to obtain zero current, the voltage has to be reversed to a certain V­₀ 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,  KE=eV₀eV0
            (e is the charge of the electron)

             Note that the stopping potential is independent to the intensity of light.

2. The Maximum kinetic energy increases with increase in 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.
   

3. All frequencies of light however cannot cause a photoelectric current to develop. Only light above a certain frequency (ν₀) 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.
   
4. Lastly the electron emission occurs instantly without any time lag.