Hertz and Lenard’s Observations of the Photoelectric Effect

The photoelectric effect is the phenomenon in which the particles, which are electrically charged, are released from a material’s surface when they absorb electromagnetic radiation. 

The photoelectric effect is generally described using the example of a metal plate on which light falls. However, the energy source can be of different types such as infrared, visible, UV light, X-rays, etc. 

Moreover, the material needn’t specifically be solid. It can be a liquid or a gas as well. The photoelectric effect plays a significant role in the formulation of modern physics due to the convoluted questions it raises related to the nature of light. 

This article will explain Hertz’s observation of the photoelectric effect. Moreover, it will also explain Lenard’s observation of the photoelectric effect. Furthermore, it will also explain the applications of the photoelectric effect.

Hertz’s observation of the photoelectric effect

Heinrich Rudolf Hertz, a German physicist, discovered the photoelectric effect in 1887. According to Hertz’s observation of the photoelectric effect, when a UV light falls on two metal electrodes with a voltage applied across them, the light develops a change in voltage at the point where the sparking occurs. 

Later, this hypothesis put forward by Hertz was cleared by another German physicist Philipp Eduard Anton von Lenard. Following his observation, Lenard said that when the light fell on the metal surface, it liberated the charged particles from the surface. These particles are similar to the electrons discovered by JJ Thompson in 1897.

Later, further research was carried out on Hertz’s observation of the photoelectric effect. It was found that classical physics cannot answer the interactions between light and matter. Moreover, further research described light as an electromagnetic wave. 

Moreover, another observation was made that when light struck the surface of the metal, no time lag was present between the arrival of light and the removal of electrons from the surface.

This observation helped Albert Einstein in 1905 to create a new theory of light. He said that the particle of light called the photon consists of a fixed amount of energy that depends on the light’s frequency. 

Specifically, according to Einstein, the light contains the energy (E) equal to the product of h and f, where h stands for the universal constant. In contrast, f is the frequency of the light.

Max Planck made another discovery based on the photoelectric effect and found the electromagnetic radiation produced by a hot body. According to Planck, E= hc/λ, where c is the speed of light and λ is the wavelength. Thus, Hertz’s observation of the photoelectric effect was the beginning of the later discoveries.

Lenard’s observation of the photoelectric effect

In 1902, Lenard built upon Hertz’s observation of the photoelectric effect. According to Lenard’s observation of the photoelectric effect, the amount of energy released by the electrons increases with the frequency of the radiation employed. 

However, the hypothesis put forward by Lenard wasn’t appropriately explained because according to Maxwell’s electromagnetic theory (that Hertz proved was right), the kinetic energy is only dependent upon the intensity of light and not the frequency. This theory was later resolved by Einstein when he explained the photoelectric effect of light.

Applications of the photoelectric effect

The applications of the photoelectric effect have several properties, including the development of current that is directly proportional to the intensity of light. One example of the photoelectric effect is the photoelectric cell. 

Initially, the photoelectric cell was a phototube, a vacuum tube containing a cathode made of metal, containing a small workpiece that emitted electrons. The electrons emitted by the cathode will be collected by an anode with a large positive voltage. 

However, the phototubes are now replaced by semiconductor-based photodiodes, which detect light, measure the intensity, control other devices for illumination and convert light into electricity. These devices work at lower voltages and are highly used in industrial process control, monitoring of pollution, solar cells, imaging, and fibre optics.

Photoconductive cells are manufactured with semiconductors containing band gaps which can be used for sensing the photon energies. Some examples of where photoconductive cells are used are photographic exposure metres, automatic switches for street lights, IR detectors typically used in night vision applications, etc.

The photoelectric effect also has its application in photovoltaic devices. The photovoltaic devices consist of a p-n junction semiconductor. These photovoltaic devices are used as solar cells made of crystalline silicon. They convert 15% of the incident light energy into electricity. Solar cells provide small power in special environments like space satellites and remote telephones. 

Conclusion

Hertz and Lenard’s observations of the photoelectric effect have witnessed several applications for humanity in the later centuries. Without Hertz’s observation of the photoelectric effect, humanity wouldn’t have thought about generating electrical power using sunlight as the source. 

Thus, solar cells, solar panels, solar water heaters, etc, wouldn’t have come into existence. Hence, to understand the working of these devices, a person must study Hertz’s observation of the photoelectric effect as well as Lenard’s observation, and the later developments that led to more discoveries.