Brief Analysis of Lenard’s Experiment

Earlier it was thought that nature is quantized. Here, quantized means discrete and non-continuous.  However, this was contrary to the equations provided by Maxwell to predict the result of the blackbody radiator. Planck tried to understand it but failed to correlate the behaviour of electromagnetic waves with the discrete nature of the blackbody radiations. It was in the year 1905 that Einstein propounded and properly explained Planck’s equation.

However, two scientists, Heinrich Hertz and Philipp Lenard made important contributions to the phenomenon of the photoelectric effect. In this article, we will discuss Lenard’s experiment in-depth.

Hertz Observation on Photoelectricity

The photoelectric effect was discovered for the first time in the year 1887 by a physicist from Germany known as Heinrich Hertz. He started working with a spark-gap transmitter, which is a radio broadcasting device. Hertz found that certain substances produced a bright spark on absorbing light of certain frequencies. In 1889, J. J. Thomson identified these sparks as excited electrons that left the metal surface after gaining kinetic energy.

Hertz observed that the oscillating electric field of the incident radiation caused the electrons present in the atoms of the metals to vibrate. Hertz derived the following conclusions. 

1. By increasing the intensity of the radiation, the electrons would vibrate more vigorously, and hence, more electrons will be emitted at a higher speed.
2. Increasing the frequency of the radiation would vibrate the electrons really fast; hence, it would cause the electrons to be emitted faster. Whereas in the dim light, it would take longer for the electrons to vibrate and get emitted.

However, Phillips Lenard observed certain other results from his experiment. Let us take a look at that.

Lenard’s Experimental Results

In 1902, a student of Hertz, Philipp Lenard, began studying how the energy of emitted electrons varied with the intensity of light.

For this, he used a carbon arc light and was able to increase the intensity a thousand-fold. He made such an arrangement so the ejected electrons hit another metal plate: the collector connected to the cathode with a sensitive ammeter. The electrons thus ejected would give a measure of the current produced by the illumination. To measure the energy of the ejected electrons, the collector plate was negatively charged by Lenard to repel the electrons that would be coming toward it. Hence, only the electrons that were ejected with a good amount of kinetic energy to overcome the repulsion would contribute to the generation of current.

With this experiment, Lenard found that there was a definite minimum voltage that stopped any electron from getting through (Vstop). However, to his surprise, Lenard discovered that Vstop did not depend upon the light’s intensity. He also observed that when the light’s intensity was doubled, the number of electrons ejected was doubled. However, it did not affect the kinetic energies of the emitted electrons. A powerful oscillating field ejected a more significant number of electrons. However, the maximum individual energy of the ejected electrons was the same as that for the field that was weaker.

Taking further the experiment conducted by Lenard, Millikan found the following results.

Millikan Experimental Results 

Physicist Robert Millikan carried out Lenard’s experiment using a powerful arc lamp. He found out that using the arc lamp, he was able to produce a sufficient amount of light intensity to separate the colours and check out the photoelectric effect by using lights of various colours. 

He found out that the maximum energy for the ejected electrons actually depended on the colour of the light—the shorter the wavelength, the higher the frequency of light, which ejected electrons with greater kinetic energies.

So from all these experiments and observations, we can conclude that the observations made from Lenard’s and Millikan’s experiments and that the observations made by Hertz were opposite from one another. 

We can, thus, conclude the following from Lenard’s and Millikan’s experiments:

• The kinetic energy of the electrons is linearly proportional to the frequency of incident radiation that was above the threshold value of v0, and the kinetic energy is independent of the intensity of radiation.
• The number of electrons (the current produced) is proportional to the intensity and is independent of the frequency of the incident radiation above the threshold value of v0.

Einstein’s Theory 

It is true that Hertz discovered photoelectrons in the year 1887; however, it was not until the year 1905 that a theory was proposed that explained its effect entirely. Einstein proposed this theory, and he claimed that electromagnetic radiation exists in the form of particles known as photons that collide with electrons on the surface and emit them. This theory was quite contrary to that of waves and was not considered until Robert Millikan, in 1916, experimentally confirmed the theory.

The photoelectric effect is the process where electromagnetic radiation ejects electrons from a certain material. Einstein was the person who proposed that photons are the quanta of electromagnetic radiation, possessing energy E=hv as the frequency of the radiation. As a result, photons make up the majority of electromagnetic radiation. Furthermore, Einstein emphasised that the photoelectric effect’s features were a result of photons interacting with particular electrons. The formula KE = hv – Φ gives the maximum kinetic energy KE of ejected electrons, where hv is the photon energy and Φ  is the work function of the electron in the particular material.

Conclusion

This article explains Lenard’s experiment. Heinrich Hertz and Philipp Lenard made important contributions to the phenomenon of the photoelectric effect. Lenard found that there was a defined and proper minimum voltage that stopped any electron from getting through (Vstop). However, to his surprise, Lenard discovered that Vstop did not depend upon the intensity of light. When the light intensity was doubled, the number of electrons was doubled. Hopefully, now you have understood Lenard’s experiment and the difference between the classical theories as well as Lenard’s experiment and its different observations.