Catching Light in Quanta: Measuring Planck’s Constant in the Teaching Laboratory

(UNESCO International Year of Quantum Science and Technology)

UNESCO-declared 2025, as the International Year of Quantum Science and Technology to celebrates a century the ideas that transformed our understanding of nature. One of the most beautiful aspects of this celebration is to tell the young generation that the foundations of quantum physics are not confined only to the high-end research facilities. However, they can be explored and experienced through hands-on experiments by students in a teaching laboratory as well.

This photoelectric effect experiment is part of the laboratory curriculum of the Physics Department, Aligarh Muslim University, Aligarh, India. It is conducted alongside many other beautiful experiments that demonstrate the fundamental ideas of quantum and nuclear physics.

One of the classic examples is the study of the photoelectric effect. In this experiment the light itself becomes the probe of quantum reality. Here, students not only observe a genuine quantum phenomenon but also measure Planck’s constant, one of the fundamental constants of nature. These same quantum principles form the backbone of nuclear physics, governing processes such as radioactive decay, nuclear energy levels, and particle interactions. Thus, a simple laboratory experiment like the study of photoelectric effect provide students with a direct understanding the quantum ideas that are relevant both in atomic and nuclear physics.

When Light Refused to Behave Classically

At the beginning of the 20th century, classical physics suggested that shining sufficiently intense light on a metal surface should eventually release electrons. Experiments, however, indicated a very different story. Electrons were found to be emitted only when the incident light had a certain minimum frequency, regardless of how intense the light beam was. This minimum value of frequency is known as the threshold frequency.

In 1905, Albert Einstein solved this puzzle by introducing a new idea. He assumed that light is composed of tiny, discrete packets of energy called photons. Each photon carries a fixed energy hν , where h is the Planck’s constant. This proposal challenged the long-held belief that light behaves only as a wave.

Einstein’s explanation of the photoelectric effect showed that light transfers energy in fixed, indivisible packets called quanta, or photons. Since this energy is delivered in discrete packets, light behaves like a stream of particles rather than a continuous wave. This is why the photoelectric effect is described as evidence of the particle nature of light. The idea of quantized energy is also fundamental in nuclear physics. Inside the nucleus, energy levels are discrete, and processes such as radioactive decay and gamma-ray emission follow strict quantum rules. Thus, the photoelectric effect not only exposed the limitations of classical physics but also helped establish quantum mechanics, the framework that governs both atomic and nuclear phenomena.

Bringing Quantum Physics into the Student Laboratory

In the laboratory class, this historic experiment is recreated using a vacuum phototube. Light of different frequencies is obtained by using filters of different colours. By illuminating a metal cathode with light ranging from red to blue, students observe that photoelectrons are emitted only above a certain threshold frequency. This is the direct evidence of the particle nature of light.

Measuring Planck’s Constant with Light

The most exciting part of the experiment comes from quantitative analysis. Students apply a retarding (stopping) potential to the phototube and gradually increase it until the photocurrent just falls to zero. This stopping potential is directly related to the maximum kinetic energy of the emitted photoelectrons.

According to Einstein’s photoelectric equation, when a photon of energy hν is incident on a metallic surface, it transfers its energy Eₑ to an electron. A part of this photon energy (hν) is used to overcome the work function  of the metal, and the remaining energy appears as the kinetic energy (Eₑ) of the emitted electrons. The emitted electrons are referred to as the photoelectrons. Thus, the maximum kinetic energy  of the photoelectrons is given by;

Eₑ = hν − Φ, where Φ is the work function of the metal.

Now, if a retarding (stopping) potential (Vₛ) is applied to just stop these photoelectrons, then the kinetic energy of the electrons is equal to the electrical potential energy acquired against this field, i.e. Eₑ = eVₛ.

Equating the two expressions above, for the kinetic energy, the stopping potential can be written as;  Vₛ = (h/e)ν − Φ/e. So if plot stopping potential as a function of frequency, it can be observed that the stopping potential varies linearly with the frequency of the incident radiation, with a slope (h/e) . The intercept determines the work function (Φ) of the metal.

By repeating this procedure for different wavelengths (by employing different colour filters) and plotting the stopping (retarding) potential against the frequency of light, one obtains a straight-line graph. The slope of this graph yields Planck’s constant, while as mentioned above the intercept provides the work function of the metal.

Figure 2: Plot of stopping potential (Vₛ) versus frequency (ν) of incident light. The linear fit verifies Einstein’s photoelectric equation and allows determination of Planck’s constant and work function.

From the analysis of data, the experimentally obtained value of Planck's constant (h) is found to be remarkably close to the accepted value of 6.63 × 10⁻³⁴ J·s. This demonstrates the power of simple laboratory experiments in revealing deep physical reality.

A Classical Law Also Verified Using the same Device

The same experimental setup also enables students to verify a fundamental result from classical physics, the inverse square law of radiation. By varying the distance between the light source and the phototube and measuring the resulting photocurrent, students also confirm that light intensity decreases with the square of the distance, as I ∝ 1/r².

Figure 3: Verification of the inverse square law of radiation. A linear variation of photoelectric current (I) with 1/r² confirms the inverse square dependence of light intensity.

Why This Experiment Matters Even Today

This experiment connects the past with the present. It shows a strange effect that classical physics could not explain. This puzzle inspired Einstein’s famous ideas. These ideas later became quantum mechanics. Today, the power technologies we use every day, such as solar cells, automatic lights, digital cameras, and medical imaging machines, involve such principles. During teaching, it is important to connect the theoretical ideas of quantum mechanics with these technological advances. Through this experiment, students see how a complex idea from physics becomes part of daily life.

As we celebrate the UNESCO-International Year of Quantum Science and Technology, the photoelectric effect experiment stands as a powerful reminder that quantum science is not just about equations and abstractions. It is about measuring, observing, and understanding nature at its most fundamental level. Such hands-on experiments highlight the practical relevance of quantum mechanics and inspire the young generation at the right time, motivating them to explore the subject further. Beginning in the teaching laboratory, these experiences help shape future scientists and innovators who will apply quantum ideas to real-world challenges.

Thank you for reading. I hope this experiment inspires you to explore the fascinating ideas of quantum physics further.

-Prof. Dr. B. P. Singh (atomicexplorers1@gmail.com)

Department of Physics

Aligarh Muslim University

ALIGARH-202002

INDIA