May 2021

Quantum Physics: The Photoelectric Effect Paradox Explained

May 5, 2021

O Level Physics Tuition

Albert Einstein may be famous for his theory of relativity, but it was his research on the photoelectric effect that won him a Nobel Prize for Physics. The photoelectric effect is a core concept of quantum physics, and it has brought about giant leaps in humanity’s understanding of the quantum nature of light and electrons. However, when the effect was first observed and subsequently studied, many physicists at the time referred to this phenomenon as a paradox.

To understand how Einstein resolved this paradox, we must first understand the photoelectric effect and the resulting observations that perplexed many physicists at the time.

What is the photoelectric effect?

The photoelectric effect is the discharge of electrons when electromagnetic radiation hits a material. Heinrich Hertz first discovered this phenomenon when he observed electric currents being produced when ultraviolet light shone on a piece of metal.

Why was there a paradox?

After Hertz’s discovery, studies were conducted by physicists on this phenomenon, and they confirmed two observations:

  • The energy of the individual photoelectrons grows when the frequency or colour of the light changes. However, no change is observed when the intensity or brightness of the light changes.
  • The photoelectric current is determined by the light’s intensity. An increase in light intensity leads to a proportional increase in the number of emitted electrons.

These two observations baffled physicists due to their misunderstanding of light as a wave phenomenon. The energy of a wave phenomenon is dependent on the amplitude of the wave and not its frequency, but this concept does not correspond with the observations made.

How was the paradox resolved?

Max Planck played an essential role in Einstein resolving this paradox. Planck’s theory that the atoms’ energy can only take on discrete values led Einstein to realise that electromagnetic waves have a particle nature.

Einstein theorised that light is comprised of photons, and the energy of the photons in each quantum of light matches the frequency multiplied by a constant h, which is referred to as Planck’s constant. The photons’ energy calculation is represented by the formula E = hf, where E is the photons’ energy, h is Planck’s constant and f is the light’s frequency.

He also deduced that shining a light source with sufficient energy on metal will cause the discharge of electrons. A photon above the threshold frequency will have sufficient energy to eject an electron, producing the photoelectric effect observed by Hertz. As such, it is the frequency, and not the intensity, that determines the emission of electrons. The kinetic energy of the displaced electron is determined by the formula Kmax = hf – Φ, where h is Planck’s constant and f is the photons’ frequency. Φ represents the work function, which is the minimum energy necessary to displace the electron.


Einstein’s revelation is critical to the foundation of quantum mechanics, which is fundamental to quantum physics. It has changed the way generations of scientists view the properties of electromagnetic radiation.

His discovery has also fueled further research into the area and resulted in another physicist, Louis De Broglie, uncovering that light contains both wave-like and particle-like characteristics. The resolution of the photoelectric effect paradox demonstrated that one discovery has the potential to lead to another. This drive to uncover new things gives us a better understanding of physics and how the universe works.

If you find yourself struggling with this topic, you can consider taking up a tuition class. We offer H2 physics tuition, and the photoelectric phenomenon is part of the core syllabus that we will be covering in our classes. We also offer O level physics tuition as well. If you are interested, you can sign up for our physics tuition classes today!

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