Photoelectric Effect and Wave Theory of Light

When light shines on a metal, electrons can be ejected from the surface of the metal in a phenomenon known as the photoelectric effect. This process is also often referred to as photoemission, and the electrons that are ejected from the metal are called photoelectrons. The wave theory of light states that light travels in the form of waves.

In this article, we will learn about the photoelectric effect, the wave theory of light and will try to answer whether they are related to each other or not.

What is Photoelectric Effect ?

In 1887, Hertz noticed that electrons are emitted from a metal surface when electromagnetic radiation falls. In 1888, Hallwachs showed experimentally that electrons are emitted from the Zinc plate when ultraviolet rays fall on the plate.

This phenomenon of emission of electrons from a metallic surface, when illuminated by the light of appropriate wavelength or frequency, is called the photoelectric effect. The electrons radiated in this process are called photoelectrons, and the current produced in the circuit is called photoelectric current. This effect has been found useful in electronic devices specialized for detecting light and precisely timed emissions of electrons. When the frequency of the incident UV light was smaller than a specified minimum value, known as the threshold frequency, then no electrons were emitted at all. This minimum frequency was dependent on the nature of the material of the emitter plate.

Einstein’s Photoelectric Equation

Albert Einstein in 1905 proposed that radiation energy is built up of discrete units called quanta of energy radiation. Each quantum of radiation has energy h, where h is the Planck’s constant and is the frequency of light radiation. A quantum of energy is absorbed by the electron and if it exceeds the minimum energy needed for the electron (Work Function 0 ). Einstein’s Photoelectric Equation is given as:

KE = hv – hv₀

where

• KE is the kinetic energy of the photoelectron,
• h is Planck’s constant,
• v is the frequency of the photon
• v₀ is the threshold frequency of material. 

What is Wave Theory of Light ?

The wave theory of light, also known as the wave theory of electromagnetic radiation, proposes that light is a form of electromagnetic wave. According to the wave theory, light exhibits wave-like behavior, characterized by properties such as interference, diffraction, polarization, and refraction. These phenomena can be explained by the interaction of light waves with each other and with the medium through which they propagate.

Interference and Diffraction

One of the key pieces of evidence supporting the wave theory of light is the phenomenon of interference, where two or more light waves superpose to form regions of constructive and destructive interference.

Diffraction, the bending of light waves around obstacles or through small openings, is another phenomenon consistent with wave behavior.

Thomas Young’s Double-Slit Experiment

In 1801, Thomas Young conducted the famous double-slit experiment, where he demonstrated interference patterns formed by light passing through two closely spaced slits. This experiment provided compelling evidence for the wave nature of light .

Does Photoelectric Effect Explain Wave Theory of Light?

No, Photoelectric effect cannot explained by wave theory of light. It is explains the particle nature of light given below:

Photoelectric Effect and Particle Nature

The key properties of the photoelectric effect that support the particle nature of light are:

Threshold Frequency: The photoelectric effect occurs only when the frequency of the incident light is above a certain threshold value (the threshold frequency).

Energy Conservation: According to Einstein’s theory, the energy of a photon is directly proportional to its frequency (E = hf, where E is energy, h is Planck’s constant, and f is frequency). When a photon is absorbed by an electron, its energy is completely transferred to the electron, causing it to be emitted with a kinetic energy equal to the difference between the energy of the photon and the binding energy of the electron.

Instantaneous Emission: The emission of photoelectrons occurs instantaneously, with no time delay between the absorption of a photon and the emission of an electron. This suggests that light behaves as discrete particles (photons) rather than continuous waves.

Modern Understanding about Light

• In modern physics, light is understood as a form of electromagnetic radiation consisting of oscillating electric and magnetic fields propagating through space.
• The wave theory of light is a foundational concept in optics and plays a crucial role in understanding a wide range of phenomena, from reflection and refraction to polarization and dispersion.

Wave-Particle Duality

The wave theory of light coexists with the particle theory of light, which suggests that light behaves like discrete packets of energy called photons. This duality is a fundamental aspect of quantum mechanics and is expressed in the principle of wave-particle duality.

Conclusion

In this article, we learned about the photoelectric effect discovered by Albert Einstein, for which he was awarded the Nobel Prize in Physics in 1921, and its principles. The photoelectric effect is a physical phenomenon that occurs when a metal surface is struck by light of a specific frequency. Heinrich Hertz discovered this phenomenon back in 1887, Lenard confirmed it in 1902. The minimum frequency required for electron emission is known as the threshold frequency. We also learned about the particle nature of light and the photon. The light quantum is associated with a particle and has a definite value of energy and momentum, and is known as the Photon. Finally, we learn that photoelectric effect confirms the particle nature of light and not its wave nature however in modern understanding we have learn that light possess dual nature that is both particle and wave nature.

Frequently Asked Questions

What is the photoelectric effect?

The photoelectric effect is the phenomenon where electrons are emitted from a material (usually a metal) when it is exposed to light or electromagnetic radiation of sufficiently high frequency.

Who discovered the photoelectric effect?

The photoelectric effect was first observed by Heinrich Hertz in 1887. However, it was Albert Einstein who provided the theoretical explanation of the effect in 1905, for which he was awarded the Nobel Prize in Physics in 1921.

What is the mechanism behind the photoelectric effect?

According to Einstein’s explanation, the photoelectric effect occurs when photons (particles of light) with sufficient energy strike the surface of a material. Each photon transfers its energy to an electron in the material, causing the electron to be ejected from the material as a photoelectron.

What is the threshold frequency in the photoelectric effect?

The threshold frequency is the minimum frequency of light required to cause the photoelectric effect for a given material. Below this frequency, no electrons are emitted regardless of the intensity of the light.

What factors affect the kinetic energy of emitted photoelectrons?

The kinetic energy of emitted photoelectrons depends on the frequency of the incident light (which determines the energy of the photons) and the work function of the material (which represents the minimum energy required to remove an electron from the material).

What is the work function of a material in the context of the photoelectric effect?

The work function is the minimum amount of energy required to remove an electron from the surface of a material and is typically denoted by ϕ. It represents the energy barrier that must be overcome for the photoelectric effect to occur.

What is the significance of Einstein’s explanation of the photoelectric effect?

Einstein’s explanation of the photoelectric effect provided crucial evidence for the particle nature of light (photons) and helped lay the foundation for the development of quantum mechanics, which revolutionized our understanding of the behavior of particles and waves at the atomic and subatomic levels.