The dual behavior of electromagnetic radiation — wave and particle — is one of the cornerstones of modern physics and chemistry. From Planck's quantum hypothesis to Einstein's photoelectric theory and de Broglie's wave nature of matter, a revolutionary concept emerged: the dual nature of electromagnetic radiation. This profound insight led to a complete reevaluation of light, matter, and the atomic world. It marked a pivotal paradigm shift, powerfully showcasing that the principles governing nature at the microscopic level frequently operate counter to our everyday intuition, thereby displacing the deterministic models of classical physics with probabilistic approaches.
Electromagnetic (EM) radiation is a form of energy that travels through space as oscillating electric and magnetic fields. These fields are perpendicular to each other and to the direction of wave propagation. Electromagnetic radiation covers a wide spectrum, from radio waves to gamma rays, including visible light, ultraviolet, infrared, microwaves, and X-rays.
These waves are characterized by:
For centuries, light was thought to be a wave. This idea gained strong support in the 19th century, when experiments demonstrated the wave-like properties of light.
These phenomena could be explained using the wave theory, which considers light as a continuous electromagnetic wave.
Interference: When two coherent light waves superimpose, they produce a pattern of constructive and destructive interference — a signature of wave nature.
Diffraction: Light bends around obstacles and spreads when it passes through narrow slits. This too can only be explained by wave theory.
Despite the success of the wave theory, there were certain observations that it could not explain:
Black Body Radiation
A black body is an idealised physical body that absorbs all incident radiation and emits a characteristic spectrum of radiation known as blackbody radiation. According to classical physics, the energy radiated by a black body should increase indefinitely with frequency (known as the ultraviolet catastrophe). However, experiments have shown that the radiation intensity peaks at a certain frequency and then decreases.
Planck's Solution: Max Planck solved this by proposing that energy is not emitted or absorbed continuously but in discrete packets called quanta. Here, E =h𝜈
This marked the beginning of quantum theory.
Photoelectric Effect
When light of a certain frequency falls on a metal surface, electrons are ejected. This is called the photoelectric effect. Classical wave theory could not explain:
Einstein’s Explanation (1905): Einstein extended Planck’s quantum theory and proposed that light consists of particles or photons.
This indicates that light, in some situations, behaves like a particle.
The contradictory nature of light’s behavior led to a revolutionary concept: Scientists have discovered that light possesses a wave-particle duality, meaning it exhibits characteristics of both waves and particles.
This duality is not confined to light alone. Even electrons and other microscopic particles exhibit similar behaviour, as proven by the Davisson-Germer experiment.
Wave Nature of Matter, inspired by Einstein’s concept, Louis de Broglie proposed that particles like electrons also possess a wave nature. His hypothesis: where,
This was later verified by experiments using electron diffraction, which confirmed that matter can also behave like waves.
The dual nature of light and matter had profound consequences:
A. Quantum Mechanics
Classical mechanics could not describe phenomena at the atomic or subatomic levels. The development of quantum mechanics, especially the Schrödinger equation, was based on the wave-like nature of particles.
B. Uncertainty Principle
Heisenberg’s Uncertainty Principle is a direct outcome of duality: It states that the position and momentum of a particle cannot both be known exactly at the same time.
C. Modern Technologies
Problem 1 : When electromagnetic radiation of wavelength 300 nm falls on the surface of sodium, electrons are emitted with a kinetic energy of 1.68 × 105 J mol–1. What is the minimum energy needed to remove an electron from sodium? What is the maximum wavelength that will cause a photoelectron to be emitted?
Solution
The energy (E) of a 300 nm photon is given by
= 6.626 × 10–34 Js x 3.0 x 108ms-1 / 300 x 10-9 m
= 6.626 × 10–19 J
The energy of one mole of photons
= 6.626 ×10–19 J × 6.022 × 1023 mol–1
= 3.99 × 105 J mol–1
The minimum energy needed to remove one mole of electrons from sodium
= (3.99 –1.68) 105 J mol–1
= 2.31 × 105 J mol–1
The minimum energy for one electron
= 2.31 x 105 J mol-1 / 6.022 x 1023 electrons mol-1
= 3.84 x 10-19 J
This corresponds to the wavelength
= 517 nm (This corresponds to green light)
Problem 2 : A 100-watt bulb emits monochromatic light of wavelength 400 nm. Calculate the number of photons emitted per second by the bulb.
Solution
Power of the bulb = 100 watt
= 100 J s–1
Energy of one photon E = hν = hc/λ
= 6.626 x10 -34 J s x 3 x 108 m s-1 / 400 x 10-9m
= 4.969 × 10–19 J
Number of photons emitted
100 Js-1 / 4 969 x10-19
= 2 012 x 10
(Session 2025 - 26)