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Home
JEE Chemistry
Dual Nature of Electromagnetic Wave

Dual nature of Electromagnetic Wave

1.0Wave-Particle Duality

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.

2.0Electromagnetic Radiation

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:

  • Wavelength (λ): Distance between two successive crests or troughs.
  • Frequency (𝜈): Number of oscillations per second.
  • Velocity (c): All EM waves travel at the speed of light in a vacuum.

3.0Classical Wave Theory of Light

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.

  • Interference
  • Diffraction
  • Polarization

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.

4.0Black Body Radiation & Photoelectric Effect

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

  • E is energy,
  • h is Planck's constant ( Js),
  • 𝜈 is frequency.

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:

  • Why are no electrons ejected below a certain frequency, regardless of intensity?
  • Why does the number of photoelectrons increase with intensity, but their kinetic energy increases with frequency?

Einstein’s Explanation (1905): Einstein extended Planck’s quantum theory and proposed that light consists of particles or photons.

  • A photon of light has energy.
  • Only if the energy of a photon exceeds the work function of the metal electrons are emitted.

This indicates that light, in some situations, behaves like a particle.

5.0Dual Nature of Electromagnetic Radiation

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.

  • When light propagates through space and shows interference or diffraction, → Wave behavior.
  • When light interacts with matter (e.g., photoelectric effect, Compton effect) → Particle behavior.

This duality is not confined to light alone. Even electrons and other microscopic particles exhibit similar behaviour, as proven by the Davisson-Germer experiment.

6.0Comparison of Wave and Particle Nature

Phenomenon

Wave or Particle Nature

Explained by

Interference

Wave

Wave Theory

Diffraction

Wave

Wave Theory

Polarization

Wave

Wave Theory

Black body radiation

Particle

Quantum Theory

Photoelectric effect

Particle

Quantum Theory

Compton effect

Particle

Quantum Theory

7.0De Broglie’s Hypothesis

Wave Nature of Matter, inspired by Einstein’s concept, Louis de Broglie proposed that particles like electrons also possess a wave nature. His hypothesis:  λ=mνh​ where,

  • λ is the de Broglie wavelength,
  • h is Planck’s constant,
  • M is the mass of the particle,
  • v is the velocity.

This was later verified by experiments using electron diffraction, which confirmed that matter can also behave like waves.

8.0Inference of Wave-Particle Duality

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

  • Electron microscopes utilise electron waves to achieve ultra-high resolution.
  • Lasers, solar cells, semiconductors, and quantum computers rely on an understanding of photon and electron duality.

9.0Real-World Applications of Wave-Particle Duality

  1. Photoelectric Cells: Used in automatic doors, solar panels, and light meters.
  2. Electron Microscopes: They rely on the wave nature of electrons to magnify tiny objects far beyond the limits of optical microscopes.
  3. X-ray Diffraction: Used to study the structure of crystals and DNA.
  4. Quantum Computers: This entire field relies on quantum phenomena, such as superposition and duality.
  5. LEDs and Lasers: Photon emission in these devices is based on the particle nature of light..

10.0Solved NCERT Problems on Dual Nature of Electromagnetic Wave 

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

∴λ=𝜈hc​

 = 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

Table of Contents


  • 1.0Wave-Particle Duality
  • 2.0Electromagnetic Radiation
  • 3.0Classical Wave Theory of Light
  • 4.0Black Body Radiation & Photoelectric Effect
  • 5.0Dual Nature of Electromagnetic Radiation
  • 6.0Comparison of Wave and Particle Nature
  • 7.0De Broglie’s Hypothesis
  • 8.0Inference of Wave-Particle Duality
  • 9.0Real-World Applications of Wave-Particle Duality
  • 10.0Solved NCERT Problems on Dual Nature of Electromagnetic Wave 

Frequently Asked Questions

It helps us understand: The structure of atoms Quantum theory How electrons behave in atoms The foundation of modern technologies like lasers, semiconductors, and quantum devices

It means that electromagnetic radiation, like light, exhibits both wave-like and particle-like properties. For example: It behaves like a wave when showing interference and diffraction. It behaves like a particle (photon) when interacting with matter, such as in the photoelectric effect.

The idea was proposed by Albert Einstein in 1905 based on Planck’s quantum theory. He suggested that light consists of energy packets called photons to explain the photoelectric effect.

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