Difference Between Electromagnetic Wave Theory and Planck’s Quantum Theory

The key difference between Electromagnetic Wave Theory and Planck’s Quantum Theory is that the Electromagnetic Wave Theory does not explain the black body radiation phenomena and photoelectric effect whereas Planck’s Quantum Theory does explain the black body radiation phenomena and the photoelectric effect.

If we heat a substance (which has a high melting point), it first turns red coloured, then converts into a yellow colour, which then begins to glow with white and blue light. Once the substance is heated like this, we call it a “black body” and the resulting radiation (that the substance emits) is “black body radiation”. However, we cannot explain how this happens using electromagnetic wave theory but the Planck’s quantum theory explains it well.

CONTENTS

1. Overview and Key Difference
2. What is Electromagnetic Wave Theory
3. What is Planck’s Quantum Theory
4. Side by Side Comparison – Electromagnetic Wave Theory vs Planck’s Quantum Theory in Tabular Form
5. Summary

What is Electromagnetic Wave Theory?

Electromagnetic Wave Theory is a theory in chemistry that was developed by James Clark Maxwell in 1864. According to this theory, there are several points about radiation emitted from a substance.

These points are as follows:

• Energy emits from any source continuously in the form of radiant energy.
• Radiation has two fields oscillating perpendicular to each other; electric field and magnetic field. Both these fields are perpendicular to the path of radiation.
• Radiation has wave characteristics and travels in the velocity of light. We call it electromagnetic radiation.
• These electromagnetic radiation does not require matter for propagation.

The “wave” described in this theory has several characteristics. The wavelength of the wave is the distance between two consecutive crests or troughs of the wave. A number of waves that pass through a point per one second are the frequency of the wave. The linear distance that the wave travels per one second is the velocity. Wavenumber is the number of waves present in one-centimetre length.

Using this theory, we can develop the electromagnetic spectrum. However, there are some limitations to this theory. These limitations are as follows:

1. It cannot explain the black body radiation.
2. And, it does not explain the photoelectric effect.
3. It cannot explain how the heat capacity varies the temperature of solids.
4. Furthermore, it cannot explain the line spectra of atoms.

What is Planck’s Quantum Theory?

Planck’s Quantum Theory is a theory in chemistry developed by Max Planck in 1900. This theory is like a modification for electromagnetic wave theory because we can explain the things that the electromagnetic wave theory couldn’t explain. The important points in this theory are as follows:

• Radiant energy emits or absorbs discontinuously as energy packets, which we call quanta.
• The energy of each quantum equals the product of Planck’s constant and the frequency of radiation.
• Always the total amount of energy that a substance emits or absorb is a whole number of quanta.

Moreover, this theory explained the phenomena of blackbody radiation and the photoelectric effect that the electromagnetic wave theory failed to explain. According to this theory, when we heat a substance, the atoms of that substance absorb energy from heat and starts oscillations to emit radiation; when we further heat the substance, it emits more and more radiation. Then the substance emits radiation with the lowest frequency of visible range which gives red colour, and the next is yellow colour and so on.

When considering the explanation for the photoelectric effect, first let us understand what a photoelectric effect is. When the radiation strikes the surface of a metal, it causes the emission of electrons in the surface of the metal. This is what we call photoelectric effect.

According to the Planck’s Quantum Theory, when light strikes on a surface, the quanta of the light radiation gives all its energy to the electrons in the surface. Hence, the electrons become detached from the surface and ejected from the surface, if the incident radiation has the energy equal to the force of attraction between the atomic nucleus and electron.

What is the Difference Between Electromagnetic Wave Theory and Planck’s Quantum Theory?

Electromagnetic Wave Theory is a theory in chemistry developed by James Clark Maxwell in 1864 whereas Planck’s Quantum Theory is a theory in chemistry developed by Max Planck in 1900. The key difference between electromagnetic wave theory and Planck’s quantum theory is that the electromagnetic wave theory does not explain the black body radiation phenomena and photoelectric effect whereas the Planck’s quantum theory does explain the black body radiation phenomena and the photoelectric effect. Moreover, another difference between electromagnetic wave theory and Planck’s quantum theory is that according to the electromagnetic wave theory, the radiation is continuous but, according to the Planck’s Quantum Theory, the radiation is discontinuous.

The below infographic presents the difference between electromagnetic wave theory and Planck’s quantum theory in tabular form.

Summary – Electromagnetic Wave Theory vs Planck’s Quantum Theory

The two theories Electromagnetic Wave Theory and Planck’s Quantum Theory explains the behaviour of radiation emitted from a substance. However, the key difference between Electromagnetic Wave Theory and Planck’s Quantum Theory is that the Electromagnetic Wave Theory does not explain the black body radiation phenomena and photoelectric effect whereas the Planck’s Quantum Theory does explain the black body radiation phenomena and the photoelectric effect.

Reference:

1. Libretexts. “4.2: Planck’s Quantum Theory.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here 

Image Courtesy:

1.”1526374″ by helder100 (CC0) via pixabay
2.”Blackbody Spectrum” By Almazi – Own work, (CC BY-SA 4.0) via Commons Wikimedia
3.”Photoelectric effect” By Wolfmankurd – en:Inkscape, (CC BY-SA 3.0) via Commons Wikimedia