The **key difference** between photon and quantum is that **photon is an elementary particle, whereas quantum is a measure of quantity.
**

Photon is an elementary particle while quantum is a discrete packet with energy stored in it. Photon and quantum are two very important concepts in modern physics. Furthermore, these concepts are useful widely in fields such as quantum physics, quantum chemistry, electromagnetic theory, optics, particle physics, etc. These concepts are also important in many real-world applications such as LASERs, high-resolution microscopy, measurements of molecular distances, quantum cryptography, and photochemistry.

### CONTENTS

1. Overview and Key Difference

2. What is Photon

3. What is Quantum

4. Side by Side Comparison – Photon vs Quantum in Tabular Form

5. Summary

## What is Photon?

Photon is an elementary particle that does not have a substructure. Elementary particles are the building blocks of the universe; all other particles are made from these particles. Photons belong to the category of elementary bosons. Albert Einstein is the father of the modern concept of photons. He used this concept to explain experimental observations that did not fit with the classical wave model of light.

A photon is a particle with a zero rest mass, but it has a relativistic mass. Also, it has no electric charge. Besides, it does not decay spontaneously in the space. Moreover, it moves at the speed of light in space. We can find the energy of a photon by E=hf, where E is energy, f is the frequency of the photon and h is the Plank’s constant. We can also give this equation in the form E=hc/λ, where the speed of light is c and λ is wavelength.

Furthermore, photons, like all other quantum objects, exhibit wave-like and particle-like properties. And, this dual wave-particle nature is the concept we call wave-particle duality of a photon. Photons are emitted in many natural processes; for example, when accelerating a charge, during a molecular, atomic, or nuclear transition to a lower level, and when a particle and its corresponding antiparticle are in annihilation.

## What is Quantum?

The term quantum comes from the Latin ‘quantus’ meaning ‘how much’. A quantum is ‘a discrete packet’ with energy stored in it. The energy of matter is not continuous. That means the transfer of any quantity of energy is not possible. Scientists discovered that energy is quantized and it transfers in discrete units (or packets) of size hf. We call each of the packets of energy as ‘a quantum’.

For example, a photon is a single quantum of light. The plural of quantum is quanta. Max Plank discovered the concept of quantization. He used this concept to explain the emission of radiation from heated objects; we call this black body radiation.

## What is the Difference Between Photon and Quantum?

Photon is the smallest discrete amount or quantum of electromagnetic radiation while quantum is a discrete quantity of energy proportional in magnitude to the frequency of the radiation it represents. Therefore, the key difference between photon and quantum is that photon is an elementary particle, whereas quantum is a measure of quantity.

Moreover, another significant difference between photon and quantum is that the photon is important as a quantum of electromagnetic radiation while quantum is important to measure quantity at the subatomic scale.

## Summary – Photon vs Quantum

We can describe quantum as a measure of quantity, but a photon is not about a measure of quantity. In fact, we can describe photon as a quantum of energy. Hence, the key difference between photon and quantum is that photon is an elementary particle, whereas quantum is a measure of quantity.

##### Reference:

1. Jones, Andrew Zimmerman. “What Is a Photon in Physics?” ThoughtCo, Sep. 3, 2018, Available here.

##### Image Courtesy:

1. “AtomicLineSpEm” (CC BY-SA 3.0) via Commons Wikimedia

2. “Wavelet” By Thierry Dugnolle – Own work (CC BY-SA 3.0) via Commons Wikimedia

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