Difference Between Photons and Phonons

Most of us get confused, when we hear the word phonon, because it is very similar to the word photon. And the question pops up in our minds: whether these two words refer to a similar thing or not. But, did you know, we are somehow partially correct? Because these two words are analogous to each other. Most of their properties are similar and represent the same thing to some extent. In this article, we will discuss both of them in detail, including their definitions, characteristics, properties, and real-life applications. We will learn that these are analogous to each other, but we will also discuss their differences. So, let’s begin…

What is Photon?

We know that when an electric charge accelerates, it produces electromagnetic waves, which are perpendicular to the direction of its propagation. These electromagnetic waves are energy waves, which contain some finite amount of energy, which is quantized in nature. These quanta of electromagnetic waves are known as Photons. These Photons show a dual nature, as they behave as waves when they are traveling in space and acts as a particle when they strike with any other particle. They are traveling with the is in the vacuum, hence the rest of the mass of the photon is zero.

The energy associated with a single photon is E = hν or ћω or hc/λ.

• where ћ = h/2П
• h = plank’s constant (h = 6.626 * 10^-34 j-s )
• ν = Frequency of radiation
• c = Speed of light
• λ = Wavelength of radiation

When a charged particle is in accelerated motion, it produces emw

What is Phonon?

As we discuss, that photon is a Quanta of electromagnetic wave energy. Similarly when we talk about a solid crystal, all the ions present in the crystal lattice show lattice vibrations. And these vibrations produce vibrational waves at the atomic level. This is due to the presence of an elastic spring type of bond between them. In reality, there is no such type of bond exists between the atoms, but their behavior shows the nature of an elastic spring. Initially, all the atoms are present at their lattice site or in an equilibrium state. When their equilibrium position is disturbed either due to internal or external force, two types of interaction occur between the atoms:

• When the distance between the atoms is less than the bond length, say (a): Both repel each other.
• When the distance between the atoms is greater than the bond length, say(a): Both attracts each other.

Due to this attraction and repulsion, atoms start vibrating about their mean position, and this vibration is known as the lattice vibration. And these vibrations produce vibrational waves, which are energy waves. And the Quanta (minimum possible energy) of these vibrational waves energy is known as Phonon. The property of a phonon is very similar to a photon because a phonon is analogous to a photon. It also shows dual nature, having the same amount of energy E = ћω.

Crystal lattice: Elastic behavior of bonds

Difference Between Photon and Phonon

Photon	Phonon
It is defined as the quanta of electromagnetic energy.	It is defined as the quanta of vibrational mechanical energy.
It is discovered by Gilbert Lewis in 1926	It is discovered by Igor Tamm in 1932
Energy Formula : E = ћω where ћ = h/2П ω = angular frequency of radiation	Energy Formula : E = ћω where ћ = h/2П ω = angular frequency of vibration
Its rest mass is Zero	Its mass is considered a negative mass with negative gravity.
Photons exist in nature	It is not proven by any experiment, that phonons exist in nature.
Types of photons are : Cosmic rays, X-rays, and all E-M ray photons	Types of phonons are : Acoustic and Optical phonons
Photons travel with the speed of light. c = 3*108m/s	Phonons travel slower than photons. ( approx 2.9 * 10-3km/s)
Photons are neutral, hence they do not deviate in an either electric or magnetic field	Phonons show some deflection in the magnetic field.
Size of photon is Approx. 34.9nm	Size of phonon is Not Defined
It is a Bosonic particle	It is also a Bosonic particle

Applications of Photons

• Photons are used in solar panels.
• They are used as a tool in the medical field like X-Rays.
• They are also used in the study of molecules at the micro-level like to measure the distance between the atoms.

Applications of Phonons

• It is used in the study of different properties of matter like thermal and electrical conductivity.
• Phonons are also used to cool down the temperatures of electronic devices by just dissipating the heat around the whole lattice.

Conclusion

We learned about both of the particles and get to know that, both photons and phonons are very similar to each other but with many differences between them. Both of them have their significance and applications in the real world. The photoelectric effect and Crompton effect verify the existence of photons in the world, on the other hand, many theories forcefully try to prove that without the presence of phonons, some of the phenomena in nature can never be explained, like the specific heat of solid at zero temperature is zero. With the evolution of science and technology, we hope in the future, we have some theories that will prove the existence of phonons in nature.

Frequently Asked Questions

Q.1: Does photon and phonon refers to the same thing?

Answer:

No, photons represent the quanta of electromagnetic energy, and on the other hand, phonons represent the quanta of vibrational mechanical energy.

Q.2. Do phonons also interact with each other, just like photons?

Answer:

Yes, phonons also interact with each other just like photons. And their interaction is considered scattering.

Q.3. What is the value of chemical potential for both photons and phonons?

Answer:

The value of chemical potential μ = 0 for both photons and phonons.

Q.4. Why the rest mass of the photons is zero?

Answer:

Because the formula for calculating the effective mass:

m = m_o/√ (1 -v²/c²) ,

• where m = effective mass
• m_o = rest mass
• v = velocity of photons
• c = speed of light

When we are calculating the rest mass, the velocity of the photon is equal to the speed of light. Therefore v=c. And hence, the mass would become infinite, which is not possible. That’s why the rest mass of the photon is considered to be zero.