Momentum of Photon Formula

Momentum of Photon

The momentum of a photon is intricately linked with its energy. Similarly, the energy of a photon is proportional to its frequency; the momentum of a photon is linked with its wavelength. The momentum of a photon can be calculated using the formula: p = h/λ


Momentum of Photon Formula

Einstein described the momentum (p) of a photon with the given formula: p = h/λ

where, 

p = the momentum of the photon (measured in Kg.m/s)

h = the Planck’s constant has a value of 6.63×10−34 J ⋅ s

λ = the wavelength of the wave bearing the photon in meters.

Note: Seeing that the value of the “h” is low the momentum of the photon is insignificant in most cases. This is validated by the fact that the mirror doesn’t draw back when the photons bounce off it.


Importance of Momentum of Photon Formula

In quantum mechanics, Photon is a crucial particle. Light and other such rays are composed of photons. Albert Einstein was the first scientist to bring into light this concept. It announced a new era in the world of physics.


Solved Example Using Momentum of Photon Formula

Example:

Calculate the momentum of a photon of light in a beam of light having a wavelength of 4x10−7 meters. Apply the Momentum of Photon Formula to find the answer.

Solution:

We already know that the momentum of the photon formula is: p=h/λ

Substituting the values, p = 6.63×10−34 J ⋅ s 4x10−7

 = 1.6575×10−27 kg⋅ m/s

FAQs (Frequently Asked Questions)

Q1. What is a Photon?

Answer: The photon is a kind of elementary particle. Light is composed of photons. They are typically Qantas of energy while having a mass of zero. Photons travel at the speed of light in a vacuum. We can use Heisenberg’s uncertainty principle in order to interpret the behaviour of photons. This should be used since we are not aware of the momentum and the position of the photon concurrently and accurately.

So, for example, find the speed of a bullet with the speed of light in a vacuum.

Speed of Light, C = 299,792,458 m/s in vacuum

So, U s/b C = 3 x 108 m/s

Not that,

C = 3 x 108 m/s

To imply,

C = 324 m/s

A bullet is faster than 324m/s

Q2. What is a Momentum?

Answer: The product of the velocity and mass of an object is said to be it’s linear or translational or simply momentum. This is according to Newtonian mechanics. It is simply a vector quantity, having a magnitude and a direction.

The momentum of objects can be calculated as follows: p = m × v

Where,

M = mass of the object

V = velocity of the object.

Q3. In What Units is the Momentum Calculated?

Answer: Momentum is measured in kilogram meters per second (kg⋅m/s) as per the SI units.


In any inertial frame, the linear momentum is preserved. The momentum of an object is computed as per the 2nd law of Newton that postulates “the rate of change of momentum of a body is directly proportional to the force exerted on it and is in that direction.”


Issac Newton, one of the principals and leading scientists in humanity formed three laws of motion. These laws regulate all the behaviour of different objects around the world. These laws are not to be applied at the atomic level and hence have to be altered before use in Quantum mechanics and Relativity. Remember that all Heavenly bodies abide by Newton's laws of motions.