# De Broglie Wavelength Calculator

By Bogna Szyk
Last updated: May 21, 2021

This de Broglie wavelength calculator will help you describe the wave-particle duality of matter. According to this principle, we can analyze light as waves, explaining its properties such as refraction. On the other hand, light must be made of particles (called photons) if we want to understand the photoelectric effect. De Broglie stated that every particle could be described as having a particular wavelength and formulated the famous de Broglie equation. If you want to know the de Broglie wavelength of an electron or a photon, keep reading!

## De Broglie equation

According to de Broglie, a beam of particles of some mass can behave as a matter wave. Its wavelength is related to the mass and velocity of the particle:

`λ = h/mv`

where:

• `m` is the mass of the particle;
• `v` is the velocity of the particle, and
• `h` is the Planck constant, equal to `6.6261*10-34 Js`.

## The units of mass

The mass of particles is usually given in kilograms, but using scientific notation since the numbers are small. In the calculator, use the drop-down menu to select the exponent of the mass. For reference, these are the value of the electron rest mass (me) and the atomic mass unit (u), which is an average of the proton and neutron masses.

`1 me = 9.10938356*10-31 kg`
`1 u = 1.660538921*10-27 kg`

## What is the de Broglie wavelength of an electron?

Let's find the de Broglie wavelength of an electron traveling at 1% of the speed of light.

1. The mass of an electron is equal to 1 me, or
`9.10938356*10-31 kg`.
2. The speed of this electron is equal to 1 c divided by 100, or `299,792,458 m/s / 100 = 2,997,924.58 m/s`.
3. Multiplying the mass and speed, we obtain the momentum of the particle: `p = mv = 2.7309245*10-24 kg·m/s`.
4. If we divide the Planck constant by the momentum, we will obtain the de Broglie wavelength: `h/p = 6.6261*10-34 / 2.7309245*10-24 = 2.426*10-10 m`.
5. Choosing proper units, we can say that the de Broglie wavelength of this electron is equal to 0.24 nm.

## How to calculate the de Broglie wavelength of a photon?

Since a photon's rest mass is zero, you might be wondering how to use our de Broglie wavelength calculator to find the de Broglie wavelength of a photon. Thanks to wave-particle duality, a photon does have a momentum associated with it. The value of this momentum can be used to calculate the de Broglie wavelength of the photon.

A question asks you to find the de Broglie wavelength of a photon which has a momentum of `6.8*10-35 kg·m/s`. Let's see how to use our calculator to answer this question.

1. Click on the unit changer menu of the momentum row and select "x10-35" from the list.
2. Enter `6.8` into the momentum row. You've now entered the correct value for the momentum of the photon.
3. You will see the result for the wavelength in nanometers by default. As it is a huge value, you'll probably want to change the units to something more manageable, such as meters. The result is `9.74 m`, rounding to 3 significant figures.
Bogna Szyk
Rest mass (m)
x10⁻³¹
kg
Velocity (v)
m/s
Momentum (p)
x10⁻²⁴
kg·m/s
Wavelength (λ)
nm
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