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The de Broglie Wavelength (CIE A Level Physics)

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The de Broglie wavelength

  • De Broglie proposed that electrons travel through space as a wave
    • This would explain why they can exhibit behaviour such as diffraction

  • He therefore suggested that electrons must also hold wave properties, such as wavelength
    • This became known as the de Broglie wavelength

  • However, he realised all particles can show wave-like properties, not just electrons
  • So, the de Broglie wavelength can be defined as:

The wavelength associated with a moving particle

  • The majority of the time, and for everyday objects travelling at normal speeds, the de Broglie wavelength is far too small for any quantum effects to be observed
  • A typical electron in a metal has a de Broglie wavelength of about 10 nm
  • Therefore, quantum mechanical effects will only be observable when the width of the sample is around that value

Calculating de Broglie Wavelength

  • Using ideas based upon the quantum theory and Einstein’s theory of relativity, de Broglie suggested that the momentum (p) of a particle and its associated wavelength (λ) are related by the equation:

lambda space equals space fraction numerator space h over denominator p end fraction

  • Since momentum p = mv, the de Broglie wavelength can be related to the speed of a moving particle (v) by the equation:

lambda space equals space fraction numerator space h over denominator m v end fraction

  • Using kinetic energy E space equals space 1 half m v squared, momentum and kinetic energy can be related by:

Energy: E space equals space 1 half m v squared space equals space fraction numerator m v squared over denominator 2 end fraction space equals fraction numerator space p squared over denominator 2 m end fraction 

Momentum: p space equals space square root of 2 m E end root

  • Combining this with the de Broglie equation gives a form which relates the de Broglie wavelength of a particle to its kinetic energy:

lambda space equals space fraction numerator space h over denominator square root of 2 m E end root end fraction

  • Where:
    • λ = the de Broglie wavelength (m)
    • h = Planck’s constant (J s)
    • p = momentum of the particle (kg m s-1)
    • E = kinetic energy of the particle (J)
    • m = mass of the particle (kg)
    • v = speed of the particle (m s-1)

Worked example

A proton and an electron are each accelerated from rest through the same potential difference.

Determine the ratio: fraction numerator d e space B r o g l i e space w a v e l e n g t h space o f space t h e space p r o t o n over denominator d e space B r o g l i e space w a v e l e n g t h space o f space t h e space e l e c t r o n end fraction

  • Mass of a proton = 1.67 × 10–27 kg
  • Mass of an electron = 9.11 × 10–31 kg

Answer:

Step 1: Determine how the proton and electron can be related via their mass

  • The only information we are given is the mass of the proton and the electron
  • When the proton and electron are accelerated through a potential difference, their kinetic energy will increase
  • Therefore, we can use kinetic energy to relate them via their mass

Step 2: Write the equation relating the de Broglie wavelength of a particle to its kinetic energy

  • The de Broglie wavelength

lambda space equals space fraction numerator h over denominator m v end fraction space equals space h over p

  • Kinetic energy

E space equals space 1 half m v squared space equals space fraction numerator m v squared over denominator 2 end fraction space equals space fraction numerator p squared over denominator 2 m end fraction

  • Kinetic energy in terms of momentum

p space equals space square root of 2 m E end root

  • Substitute the expression for momentum into the do Broglie wavelength equation

lambda space equals space fraction numerator h space over denominator square root of 2 m E end root end fraction

Step 3: Find the proportional relationship between the de Broglie wavelength and the mass of the particle

lambda space proportional to space fraction numerator 1 over denominator square root of m end fraction space space rightwards double arrow space lambda space equals space k fraction numerator 1 over denominator square root of m end fraction

Step 4: Calculate the ratio

fraction numerator d e space B r o g l i e space w a v e l e n g t h space o f space t h e space p r o t o n over denominator d e space B r o g l i e space w a v e l e n g t h space o f space t h e space e l e c t r o n end fraction space equals space fraction numerator 1 over denominator square root of m subscript p end root end fraction space divided by fraction numerator space 1 over denominator square root of m subscript e end root end fraction

square root of m subscript e over m subscript p end root space equals space space square root of fraction numerator 9.11 space cross times space 10 to the power of negative 31 end exponent over denominator 1.67 space cross times space 10 to the power of negative 27 end exponent end fraction end root space equals space 2.3 space cross times space 10 to the power of negative 2 end exponent

  • This means the de Broglie wavelength of the proton is 0.023 times smaller than that of the electron OR the de Broglie wavelength of the electron is about 40 times larger than that of the proton

Examiner Tip

Particles with a greater mass, such as a proton, have a greater momentum. The greater the momentum, the smaller the de Broglie wavelength. Always perform a logic check on your answer to check that makes sense.

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.