What does electron diffraction show about the behaviour of electrons?

It shows that electrons display quantum behaviour, arriving with a probability obtained by combining contributions from all possible paths, exactly as photons do.

An electron has a momentum of 1.0 × 10⁻²⁴ kg m s⁻¹. Calculate its de Broglie wavelength.

λ = h/p = 6.63 × 10⁻³⁴ / 1.0 × 10⁻²⁴ = 6.6 × 10⁻¹⁰ m.

What happens to the diameter of the electron diffraction rings when the accelerating voltage is increased?

The rings shrink, because faster electrons have greater momentum and therefore a shorter wavelength, which gives a smaller diffraction angle.

OCR A-Level Physics B H557

Planning

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Implementing

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Evaluation

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Imaging and Signalling

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Waves and Quantum Behaviour

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Charge and Field

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Probing Deep Into Matter

Topic overview Particle accelerators and high-energy beams Scattering and the nuclear atom Paths of scattered particles and closest approach Discrete energy levels in atoms Electron standing waves and the confined electron Quarks, hadrons and leptons Conservation laws and nuclear equations Charged particles in detectors and magnetic fields Relativistic energy and the relativistic factor

Ionising Radiation and Risk

Topic overview The nature of ionising radiations Effects on living tissue and radiation safety Absorbed dose, effective dose and risk Binding energy and nuclear stability The Nuclear Valley and modes of decay Fission, fusion and nuclear power Activity, half-life and the decay constant Investigating absorption and half-life

Waves and Quantum Behaviour

Electron diffraction and the de Broglie wavelength

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the ring pattern from graphite and what it shows, de Broglie wavelengths of electrons, protons and everyday objects, the effect of accelerating voltage, and microscope resolution.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is observed when a beam of electrons passes through a thin film of polycrystalline graphite?

Key term

Why does electron diffraction through graphite give rings rather than separate spots?

Key term

State the de Broglie relationship and define its terms.

Questions from this subtopic

Each one with the model answer underneath.

1
What is observed when a beam of electrons passes through a thin film of polycrystalline graphite?
Model answer
Concentric bright rings appear on the fluorescent screen beyond the film.
2
Why does electron diffraction through graphite give rings rather than separate spots?
Model answer
The many small graphite crystals are randomly oriented, so every direction around the beam axis offers crystals at the diffracting angle.
3
What does electron diffraction show about the behaviour of electrons?
Model answer
It shows that electrons display quantum behaviour, arriving with a probability obtained by combining contributions from all possible paths, exactly as photons do.
4
State the de Broglie relationship and define its terms.
Model answer
The relationship is λ = h/p, where λ is the wavelength associated with a particle, h the Planck constant and p the momentum of the particle.
5
An electron has a momentum of 1.0 × 10⁻²⁴ kg m s⁻¹. Calculate its de Broglie wavelength.
Model answer
λ = h/p = 6.63 × 10⁻³⁴ / 1.0 × 10⁻²⁴ = 6.6 × 10⁻¹⁰ m.
6
What happens to the diameter of the electron diffraction rings when the accelerating voltage is increased?
Model answer
The rings shrink, because faster electrons have greater momentum and therefore a shorter wavelength, which gives a smaller diffraction angle.

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