OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(b) diffraction of electrons travelling through a thin slice of…
Full specification wording
Wave–particle duality: (b) diffraction of electrons travelling through a thin slice of polycrystalline graphite by the atoms of graphite and the spacing between the atoms
When electrons pass through a thin slice of polycrystalline graphite, they are diffracted by the regular arrangement of carbon atoms.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When electrons pass through a thin slice of polycrystalline graphite, they are diffracted by the regular arrangement of carbon atoms. The spacing between adjacent atomic planes acts like a diffraction grating, producing a pattern of concentric rings on a fluorescent screen. The ring radii depend on the electron momentum and the atomic spacing. The de Broglie wavelength λ = h/p links the electron momentum to the diffraction pattern. Increasing the accelerating voltage increases momentum, decreases the de Broglie wavelength, and makes the rings smaller. This experiment provides direct evidence that electrons have wave-like properties and that the atomic spacing in graphite is of the order of 10⁻¹⁰ m.
What earns the marks
- Electrons travelling through a thin slice of polycrystalline graphite are diffracted by the atoms of graphite.
- The regular spacing between carbon atoms acts like a diffraction grating for the electron waves.
- The diffraction pattern consists of concentric rings on a fluorescent screen.
- The ring radii depend on the electron momentum and the spacing between the atoms of graphite.
- The de Broglie wavelength λ = h/p relates the electron momentum to the diffraction pattern.
- Increasing the accelerating voltage increases electron momentum, decreases the de Broglie wavelength, and makes the diffraction rings smaller.
Common mistakes and how to put them right
Thinking that the graphite atoms emit electrons.
Put it right: The atoms diffract the electrons; they do not emit them.
Believing that the diffraction pattern is due to electrons passing through a single slit.
Put it right: The pattern arises from diffraction by the regular atomic planes in the polycrystalline graphite.
Assuming that the atomic spacing is much larger than the electron wavelength.
Put it right: The atomic spacing is of the order of 10⁻¹⁰ m, comparable to the de Broglie wavelength of the electrons, which is why diffraction occurs.
Confusing the effect of accelerating voltage on the pattern.
Put it right: Higher accelerating voltage gives higher momentum, shorter de Broglie wavelength, and smaller diffraction rings.
Exam tips
- State that the graphite is polycrystalline and that the atomic spacing acts as a diffraction grating.
- Describe the pattern as concentric rings and explain how the ring radii relate to the de Broglie wavelength.
- Explain how changing the accelerating voltage affects the electron momentum and the observed ring pattern.
Try the lesson
The start of the Wave–particle duality: (b) diffraction of electrons travelling through a thin slice of polycrystalline graphite by the atoms of graphite and the spacing between the atoms lesson, made from checked teaching notes for this specification point.
Listen
When electrons pass through a thin slice of polycrystalline graphite, they are diffracted by the regular arrangement of carbon atoms. The spacing between adjacent atomic planes acts like a diffraction grating, producing a pattern of concentric rings on a fluorescent screen. The ring radii depend on the electron momentum and the atomic spacing. The de Broglie wavelength λ = h/p links the electron momentum to the diffraction pattern. Increasing the accelerating voltage increases momentum, decreases the de Broglie wavelength, and makes the rings smaller. This experiment provides direct evidence that electrons have wave-like properties and that the atomic spacing in graphite is of the order of 10⁻¹⁰ m. A strong answer earns marks for these points. Electrons travelling through a thin slice of polycrystalline graphite are diffracted by the atoms of graphite. The regular spacing between carbon atoms acts like a diffraction grating for the electron waves. The diffraction pattern consists of concentric rings on a fluorescent screen. The ring radii depend on the electron momentum and the spacing between the atoms of graphite. A common slip: Thinking that the graphite atoms emit electrons. Instead, the atoms diffract the electrons; they do not emit them. Exam tip: State that the graphite is polycrystalline and that the atomic spacing acts as a diffraction grating.
Key words
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Electron diffraction
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Polycrystalline graphite
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Atomic spacing
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De Broglie wavelength
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See every definition
- Electron diffraction
- Electrons travelling through a thin slice of polycrystalline graphite are diffracted by the atoms of graphite.
- Polycrystalline graphite
- A thin slice of this material contains a regular arrangement of carbon atoms that diffract the electrons passing through it.
- Atomic spacing
- The regular spacing between carbon atoms acts like a diffraction grating for the electron waves and is of the order of 10⁻¹⁰ m.
- De Broglie wavelength
- Given by λ = h/p, it links the electron momentum to the diffraction pattern, and increasing the accelerating voltage decreases it.
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