(a) electron diffraction, including experimental evidence of this effect

The idea

Electron diffraction is the spreading of electrons when they pass through a narrow gap or around an obstacle, producing patterns of maxima and minima. Experimental evidence comes from firing electrons through a thin graphite film or a narrow slit and observing a diffraction pattern on a fluorescent screen. The pattern is similar to that formed by light or other waves, showing that electrons have wave-like properties. The de Broglie wavelength λ = h/p links the electron momentum to the pattern spacing. Increasing the accelerating voltage increases electron momentum, decreases the de Broglie wavelength, and makes the diffraction pattern narrower. This evidence supports wave–particle duality: electrons behave as waves in diffraction but as particles in other experiments.

What earns the marks

• Electron diffraction is the spreading of electrons after passing through a narrow gap or around an obstacle, producing a pattern of maxima and minima.
• Experimental evidence includes firing electrons through a thin graphite film or narrow slit and observing a diffraction pattern on a fluorescent screen.
• The diffraction pattern for electrons is similar to that for waves, demonstrating the wave-like nature of electrons.
• The de Broglie wavelength λ = h/p relates the electron momentum to the spacing of the diffraction pattern.
• Increasing the accelerating voltage increases electron momentum, decreases the de Broglie wavelength, and makes the diffraction pattern narrower.

Common mistakes and how to put them right

• Thinking that electrons only behave as particles.

  Put it right: Electron diffraction shows that electrons also have wave-like properties.

• Believing that the diffraction pattern is due to electrons interacting with each other.

  Put it right: The pattern arises from the wave nature of each electron passing through the graphite or slit.

• Assuming that increasing the accelerating voltage broadens the diffraction pattern.

  Put it right: Higher voltage gives higher momentum, shorter de Broglie wavelength, and a narrower pattern.

• Confusing electron diffraction with the photoelectric effect.

  Put it right: Electron diffraction demonstrates the wave nature of electrons, while the photoelectric effect demonstrates the particle nature of light.

Exam tips

• Describe the experimental setup clearly: electron gun, thin graphite film or narrow slit, fluorescent screen, and vacuum.
• Explain how the observed pattern provides evidence for wave behaviour, using the terms maxima and minima.
• Link the pattern spacing to the de Broglie wavelength and explain how changing the accelerating voltage affects the pattern.