De Broglie Analysis and Revision A LEVEL PHYSICS Year 1 De Broglie Analysis and Revision C B A A* I can explain why we can change the wavelength of a matter particle but not a photon (Grade A) Be able to explain how and why diffraction changes when the momentum of a particle changes (Grade A).

What can we change to increase the de Broglie wavelength of an electron? High velocity – dividing by a ‘big’ number – small wavelength (remember h and m are constant). Low velocity – diving by a ‘small’ number – big wavelength (remember h and m are constant).

Would slower or faster moving electrons give widely spaced rings? Wave Theory The spread of lines in the diffraction pattern increases if the wavelength is greater. Would slower or faster moving electrons give widely spaced rings? A smaller accelerating voltage leads to slower electrons (big wavelength) which gives widely spaced rings

What happens to the diffraction pattern as the electron speed is increased? Velocity is higher Wavelength is shorter Spread of the lines is smaller

Explain what is happening here Explain what is happening here? Why does the amount of diffraction change? This is the same for all diffraction: it only happens if a particle interacts with an object (this could be a gap) about the same size as it de Broglie wavelength.

How would the diffraction patterns compare for fast moving electrons and slower moving electrons? How would the diffraction patterns compare for a particle with greater mass than an electron (for example a neutron) if they were travelling at the same speed? What is the relationship between the diffraction pattern and the momentum of the particle? (Hint: Start by writing out the equation for momentum) Challenge Q: Use your phone (to research) and the textbook to explain why electron microscopes can resolve finer detail than a light microscope.

Slower electrons – longer wavelength - widely spaced Faster electrons – shorter wavelength - tightly packed Smaller mass– longer wavelength - widely spaced Greater mass – shorter wavelength - tightly packed As momentum increase, wavelength decrease so the diffraction pattern is more tightly packed. Pg.61 of the textbook.

Multiple Choice Revision Questions

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