Download Lesson 9 – De Broglie Analysis

A LEVEL PHYSICS
Year 1
De Broglie Analysis and Revision
A*
A
B
C
• 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).
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? 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.
1. How would the diffraction patterns compare for
fast moving electrons and slower moving
electrons?
2. 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?
3. What is the relationship between the diffraction
pattern and the momentum of the particle?
(Hint: Start by writing out the equation for
momentum)
4. Challenge Q: Use your phone (to research) and
the textbook to explain why electron
microscopes can resolve finer detail than a light
microscope.
1. Slower electrons – longer wavelength widely spaced
Faster electrons – shorter wavelength tightly packed
2. Smaller mass– longer wavelength - widely
spaced
Greater mass – shorter wavelength - tightly
packed
3. As momentum increase, wavelength
decrease so the diffraction pattern is more
tightly packed.
4. Pg.61 of the textbook.
Multiple Choice Revision Questions
Next Lesson: Quantum Physics
Assessment
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