1. To start
Exam question: A fluorescent light tube contains mercury vapour at low pressure. The tube is coated on the inside, and contains two electrodes. Explain why the mercury vapour is at low pressure (1 mark)
A low pressure is needed to make it a vapour – its usually a liquid at room temperature (think of thermometers).

2. Wave-Particle Duality
A LEVEL PHYSICS Year 1
Wave-Particle Duality
I can explain what is meant by the term ‘wave-particle’ duality (Grade C)
I can discuss the evidence that matter particles have dual nature (Grade B)
Understand that electron diffraction suggests the wave nature of particles and explain how diffraction changes with momentum (Grade A).
I can explain why we can change the wavelength of a matter particle but not a photon (Grade A) C B A A*

3. Waves and particles have some similar but many different characteristics Note down as many similarities and differences as possible:
Similarities
Differences

4. Wave-like nature
Particle-like nature
Diffraction of light
When passing through a narrow slit the emerging light is spread out (diffracted).
A narrow gap/longer wavelength leads to larger diffraction.
Photoelectric effect
Light directed at a metal surface.
Electrons at the surface absorbs KE of Photon.
Photoelectron escapes surface of metal if work function is overcome.

5. Diffraction
When a beam of light passes through a narrow gap it spreads out. This can only be explained using waves. What would happen in this situation if light was only a particle?
A low pressure is needed to make it a vapour – its usually a liquid at room temperature (think of thermometers).

6. You will learn about this in more detail later

7. Wave-Particle Duality
Wave-like nature
Particle-like nature
Diffraction of light
When passing through a narrow slit the emerging light is spread out (diffracted).
A narrow gap/longer wavelength leads to larger diffraction.
Photoelectric effect
Light directed at a metal surface.
Electrons at the surface absorbs KE of Photon.
Photoelectron escapes surface of metal if work function is overcome.

8. Electron Diffraction

9. Electron Diffraction
Electrons are accelerated to high velocities in a vacuum then passed through a graphite crystal. As it passes through the spaces between the crystal atoms they diffract just like waves. They produce a pattern of rings.

11. By comparing the diffraction of light with the diffraction of electrons, explain why it was surprising that they behaved in a similar way.
You may want to start by explaining that light can be considered as both a wave and a particle (use the examples of the photoelectric effect and light diffraction).
Explain then would you learnt from the Dr.Quantum video.
Why then is it surprising that electrons (particles) also behave this way?

12. If ‘wave-like’ light shows particle properties then maybe ‘particles’ (like electrons) should be expected to show wave-like properties.

13. De Broglie wavelength
Look closely at the equation for de Broglie wavelength above and write down an equation for momentum (you may also remember it from GCSE).
This equation does not apply to electromagnetic waves – can anyone think why?

14. Can be interpreted as a ‘probability wave’ – peaks represent a greater likelihood that the particle will be in that place.
The scientific community were not initially impressed with de Broglie’s theory. It went through an intense ‘peer review’ process before being published and then was tested experimentally (electron diffraction).
Once evidence was found to back up the theory it was validated.

15. Practice Questions
Questions 2 and 3 from the practice questions (application) on page 61 of the textbook.
Check and mark your answers (answers in the back of the textbook).

16. Homework
Wave-particle Duality and De Broglie Wavelengths Homework Due Tuesday 10th November

17. Next Lesson: De-Broglie Wavelength Analysis and Revision
Lesson after that: Quantum Physics Assessment