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W.A.L.T

W.A.L.T. 2.5 Wave-particle duality W e A re L earning T o. Understand

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W.A.L.T

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  1. W.A.L.T 2.5 Wave-particle duality We Are Learning To • Understand • Candidates should know that electron diffraction suggests the wave nature of particles and the photoelectric effect suggests the particle nature of electromagnetic waves; details of particular methods of particle diffraction are not expected. • de Broglie wavelength = h • mv • where mvis the momentum.

  2. Starter Newton Vs. Huygens

  3. In the 1600s, Christiaan Huygens and Isaac Newton proposed competing theories for light's behavior. Huygens proposed a wave theory of light while Newton's was a "corpuscular" (particle) theory of light. Huygens' theory had some issues in matching observation. Newton's prestige helped lend support to his theory, so for over a century his theory was dominant. Christiaan Huygens (1629 – 1695) Sir Isaac Newton (1643 – 1727)

  4. Reflection:- can be explained by treating light as a wave or a particle

  5. Interference:- this can be explained by the wave theory of light (treating light as a wave)

  6. Refraction of light:- can be explained by treating light as wave or particle

  7. Interference of light:- can only be explained by treating light as a wave

  8. TOMAS YOUNG 1805 INTERFERENCE EXPERIMENT constructive interference destructive interference

  9. Diffraction:- can be only explained by treating light as a wave

  10. Main Demonstrate diffraction of light

  11. circular aperture light Light can be diffracted Light must be a wave

  12. Photoelectric Effect:- can only be explained if treat light as a particle (photon)

  13. Main Activity Particles

  14. Behavior of Electrons Particle behavior cathode anode

  15. Electrons travel in a straight line when they meet an obstacle they cast a sharp shadow, no diffraction is observed as would be with waves. Also accelerated electrons can be deflected by magnetic fields and electric fields, waves are not affected by these fields. So electrons are particles, right?

  16. Wave Behaviour of electrons heater vacuum graphite target YES,ELECTRONS DO HAVE A WAVE NATURE Test: Can electrons be diffracted? e

  17. Wave behavior of electrons The particle theory predicts we should observe a single blurred spot where the electrons hit the screen. In fact we see a interference pattern which can only be explained by the wave theory. The regular atoms in the graphite diffract the electrons which interfere to produce regions of maximum and minimum intensity

  18. TOMAS YOUNG 1805 INTERFERENCE EXPERIMENT constructive interference destructive interference

  19. Plenary Duality

  20. Light behaves like water waves in a ripple tank min max min max min max Light must be a wave min

  21. circular aperture light Light can be diffracted Light must be a wave

  22. potassium metal e LIGHT MUST BE A PARTICLE! Photoelectric Emission! e Photon of Light

  23. LIGHT PARTICLE OR WAVE OR WHAT?

  24. PRE 1900 CLASSICAL THEORY ENERGY IS CONTINUOUS VARIABLE AT THE ATOMIC LEVEL ENERGY IS QUANTISED, IT CANNOT HAVE ANY VALUE 1900 PLANCK’S QUANTUM THEORY

  25. PHOTONS Light comes in packets of energy. EINSTEIN (1905): E = h f ENERGY OF A PHOTON is but c = f λ

  26. combining and i.e. the wavelength of a photon is Planck’s constant divided by its momentum, p .

  27. Electrons cast sharp shadows and are affected by magnetic and electric fields, unlike wave Electrons must be Particles

  28. Electrons must be Wave-like Electron diffraction can only be explained if electrons behave like waves

  29. 1923 : Louis de Broglie : “If a photon behaves as particle with mass, then a particle should have an associated wavelength given by where v is the particle’s velocity

  30. Summary • The wave-like nature of light is observed when diffraction takes place • The particle-like nature of light is observed in the photoelectric effect • The particle-like nature of electrons is observed by magnetic and electric deflection • The wave-like nature of electrons is observed in electron diffraction • Particles have a wavelength given by:

  31. What’s your wavelength? According to De Broglie, you have a wavelength! What do you notice about it? What are the implications?

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