Wave-particle duality (22.3)

What is the the evidence that light can behave as particles?

The photoelectric effect - this is because photoelectrons are emitted regardless of intensity (only are emitted above a certain frequency) - if wave model was used this effect would have to be due to energy transfer to the metal surface (increase in intensity)

What is the evidence that light can behave as a wave?

Light can diffract and interfere - these require a wave model and it is related to phase difference which is a property of waves

What is evidence that particles can behave as waves and what is the setup of this evidence?

Electron diffraction is evidence that electrons (and particles) can behave as waves

This shows that electrons also have a wavelength and display wave behaviour

Set up:

• A beam of high speed electrons are fired at a thin graphite crystal (atoms act as diffraction gratings)

• Diffraction rings are formed (diffraction and interference are property of waves)

What is de Broglie wavelength, and what is its relationship with Plank’s constant and momentum?

de Broglie wavelength - wavelength associated with a moving particle (wavelength acts the same as regular wavelength in terms of diffraction (for pronounced diffraction, gap size must match wavelength))

λ_m = h/p

Where λ_m is de Broglie wavelength (m)

Where h is Plank’s constant (Js)

Where p is momentum (kgms^-1)