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Photon
A quantum of electromagnetic energy
Work function
The minimum energy needed to remove a single electron from the surface of a particular metal
Threshold frequency (f0)
The minimum frequency of the electromagnetic radiation that will cause the emission of an electron from the surface of a particular metal
Planck’s constant
6.63 × 10-34 Js (in formula book)
Electron volt
Energy transferred to or from an electron when it passes through a p.d. of 1 volt
1 eV = 1.60×10-19J
Photoelectric effect
The emission of photoelectrons from a metal surface when electromagnetic radiation above a threshold frequency is incident on the metal
Photoelectron
Electrons emitted from the surface of a metal by the photoelectric effect
Evidence to suggest electromagnetic radiation is a wave
Evidence:
It can diffract, reffract and interfere with other EM ___
Evidence to suggest light is a particle
Evidence:
The photoelectric effect
Evidence to suggest electrons are waves
Evidence:
They can be diffracted
Evidence to suggest electrons are particles
Evidence:
They can be accelerated and deflected by magnetic and electric fields
Equation for photon energy in terms of frequency
E = hf
Equation for photon energy in terms of wavelength
E = hc/λ
Effect of increasing intensity of the light source while keeping the frequency constant on the photoelectric effect
If the frequency is greater than or equal to the threshold frequency:
Increases the rate of emission of photoelectrons.
Has no affect on their maximum KE
Otherwise, when less than the threshold frequency:
No change; still no electrons emmitted
Eintstein’s photoelectric effect equation
hf = ϕ + KEmax
What must be true for the photoelctric effect to happen
Energy of photon > work function / frequency of photon > threshold frequency
The behaviour of electrons which demonstrates that they have wave properties.
When electrons are passed through a thin slice of graphite they produce circular interference fringes on a flourescent screen. This demontstrates they are diffracted.
Diffraction of electrons occurs when the wavelength is similar to the gap size
Changes in the electron's speed/energy change the size of the ring / interference fringe spacing
Electrons have a (de Broglie) wavelength given by λ=h/p
How does increasing the electron’s speed/energy affect the circular interference pattern produced
Higher speed/energy means smaller de Broglie wavelength
Smaller wavelength means less diffraction
Smaller wavelength gives shorter path difference between areas of constructive and destructive interference
As such the rings become closer together (not just ‘smaller’)
They also become brighter because the electrons have more energy to transfer to the flourescent screen
How does decreasing the electron’s speed/energy affect the circular interference pattern produced
Lower speed/energy means larger de Broglie wavelength
Larger wavelength gives longer path difference between areas of constructive and destructive interference
As such the rings become further apart
They also become dimmer because the electrons have less energy to transfer to the flourescent screen
How is the work function related to the threshold frequency?
A photon with less than the threshold frequency f0 cannot cause electron emission
As such work function = h (threshold frequency)
Explain how Einstein's theory explains the observations of the photoelectric effect and how they demonstrate the particle-like behaviour of electromagnetic waves.
Individual photons are absorbed by individual electrons in the metal surface in a one to one interaction.
Only photons with energies above the work function energy will cause photoelectron emission
Photon energy is proportional to frequency
Hence UV photons or blue photons with higher frequency have higher energy so will cause photoemission but red photons will not.
Number of electrons emitted depends on light intensity
A wave model does not explain instantaneous emission of electrons.
A wave model does not explain a threshold frequency for emission to occur
hf - ϕ = KEmax is the equation resulting from conservation of energy. This can be rearranged to get Einstein’s photoecetric effect equation.