What does the photon model of EM radiation propose?
That electromagnetic energy exists in discrete packets of energy (quanta) called photons
How is the energy of a photon calculated? (2)
E = hf E = hc / λ
What does the energy of a photon depend on? (2)
Frequency - energy is directly proportional to frequency
Wavelength - energy is inversely proportional to wavelength
What is h?
Planck constant (6.63x10^-34 Js)
What is the electronvolt? (2)
The electronvolt (eV) is is a small unit of energy used for the quantum scale eV = joules x 1.6x10^-19
Define 1 eV
The energy transferred to or from an electron when it moves through a potential difference of 1V
How can LEDs be used to determine the value of Planck constant? (4)
Record the wavelength of the LED being used
Use a potentiometer to vary the PD in a circuit until the LED just lights (threshold PD - the point where one electron supplies enough PD for one photon to emit light)
Since the energy of the electron = eV and energy received by photon = hc / λ , eV = hc / λ which can be rearranged to give V = hc/e x 1/λ
Plotting a graph of voltage against 1/λ for a range of LEDs will give a gradient of hc/e where c and e are known constants
What is the photoelectric effect?
Specific sorts of EM radiation can cause electrons (or photoelectrons) to be emitted from the surface of a metal when it shines on it.
How does a gold-leaf electroscope demonstrate the photoelectric effect? (4)
Briefly touching the top plate with a negative electrode charges the electroscope by depositing electrons onto the plate and stem
As both the stem and gold leaf have the same charge, they repel, causing the gold leaf to lift away from the stem
If a piece of zinc is placed on the top plate and UV radiation shines onto the surface, the gold leaf slowly falls back to the stem
This shows that electrons have been emitted from the zinc, causing the stem to lose the negative charge
What observations can be made from the photoelectric effect experiment? (3)
Photoelectrons were only emitted if the radiation was above a threshold frequency
Photoelectrons were emitted instantaneously when EM radiation shone on the surface of the metal
Increasing intensity caused more photoelectrons to be emitted but their maximum KE stayed the same unless frequency was changed
Photoelectric Effect Predictions vs. Conclusions - “Very low intensity light should have no effect”
Observation - electrons were still emitted but fewer of them
Photon model explanation - low intensity means fewer photons, not photons with less energy
Photoelectric Effect Predictions vs. Conclusions - “High frequency light should work just as well”
Observation - increasing frequency increased the energy of the electrons
Photon model explanation - photons of higher frequency have more energy
Photoelectric Effect Predictions vs. Conclusions - “Greater intensity should give electrons more energy”
Observation - intensity had no effect on the energy of the electrons
Photon model explanation - intensity does not affect the energy of the photons so it can’t affect the energy of the electrons
Photoelectric Effect Predictions vs. Conclusions - “Low frequency light should work”
Observation - there is a lower limit to frequency, below which no electrons are emitted
Photon model explanation - photons of low frequency have insufficient energy to remove an electron
Photoelectric Effect Predictions vs. Conclusions - “Low intensity light should take time to remove electrons”
Observation - electrons are emitted instantaneously
Photon model explanation - a single photon is enough to release one electron (1:1 ratio)
Photoelectric Effect Predictions vs. Conclusions - “Greater intensity means more energy, so more electrons should be released”
Observation - greater intensity released electrons at a greater rate
Photon model explanation - greater intensity means more photons per second so more electrons are emitted per second
What is the photoelectric equation?
max KE = hf - φ
What is the work function?
φ - the minimum energy required by an electron to escape the surface of a particular metal
What is the threshold frequency? (2)
The minimum frequency of radiation that releases an electron
Since max KE = 0, f = φ/h
How are maximum KE of electrons and intensity of incident radiation related?
Independent of each other
How is the rate of emission of photoelectrons related to the intensity of radiation?
Directly proportional (above threshold frequency)
What is wave-particle duality?
A theory that states that matter has both particle and wave properties
How does light show wave-particle duality? (2)
As a wave - if wavelength is comparable to the dimensions of the object it interacts with, light will diffract
As a particle - when interacting with matter (e.g. photoelectric effect)
How do electrons show wave-particle duality? (2)
As a wave - when moving through slits (e.g. electron diffraction)
As a particle - when interacting with matter (e.g. electric/magnetic field deflection)
How does electron diffraction take place? (3)
If an electron gun fires at polycrystalline graphite, the electrons pass through gaps between individual carbon atoms
The gap is similar to the wavelength of the electrons so they diffraction forming a wave-like diffraction pattern
The behave as particles as they are accelerated across a PD and then as waves when they diffract before acting like particles again when they hit the screen with discrete impacts
What size is the gap between atoms?
What is the de Broglie equation?
λ = h/mv (momentum)