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How does electromagnetic radiation travel through space?
As a continuous wave
What are photons?
Discrete energy packets
What is the equation linking energy of a photon to frequency of the electromagnetic radiation?
E = hf = hc / lamda
What is h?
Planck constant
6.63 ×10^-34
What is 1 electronvolt?
The energy transferred when an electron travels through a potential difference of 1 volt
How can you use LEDs to estimate the value of the Planck constant?
LEDs emit light when pd exceeds threshold pd
Set up potential divider to vary pd through LED
Place black tube over LED to make it clear when LED lights up
Determine threshold pd. we know wavelength due to colour of light
Use eV=hc/lamda
Repeat with different coloured lights
What is the photoelectric effect?
When electromagnetic radiation is shone on to a metal electrons are released from the surface
How can the photoelectric effect be demonstrated using a gold leaf electroscope?
zinc plate on top of a negatively charged stem, negatively charged leaf attached to stem
Leaf and stem repel
UV light shone onto zinc plate, electrons released from surface, negative charge lost
Leaf falls back to the stem
What is the work function of a metal?
Minimum energy required to free an electron from the surface of the metal
What must a photon have in order to free an electron from a metal surface?
Energy at least as great as the work function
What is threshold frequency?
Minimum frequency required of the electromagnetic radiation to free electrons from the surface of the metal
Does visible light produce the photoelectric effect?
No as the frequency is too low
What is the photoelectric equation?
hf = phi + KEmax
phi = work function of the metal
KEmax = max ke of released electron
What determines the KE of a released electron?
Amount of energy left after being released (after using up work function amount)
How does intensity of radiation relate to electron emission if frequency < threshold frequency?
no electrons will be released regardless
How does intensity of radiation relate to electron emission if frequency > threshold frequency?
increasing intensity increases rate of electron emission as it increases the number of photons available to interact with the electrons
How can you increase the kinetic energy of the released electrons?
Increase the frequency of radiation further above the threshold so there's more energy left over
What is wave particle duality?
diffraction and superposition of light relies on the radiation acting as a wave
Photoelectric effect relies on it acting as discrete photons
Therefore electromagnetic waves have a dual nature
What is the de broglie equation?
Lamda = h/p = h/mv
How can electrons be used as evidence for wave-particle duality?
electrons are particles with mass and charge, they can be accelerated and deflected by magnetic and electric fields
However they can also be made to diffract which is s property of waves
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