Chapter 5.4 - Wave-particle duality Flashcards
What is Huygens’s principle
Huygens principle is for predicting the future movement of waves. Giving we know the current position of the wave front.
What must be considered of every point on a wave front in Huygens’s principle
That every point on the wave front is a new source of circular waves travelling forwards from that point
What is found when the superposition is considered in the myriad circular waves of Huygens’s principle
The resultant wave will be the new position of the original wave front
What is the evidence that light is a wave?
Young’s two slit experiment and the interference pattern produced via diffraction both require superposition of wave displacements to generate the standing wave pattern seen.
This is only possible if light is behaving as a wave and has the appropriate repeating cycles of displacement that cause ongoing superposition to maintain constructive and destructive interference. Producing nodes and antinodes.
Particles cannot superpose In this way
What is a phenomenon exhibited by electromagnetic waves that can only be explained in classical physics by using the idea of waves?
Polarisation
Formula to work out the photon energy
Planks constant x frequency
If an electron is made to travel at high speeds through a gap what can be seen and what does this prove?
The electrons provide a diffraction pattern proving that electrons can behave as waves
What are the evidences for wave movement in:
Light
Electrons
Light - diffraction, interference, polarisation
Electrons - Diffraction, interference
What are the evidences for particle movement in:
Light
Electrons
Light - Photoelectric effect
Electrons - ionisation
What are photons?
Packets of electromagnetic radiation energy.
What is quantisation?
The concept that there is a minimum smallest amount by which a quantity can change.
What is wave-particle duality?
The principle that the behaviour of electromagnetic radiation can be described in terms of both waves and photons
The photoelectric effect experiment
Ultraviolet light is shone onto -tively charged zinc plate
Plate looses charge - light causes electrons to leave the metal removing the charge.
Gold leaf falls immediately to the zinc plate
If electromagnetic radiation only travelled as waves what could be observed in the photoelectric effect? What instead would be observed
If the wave was a small amplitude it could shine for longer and slowly pass energy to the electrons until they gained the energy to leave the zinc
This is not observed however, no mater how long or at what amplitude red light is shone on the zinc plate, no photo electrons will be emitted.
What is the setup of the photoelectric effect?
A gold leaf electroscope with a negatively charged zinc plate and a UV light source
What are the observations of the photoelectric effect?
Light travels as photons, with a photons energy level being proportional to the frequency
When a photon encounters and electron, it transfers all its energy to the electron (the photo creases to exist)
If an electron gains sufficient energy - more than the work function - it can escape the surface of the metal as a photoelectron
Brighter illumination (amplitude) means more photons per second which means greater number of photoelectrons per second
If electron doesn’t gain sufficient energy from an encounter with a photon to escape the metal surface it will transfer the energy gained from the photon to the metal as a whole before it can interact with another photon. Thus, if the photon energy is too low no photoelectrons are observed
Photoelectric effect equation
Work function = (planks x freq) - 0.5xmassxvelocity^2
What is the photoelectric cell experiment and how does it work?
Photoelectric cell can be used to measure planks and the work function for a metal.
In a vacuum place the metal as an anode in the cell with a gap t othe cathode. Shine light onto anode and current will be registered n ammeter. Turning up the pd will eventually make anode positive to the point photo electrons are attracted back giving stopping voltage (max Ek)
How can a graph be used to find planks constant?
Gradient of the graph where y axis= photoelectrong max Ek and x axis= Frequency
What are photoelectrons?
Electrons released from a metal surface as a result of its exposure to electro magnetic radiation
What is the work function?
Minimum level of energy needed by electron at the surface of a metal to escape from the metal
What is the threshold frequency?
For a given metal is the minimum frequency of electromagnetic radiation that can cause emission of photoelectrons
What is the stopping voltage
In an appropriately illuminated photoelectric cell is the minimum voltage needed to reduce current to zero
What is the de Broglie equation?>
Electron wavelength = Planks / momentum
How can we investigate electron diffraction?
Utilising an electron beam diffraction tube.
This accelerates a beam of electrons through a high voltage and passes the beam through a sliver of graphite. The array of carbon atoms in the graphite act as diffraction grating in two dimensions which produces a circular diffraction pattern. The front of tube has a phosphorescent screen that will show up the diffraction pattern
Radiation intensity formula
Intensity = power / area
What is the ground sate?
The lowest energy level for a system
What is excitation
An energy state for a system that is higher energy that the ground state
What is ionisation energy?
Minimum energy required by an electron in an atom’s ground state in order to remove the electron completely from the atom
What is a line spectrum
A series of individual lines of colour showing the frequencies present in a light source
