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Particles, Antiparticles & Photons (AQA AS Physics)
Revision Note
Antimatter
- The Universe consists of matter in the form of particles i.e. protons, neutrons, electrons etc.
- All particles of matter have an antimatter counterpart
- Antimatter particles are identical to their matter counterpart but have an opposite charge
- This means if a particle is positive, its antimatter particle is negative and vice versa
- Common matter-antimatter pairs are shown in the table below:
Matter-Antimatter Table
- Apart from electrons, the corresponding antiparticle has
- The same name with the prefix ‘anti-’
- A line above its corresponding matter particle symbol
Properties of Antiparticles
- Corresponding matter and antimatter particles have
- Opposite charges
- The same mass
- The same rest mass-energy
- The rest mass-energy of a particle is the energy equivalent to the mass of the particle when it is at rest
- Some values of common particle masses and rest mass-energies are shown in the table below:
Mass & Rest Mass Energy Table
Examiner Tip
In the exam, don't forget that the datasheet provides masses in kg and rest-mass energies in MeV for a proton, neutron, electron and neutrino
The Photon Model
- Photons are fundamental particles which make up all forms of electromagnetic radiation
- A photon is defined as:
A massless “packet” or a “quantum” of electromagnetic energy
- This means that energy is not transferred continuously but as discrete packets of energy
- In other words, each photon carries a specific amount of energy, or "quanta", and transfers it all in one go, rather than supplying it consistently
Calculating Photon Energy
- The energy of a photon can be calculated using the formula:
- Using the wave equation, photon energy can also be written:
- Where:
- E = energy of the photon (J)
- h = Planck's constant (J s)
- c = the speed of light (m s-1)
- f = frequency (Hz)
- λ = wavelength (m)
- This equation tells us:
- The higher the frequency of EM radiation, the higher the energy of the photon
- The energy of a photon is inversely proportional to the wavelength
- A long-wavelength photon of light has a lower energy than a shorter-wavelength photon
The energy of a photon is linked to the frequency of the light wave by the Planck constant, h
Worked example
Light of wavelength 490 nm is incident normally on a surface, as shown in the diagram.
The power of the light is 3.6 mW. The light is completely absorbed by the surface.
Calculate the number of photons incident on the surface in 2.0 s.
Examiner Tip
Make sure you learn the definition for a photon: discrete quantity / packet / quantum of electromagnetic energy are all acceptable definitions.The values of Planck’s constant and the speed of light will always be available on the datasheet, however, it helps to memorise them to speed up calculation questions!
Annihilation & Pair Production
- Two important interactions involving particles, antiparticles and photons are:
- Annihilation
- Pair production
Annihilation
- When a particle meets its corresponding antiparticle, the two will annihilate
- Annihilation is defined as:
When a particle meets its corresponding antiparticle they both are destroyed and their mass is converted into energy in the form of two gamma-ray photons
- The two most common particle-antiparticle pairs that are seen are:
- Proton-antiproton annihilation
- Electron-positron annihilation
When an electron and positron collide, their mass is converted into energy in the form of two photons emitted in opposite directions
- The minimum energy of one photon after annihilation is the total rest mass energy of one of the particles:
- Where:
- = minimum energy of one of the photons produced (J)
- = Planck's Constant (J s)
- = minimum frequency of one of the photons produced (Hz)
- = rest mass energy of one of the particles (J)
- To conserve momentum, the two photons will move apart in opposite directions
- As with all collisions, the mass and energy is still conserved
Pair Production
- Pair production is the opposite of annihilation, it is defined as:
When a photon interacts with a nucleus or atom and the energy of the photon is used to create a particle–antiparticle pair
- In order to achieve the creation of a particle-antiparticle pair, a single photon must have enough energy to create both particles
When a photon with enough energy interacts with a nucleus it can produce an electron-positron pair
- The minimum energy required for a photon to undergo pair production is equal to the total rest mass energy of the particles produced:
- Where:
- = minimum energy of the incident photon (J)
- = Planck's Constant (J s)
- = minimum frequency of the photon (Hz)
- = rest mass energy of one of the particles (J)
- To conserve momentum, the particle and anti-particle pair move apart in opposite directions
Worked example
Calculate the maximum wavelength of one of the photons produced when a proton and antiproton annihilate each other.
Examiner Tip
Since the Planck constant is in Joules (J) remember to always convert the rest mass-energy from MeV to J.
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