Particles and Radiation

• The strong nuclear force is the strongest of the four fundamental forces. However, it can only be felt over a very short range (0.5 - 3 fm). The strong nuclear force is only experienced by hadrons (e.g. protons and neutrons). The strong nuclear force is attractive for separations above about 0.5 fm, but strongly repulsive for separations less than about 0.5 fm.
• The weak nuclear force affects all types of particles, and is responsible for beta-plus and beta-minus decay, as well as electron capture interactions. It is a very weak, very short range force.
• Each fundamental force has its own exchange particle:

  • The gluon/pion is the exchange particle for the strong nuclear force.
  • The exchange particle of the electromagnetic force is called a 'virtual photon' (virtual because they only exist for a very short time).
  • The weak nuclear force has three exchange particles: the W+, W- and Z0 bosons.
  • Exchange particles are sometimes called gauge bosons.
• • When a particle and its corresponding antiparticle collide, they annihilate each other.
  • Their masses are converted into pure energy, producing a pair of gamma photons.
  • The energy carried away by the gamma photons must equal the total energy of the particles to begin with (kinetic energy plus rest mass energy).
  • So each gamma photon must carry away at least the rest mass energy of one particle.
• • Pair production is the opposite of annihilation.
  • Pair production is when one high energy photon spontaneously turns into a particle-antiparticle pair.
  • The energy of the photon must be at least the total rest mass energy of the particle-antiparticle pair it creates.
• • Unlike other quantum numbers, strangeness is only conserved in the strong interaction.
  • Strange particles, such as kaons, are produced via the strong interaction but decay via the weak interaction.
  • This is why kaons are always produced in pairs (K+ and K-).
  • The strangeness of each particle (-1 and +1) cancel out.
  • In weak interactions, such as kaon decay, strangeness can change by -1, 0 or +1.
• • Electrons are only emitted from a metal when the light hitting it is above a certain frequency.
  • Below this frequency, no electrons are emitted.
  • This frequency is called the threshold frequency.
  • Threshold frequency is different for different types of metal but is usually in the UV range.
• • The maximum kinetic energy of an emitted electron is dependent on the frequency of radiation but not its intensity (energy transferred per second).
• • When the light (EM radiation) is above the threshold frequency of the metal, the number of electrons emitted is proportional to the intensity of the light.
• • According to the wave theory of light, energy should be spread out evenly across the surface of the metal.
  • The energy absorbed by each electron would gradually increase so that after a while the electrons would have enough energy to be emitted.
• • According to the wave theory of light, the intensity should determine the energy transferred to the electrons each second.
  • So we would expect that the maximum kinetic energy of emitted electrons would increase with intensity (not frequency).
• • Einstein suggested that each photon had a one-on-one interaction with an electron.
  • The electron absorbs all the energy of one photon.
  • This explained why the maximum kinetic energy is independent of the intensity.
  • Intensity is the number of photons arriving per second.
  • It doesn't matter how many photons arrive per second because the electron only interacts with one.
• • For an electron to leave a metal surface, it needs to overcome the bonds holding it down.
  • The energy needed to break these bonds is called the work function, φ.
  • The work function is different for different metals.
• The equation for stopping potential is: $eV_s =$$E_{k(max)}$​

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• A fluorescent tube has mercury vapour inside it. The inside of the tube is coated with phosphor. A high voltage accelerates the free electrons inside the tube, which collide with other electrons in the mercury atoms, exciting them. The excited electrons move back down to their ground state by releasing a high frequency UV photon.

  These photons then collide with the phosphor coating, which excites electrons in the phosphor atoms. When they de-excite, they release photons with a frequency in the visible light range.
• Absorption spectra are formed when white light (which forms a continuous spectrum) is passed through a cool gas (the gas is cool to ensure atoms are in ground state). Electrons in the gas absorb certain frequencies of light to become excited. Most frequencies of the light are not absorbed, and so if we split the light coming out of the gas with a prism, we see a spectra with black lines where light has been absorbed.
• Emission spectra are formed when an excited gas de-excites and emits photons of specific frequencies. When this light is split via a prism, we see a series of bright white lines.
• • Experiments such as shining light through a diffraction grating show us that light must be a wave.
  • Light must diffract through the grating and then interfere constructively and destructively to produce bands of light and dark.
  • Only a wave would be able to do this.
• • The photoelectric effect displays that light must be a particle.
  • The experiment shows that light particles (photons) have a one-on-one interaction with an electron on the metal surface.
• • Diffraction patterns are only visible if the wavelength of the particle is roughly equal to the width of the diffraction grating.
  • Electrons are tiny, so it is possible to reduce their de Broglie wavelength to the distances between atoms in a crystal (a few hundred nanometres).
  • This means we can observe diffraction effects in electrons.