Review Lepton Number Particle Lepton number (L) electron 1 neutrino positron -1 antineutrino proton neutron Lepton Number

Conservation of Lepton Number In any process: lepton number (and electrical charge) MUST BE CONSERVED Which of these processes is possible?

Rest Energy of Particles All particles have mass when they are at rest. Because mass is a concentrated form of energy we can talk about their rest mass in energy units often referred to as their rest energy measures either in Joules or MeV (1 eV is 1.6 x10-19J 1MeV = 1.6 x 10-13J) The rest energy of an electron is 0.511MeV The rest energy of a proton is: 938.27 MeV

Particles and antiparticles For every particle there is an antiparticle with opposite charge and spin but with the same mass particle antiparticle

Light is made up of particles Light (all electromagnetic radiation) is made up of neutral particles called photons whose energy depends only on the frequency of the light. Increasing energy of photons

Calculating the Energy of Photons You can calculate the energy of a photon given the frequency of the light using Planck’s formula Where h is the Planck constant 6.63 x 10-34Js An infra red wave with a frequency of 1.00 x 1014 Hz is made up of particles called photons photons Which each carry an energy of E = hf E = 6.63 x 10-34 x 1 x 1014 = 6.63 x 10-20J An electromagnetic wave is made up of particles called photons γ γ γ

It is also possible to work out the energy of a photon given the wavelength of the light because if we know its wavelength we know its frequency as c=λf (where c is the velocity of light and λ is the wavelength) Example What is the energy of photons of yellow light of wavelength 600nm (6.00 x 10-9m) Here we can use 2 steps Work out the frequency of the light 2. Now calculate the eneregy of the photon using E = hf Alternately We can just combine the two equations as So