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

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

3. 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 MeV = 1.6 x 10-13J)
The rest energy of an electron is 0.511MeV
The rest energy of a proton is: MeV

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

5. 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

6. 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 x 1 x 1014
= 6.63 x 10-20J
An electromagnetic wave is made up of particles called photons γ γ γ

7. 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