photons and pair formation

Question 1

what is light?

Answer 1

18th century:
Sir Isaac Newton - particles?
Christian Huyges - wave?

Question 2

evidence for wave properties of light

Answer 2

• can be reflected
• can be refracted
• can be diffracted
• interfere with each other

Question 3

wave properties of light

Answer 3

• wavelength: 3 to 8 x10^-7m or 300 to 800 nm
• frequency: about 10^15 Hz
• the colour of light depends on wavelength

Question 4

particle properties of light

Answer 4

the wave theory of light could not explain some phenomena:

• the radiation coming from hot objects
• the emission of electrons from the surface of a metal illuminated by light

Question 5

photon theory

Answer 5

Max Planck and Albert Einstein

- light is not a continuous wave, but a series of small “bursts” of “quanta” of energy

Question 6

the PHOTON

Answer 6

:a “packet” of light carrying a certain amount of energy
- the energy of the photon depends on the frequency
- lots of photons acting together “merge” their properties, so light behaves like a continuous wave in most circumstances
E=hf

Question 7

photons and the electromagnetic spectrum - higher frequency

Answer 7

as the frequency gets higher (left):

• photon energy gets larger
• behave more like particles, less like waves
• gamma and x-ray photons can ionise atoms
• dangerous

Question 8

photons and the electromagnetic spectrum - lower frequency

Answer 8

as the frequency gets lower (right):

• photon energy gets smaller
• effect of individual photons becomes undetectable & radiation can only be detected when very large numbers of photons work together
  
  • radio waves - no particle effects and behave purely as waves

Question 9

pair production

Answer 9

energy and mass can be considered as different forms of the same thing
E=mc^2
- destroying mass releases energy (annihilation)
- if there is enough energy, mass can be produced
- always in the form of a particle and its antiparticle

Question 10

pair production 2

Answer 10

• a very high energy photon with energy E can create a pair of particles
• the more energy the photon has the more mass the particles can have
• max total energy of the particle pair is given by m=E/c^2
• any left-over energy can be released as Ek of the particles
  e. g. y –> e^- + e^+

Question 11

particle accelerators

Answer 11

• destroy mass by colliding particles at very high speeds (99.99999% of c)
• energy converts into particle/antiparticle pairs
• the energy densities approach those predicted very shortly after the Big Bang