The Student Room Group
Reply 1
desm0nd
Hello, I'm stuck on this relatively easy question but can't seem to find the answer in either my CGP revision guide, nor the rubbish Nelson thormes AQA A-level Physics book. Any help would be most grateful.


(b) (i) Explain why there is a minimum energy of the λ photon for this conversion to take place and what happens when a λ photon has slightly more energy than this value.

do you think we have telepathic ability to find out what conversion took place ..Post the whole q please..
I'm guessing this is because the particle pairs have a certain mass, and this mass is related to the energy of the photon by e=mc^2. The energy is related to λ by, e = hc/λ

This leads us to hc/λ = mc^2
which gives, h/λ = mc
h and c are constants, so λ is inversely proportional to m. You can't create the particles, unless you have enough energy. There's a minimum energy requirement. However, if you have more than necessary energy, this "extra" energy is the kinetic energy of the particles.

All of this, of course, is just a guess.
Reply 3
The conversion is from part a of the question where a Y photon(virtual) is converted into an electron and positron. (Basically pair production)
this is due to Einsteins mass - energy equivalence theory.
The gamma photon must have enough energy to provide the rest masses of the electron and positron. Any extra energy will provide the particles with kinetic energy