Evans SPH 4U1

Physics Grade 12

Unit 9: Quantum Theory

Note 4: The Compton Effect (Qualitative only)


Reference: Chapter 12.1


In the Photoelectric effect, so far, we have been talking about photons freeing electrons and getting absorbed in the process. ...

 However, in 1923 Compton performed experiments in which a thin metal foil was bombarded with high energy photons (actually X-rays) with a short wavelength. He noticed not only ejected electrons but ALSO lower energy X-ray photons. 


The metal used had a very small work function. As a result, the incident photons were not absorbed.  Instead an elastic collision took place in which electrons were ejected from the metal foil and the photons bounced off with lower energy. The photons after the collision had a lower frequency (greater wavelength) than the incident photons. (Actually, very little energy is transferred to the electron. In some cases the electron may not even escape from the surface of the metal foil).

Energy is conserved in this interaction.

Momentum is also conserved in this type of collision.  It was Compton who derived the expression for the momentum of a photon

using Einstein's equation E = mc2


Refer to vector diagrams from class or figure 19 in text.


ONCE AGAIN the Wave Theory could not explain this Compton effect.  In this case light was behaving like a particle.


Pair Production:

When a high energy photon (gamma rays and X-rays)  collides with a heavy nucleus, it is destroyed and an electron and a positron are formed.  A positron has the same mass as an electron but its charge is positive.  Here matter is created from the energy of the photon in accordance with Einstein's equation E = mc2 .  This process where a photon becomes an electron-positron pair is called pair production.


Pair Annihilation:

When a positron collides with an electron they destroy each other and produce photons. In this case matter is transformed into energy, E = mc2 .


In 1966 Starship enterprise (Startrek - fiction) was powered by antimatter and matter.


5 Different Types of Interactions of Photons:

  1. Reflection -photons undergo perfect elastic collisions

  2. Photoelectric Effect - photon frees an electron and gets absorbed in the process

  3. Compton Effect - photon frees an electron AND photon then emerges with lower frequency (less energy).

  4. Pair Production - photon disappears creating electron and positron

  5. Exciting an atom - photon disappears and transfers ALL its energy to can atom causing the atom to be in an excited state.