Photon-Matter Interactions SPH4U

The Compton Effect In 1923, Compton directed high-energy x-rays at a thin metal foil. Ejected electrons were observed, consistent with the photoelectric effect.

The Compton Effect However, Compton also observed the emission of lower energy/frequency x-rays.

The Compton Effect These x-rays were emitted at an angle to the ejected electrons (“scattered”).

Elastic Collisions? Compton proposed that the x-rays were acting like particles that were colliding elastically with the electrons:

Conservation of Momentum? But how can you have conservation of momentum if one of the particles is massless?

Conservation of Momentum? But how can you have conservation of momentum if one of the particles is massless? Photons do have mass equivalence:

The Momentum of a Photon

The Momentum of a Photon

The Momentum of a Photon

Example Calculate the magnitude of the momentum of a photon with a wavelength of 1.2 x 10-12 m.

Example Calculate the magnitude of the momentum of a photon with a wavelength of 1.2 x 10-12 m.

Example Calculate the magnitude of the momentum of a photon with a wavelength of 1.2 x 10-12 m.

Change in Wavelength The change in wavelength of the photon is given by the equation: (the derivation of which includes relativistic effects and is not given here).

So light and matter can interact in 5 different ways. . .

Possible Interaction #1 At low energies (visible light), photons can elastically collide with a surface and reflect.

Possible Interaction #2 At particular energies corresponding to the difference in energies between electron orbits, the light may be absorbed by the material. (more on this later)

Possible Interaction #3 At higher energies, the light may eject electrons from the material (the photoelectric effect).

Possible Interaction #4 At still higher energies, the light may not only eject an electron but also scatter off the material, losing energy and momentum in the collision.

Possible Interaction #5 At extremely high energies, the light may interact with nuclei and be transformed into matter: an electron and its antiparticle, the positron. This is called pair production. (more on this later too)

More Practice “The Photoelectric Effect Activity” Textbook Questions