Particle Theory of Light

See also: X-Rays

Particle Theory of Light is the third lecture within the Quantum Phenomena section of PH1012. It covers the wave-particle duality of light.

Previous: The Photoelectric Effect

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Light as a wave
Maxwell held the theory that light exists as an electromagnetic wave, backed up by phenomena such as Young's slits. Plotting the value of an electric or magnetic field by time or position shows a progressing sine graph, with wave energy proportionate to the light intensity and the squared amplitude of the electric field. This gives wave energies which are not quantised.

Xray tubes
Xray tubes produce xrays of approximately 1 Angstran wavelength by eatinng a coiled filament. This input of energy releases electrons through the thermoionic effect (heat causes ionisation) which are accelerated via a potential difference to very high speeds until they strike the given target. This strike causes the electrons to decelerate abruptly, releasing their kinetic energy as a photon. Quantisation is shown here by the fact that xrays can be released as single photons rather than a continuous stream.

The spectra produced show a curve with a distinct cut-off point and irregualr sharp peaks. The cutoff point is the minimum wavelength of xrays that can be produced, given by c/vmax, where hvmax is the total energy of incoming electrons. These xrays are caused by impact of electrons which lose all energy in a single impact. The rest of the xrays emitted have a lower energy as their electrons have lost energy through multiple collisions. The peaks are specific to the electronic configuration of the target material - when electrons strike a nucleus directly they cause an atmoc electron to be ejected. This leaves a gap in the shell which must be filled by a higher electron dropping down, which has an associated release of energy.

Summary
Phenomena such as Young's slits show light to be a wave. Phenomena such as xray tubes show light to be a particle. Light can be emitted from atoms by inputting energy from electrons at high speeds.