Introduction to Quantum Physics

An Introduction to Quantum Physics is the first lecture within the Quantum Phenomena section of PH1012. It covers a brief history of quantum concepts and the Ultraviolet Catastrope.

Previous: N/A

Next: The Photoelectric Effect

A Brief Timeline
Prior to the nineteen hundreds, physics operated entirely on a classical understanding of the world. However as these theories began to disagree with experimental results a new set of observations began to fit together to form our current understanding of physics with quantum effects. The most prominent of these were as follows:


 * in 1900, Planck noted that the energy of electromangetic waves is quantised (only exists with discrete energies). He used this to explain the theory behind thermal radiation.
 * in 1905, Einstein proposed that light waves exist as a collection of individual particles (later christened photons). This was used to explain the photoelectric effect.
 * in 1913, Bohr proposed a new structure of the hydrogen atom. Although Bohr's model is understood now to be flawed, it is still used as a good model of a simple atom.
 * in 1923, De Broglie proposed that matter can behave as waves do (a likewise to the earlier understanding that light waves can behave as particles).

Beyond then Schrödinger and Heisenberg developed further important theories, and physics moved into the quantum age.

The Ultraviolet Catastrope


All bodies can be observed to emit radiation. The concept of a "blackbody" is an object which is a perfect radiator - and an emission spectrum for any hot body can be drawn. Bodies that are relatively warm have peak wavelengths in the visible spectrum, which causes them to initially glow red through to blue. However as the temperatures are increased this peak shifts toward lower wavelengths (although the overall light being produced causes the object to glow white as enough of the visible spectrum is present). Classical physics fails here as it did not predict a peak wavelength but rather that for any temperature therer would be an exponentially larger quantity of UV rays produced.

Quatisation of energy can be described by the relationship E = nhν, n being an integer, h Planck's constant and v the radiation's frequency.

Summary
Classical physics was debunked to a large extent in the early nineteen hundreds. Energy quantisation, light as particles and matter as waves are all theories born of quantum physics.