Optics and the nature of light [1]
Before moving on to consider Planck's and Einstein's hypotheses that radiation and light are composed of discrete packets - "quanta", we must say say something about light itself and the way it was perceived by scientists in the years beforehand. This topic falls under the generic rubric of optics - that branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it.
Optics usually describes the behaviour of visible, ultraviolet and infrared light, but because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves and radio waves exhibit similar properties.
Most optical phenomena can be accounted for using the classical electromagnetic description of light. However, complete electromagnetic descriptions of light are often difficult to apply in practice. Practical optics is usually done using simplified models:
The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics, on the other hand, is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light.
Progress in electromagnetic theory in the 19th century led to the discovery that light waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both wave-like and particle-like properties, the explanation of which requires quantum mechanics. When considering light's particle-like properties, light is modelled as a collection of particles called photons.
Newton’s corpuscular theory of light
In the late 1660s and early 1670s, Isaac Newton developed a corpuscular theory of light, famously determining that white light was a mix of colours which can be separated into its component parts with a prism.
Then in 1690 Christiaan Huygens, proposed a wave theory for light based on suggestions that had been made by Robert Hooke in 1664. Hooke himself publicly criticised Newton's theories of light and the feud between the two lasted until Hooke's death.
In 1704, Newton published Opticks and, at the time, partly because of his success in other areas of physics and he was generally considered to be the victor in the debate over the nature of light.
The wave theory of light
Newtonian optics was generally accepted until the early 19th century when Thomas Young and Augustin-Jean Fresnel conducted experiments on the interference of light using the former's famous double-slit experiment that firmly established light's wave nature. This work led to a theory of diffraction for light and opened an entire area of study in physical optics. Wave optics was successfully unified with electromagnetic theory by James Clerk Maxwell in the 1860s. |
The exchange of energy in discrete amounts in blackbody radiation
The next development in optical theory came in 1901 when Max Planck correctly modelled blackbody radiation[2] by making an ad hoc assumption that the exchange of energy between light and matter only occurred in discrete amounts he called quanta. Planck hypothesised that the frequency of light emitted by a black body depended on the frequency of the oscillator that emitted it, and the energy of these oscillators increased linearly with frequency according to his constant h, where E = hν. [3] |
Einstein and the quantisation of light itself.
In 1905, taking Planck's black body as his model, Einstein proposed that it is the electromagnetic radiation itself that is quantised, and not the energy of radiating atoms, and used this to produce his solution to another outstanding problem of the day: the photoelectric effect, a hitherto troubling experiment that the wave theory of light seemed incapable of explaining, whereby electrons are emitted from atoms when they absorb energy from light. in so doing, he firmly established the quantisation of light itself. [4] |
Niels Bohr – atoms can only emit discrete amounts of energy
In 1913 Niels Bohr showed that atoms could only emit discrete amounts of energy, thus explaining the discrete lines seen in emission and absorption spectra. This is explained in more detail on the page Particles and forces |
de Broglie and the wave-nature of matter
In 1924, the French nobleman Prince Louis-Victor de Broglie formulated a hypothesis that all matter, not just light, has a wave-like nature. In other words, both matter and light can therefore have both particle and wavelike properties. The relationship between Planck’s and Einstein’s hypothesis and that of de Broglie dealing with the wave nature of a particle of matter is as follows:
- For the energy of the particle (photon) of light: E = hf (Planck’s hypothesis) where E is the energy, f is the frequency (= c/λ where λ is the wavelength) and h is a proportionality factor (Planck’s constant).
- For the wavelength of a particle: λ= h/p (de Broglie’s hypothesis) where p is the momentum and h is the same Planck’s constant[5].
In formulating his hypothesis, de Broglie had taken a result inherent in an equation from Einstein’s Theory of Relativity and turned it around to produce a formula whereby one could obtain a particle’s wavelength of in terms of its momentum. He was awarded the 1929 Nobel Prize in Physics for his insight into the wave nature of electrons.
The wave nature of particles was first seen (accidentally) by Davisson and Germer in 1926 while they were performing a diffraction type experiment using a beam of electrons whose momentum was known and hence their wavelength predicted. However, they obtained diffraction patterns similar to those obtained using X-rays.
But what actually is wave-particle duality?
This is the question posed by Associate Professor Michael Box [6]. It means that:
“if you conduct and experiment to look for wave-like properties (eg diffraction), you will find wave-like properties. But if you conduct an experiment to look for particle-like properties (eg collisions) you will find particle-like properties. … The answer is that at human scales, everything usually is particle or wave – black or white. However, at atomic scales that distinction no longer applies – the two coexist. We must be very careful not to take our preconceptions with us when we enter a world ten orders of magnitude smaller than our own”. On wave-particle duality, see also the page entitled Double slit and the useful animated graphic which there appears.
This is the question posed by Associate Professor Michael Box [6]. It means that:
“if you conduct and experiment to look for wave-like properties (eg diffraction), you will find wave-like properties. But if you conduct an experiment to look for particle-like properties (eg collisions) you will find particle-like properties. … The answer is that at human scales, everything usually is particle or wave – black or white. However, at atomic scales that distinction no longer applies – the two coexist. We must be very careful not to take our preconceptions with us when we enter a world ten orders of magnitude smaller than our own”. On wave-particle duality, see also the page entitled Double slit and the useful animated graphic which there appears.
These discoveries form the foundation of quantum mechanics
The understanding of the interaction between light and matter which followed from these developments not only formed the basis of quantum optics but also was crucial for the development of quantum mechanics itself. The story is told in the ensuing pages. |
Quantum electrodynamics
The ultimate culmination, the theory of quantum electrodynamics, explains all optics and electromagnetic processes in general as the result of the exchange of real and virtual photons. [7] |
[1] Unless otherwise stated, this is an edited summary of the material which appears on the site https://en.wikipedia.org/wiki/Optics
[2] In physics, a blackbody is a surface that absorbs all radiant energy falling on it. The term arises because incident visible light will be absorbed rather than reflected, and therefore the surface will appear black.
[3] https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality
[4] Ibid.
[5] http://www.physics.uwo.ca/~mgc/Mat%20Sci%20notes-1.pdf (University of Western Ontario), Physics 2800 course outline notes (2nd term)
[6] In What are atoms composed of? WEA course, Session 1, 26 October 2016.
[7] Quantum electrodynamics is considered in the page Feynman's sum over paths - a more detailed analysis