A Light Perspective

lightToday, a science book may well describe light in terms of a transverse electromagnetic wave operating within a certain frequency range,  which is visible to the human eye, but  exists within a much larger electromagnetic spectrum that is not. However, in another book, light may be described in terms of a stream of particle-like photons that represent discrete quanta of energy at a specific frequency.  However, this apparent paradox, often referred to as the wave-particle duality of light, is not the primary focus of this section, as it is a topic for discussion under the heading of ‘Quantum Theory. As such, this section will focus on the description of light within the restrictions an electromagnetic wave.

In 1678, Christiaan Huygens published a paper called ‘Traité de la Lumiere’ in which he argued in favour of the wave nature of light. Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase. Unfortunately for Huygens, Isaac Newton disagreed with him and argued for what is known as the ‘corpuscular theory of light’ and Newton's reputation was enough to sway most to accept his theory. As such, the issue that has become known as the wave-particle duality with quantum physics, actually has a much longer history. While Newton and Huygens disagreed on the nature of light, it is not often realised that both assumed that the wave and the corpuscle required some form of ether through which to propagate. Within Newton’s theory, the existence of the ether was required to transmit forces between the particles of light, while Huygens’ theory needed the ether to act as a propagation medium.

In the previous section, we discussed the nature of mechanical waves that this section now attempts to expand in order to identify the differences between a mechanical wave, which depends on some form of physical medium, and light waves that appear to self-propagate and are therefore capable of travelling vast distances through the vacuum of space. In order to make this comparison, we need to discuss the subject of electrodynamics in the form of Maxwell’s equations, which were first published in 1864, although not in the form we would necessarily recognise today. In many ways these equations have come to represent the transition from classical physics in the form of Newtonian laws of motion and gravitation into what we might describe as modern science with all its mathematical abstractions.

The Perception of Light

  • The wave nature of light is often argued based on the observations of diffraction and interference and it apparent transverse nature in respect to the effects of polarization.

  • Light is produced by one of two methods. Incandescence is the emission of light from hot matter, i.e. temperatures greater than 800K, while luminescence is the emission of light when excited electrons fall to lower energy levels.

  • The speed of light in a vacuum is represented by the letter [c] coming from the Latin word ‘celeritas’ meaning swiftness.

  • Measuring the speed of light was first, albeit unsuccessfully, attempted by Galileo based on 2 points less than 1 mile apart. However, in 1676, Ole Christensen Rømer was observing the transits of Jupiter's moon ‘IO’ and determined that the times between eclipses got shorter as the Earth approached Jupiter, and longer as Earth moved further away. He hypothesized that this variation was due to the time it took light to travel the varying distance and estimated that the time for light to travel the diameter of the Earth's orbit was 22 minutes.

  • Today, the speed of light in a vacuum is fixed at 299,792,458 m/s and is considered to be a universal constant in all reference frames. However, the speed of light in a medium is always slower the speed of light in a vacuum and depends on the characteristics of the medium.

  • The amplitude of a light wave is related to its intensity. Intensity is considered to be an absolute measure of a light wave's power density, while its brightness is the relative intensity as perceived by the average human eye.

  • The frequency of a light wave determines its colour, although frequency is also directly proportional to its energy. Visible light can be extended to include ‘ultraviolet light’ and ‘infrared light’, although even collectively, they represent only a small fraction of the electromagnetic spectrum.

  • Phase differences between light waves can produce visible interference effects.

As indicated, in the context of the development timeline of foundation science, the focus of this entire section will be orientated towards a description of light as an electromagnetic wave. However, in terms of this opening introduction it might be worth simply outlining some of the analomies that may require further consideration when eventually describing light, either as a wave or as a photon or both. One common attribute shared by all these descriptions is the speed of light [c=3*108m/s], although this this speed is specific to a vaccum, which can be reduced when passing through a transparent medium, such as a gas or glass. However, while there is empirical agreement on this speed, the mechanism by which this slow-down occurs appears to differ in the details of each model, i.e. wave or photon. For example, within the photon model, it appears to be suggested that a photon always travels at [c], which can be delayed due to collisions i.e. absorption and emission, within the atoms and molecules of the material. In these terms, it is assumed that the idea of a photon slowing down due to the refractive index of the material must be a statistical average of the time for [n] photons to pass through the material. In contrast, within the EM model, it appears to be suggested that the EM wave's propagation velocity is slowed within a material due to the disturbance caused by the wave's own electrical field as it propagates pass charged particles within the material on route. Typically, these particles will be electrons rather than protons due to the large difference in mass-energy and this effect is sometimes described in terms of the electric susceptibility of the medium. By a similar argument, the magnetic field of the EM wave also creates a disturbance proportional to the magnetic susceptibility of the medium. So, as the electromagnetic fields oscillate within the EM wave, charge particles in the material also resonant at the same frequency. As such, there is a superposition of different oscillating fields with the same frequency, but not necessarily the same phase. As a consequence, a resulting superposition wave may have the same frequency, but a shorter wavelength, which results in slower phase velocity [vp=fλ]. At this stage, these differences are only being highlighted to provide a back-drop to some of the wider implications extending beyond the scope of foundation science.