The Nature of Light

Today, we are aware of the paradox caused by the wave-particle duality description of light. However, in many ways, the following table suggests that the characteristics of light might appear to align better to a wave nature rather than a particle nature:

Effect Wave Particle
Reflection Yes Yes
Refraction Yes No
Interference Yes No
Diffraction Yes No
Polarization Yes No
Photoelectric No Yes

In this subsection, as a whole, we shall try to introduce some of the basic attributes and behaviour of electromagnetic waves. As indicated, this discussion will be biased towards a wave description because this was the consensus of scientific opinion within the timeline under consideration, i.e. 19th century classical physics. As such, the main discussion of light, as a photon stream, will be deferred until the discussion of ‘Quantum Theory, although some aspects of the particle nature will be touched upon throughout this discussion.

So what is the nature of a light wave?

For most of human history, the scope of the electromagnetic spectrum was constrained to our physiological sense of sight, which only allowed us to `see` the visible spectrum of light, i.e. red through violet. Initially, the early Greeks had believed that everything was composed of four elements, i.e. fire, air, earth and water, including the human eye. This idea led to the notion that there was a ‘fire’ element within the eye that shone outwards and illuminated the world for that person.  However, as early as 300 BCE, Euclid had mathematically postulated that light travelled in straight lines and outlined the basic laws of reflection in his work ‘Optica . He also indirectly challenged the earlier idea that a light shone out of the eye by asking the following insightful question:

How can we immediately see light from the stars on opening our eyes?

Later, in ~60 CE, Heron is attributed with the observation that light reflected by a mirror appears to travel along a path of least length. Some 80 years later, Ptolemy’s accurate measurement of star positions led him to realise that light was being refracted by the atmosphere. However, between 1011-1021, the Arabic scholar al-Haytham's wrote 7 volumes on optics, which some have ranked along side Newton's Principiaas one of the most influential works in early physics. This work would eventually trigger a revolution in optics, and the understanding of visual perception, based on the argument that sight was due, not to an inner light, but rather to light entering the eye. He even used the basic idea of the pinhole camera to support his arguments. Of course, today, we now understand that the nature of light has to be positioned, and described, within the much wider remit of the electromagnetic spectrum.

spectrum

Based on the diagram above, the human perception of light does not even account for 1 order of magnitude of the total electromagnetic spectrum, which covers more than 18 orders of magnitude. However, we will begin, as we did for mechanical waves, by first trying to outline some of the basic attributes of a wave that can be applied generally to all electromagnetic waves. After this, we shall try to extend the discussion of the wave nature of light to cover some of the basic behaviours listed in the table above. While these observed behaviours will ultimately have to be discussed within the wider scope of the wave-particle duality debate, it can be seen from the table above that 4 of the 6 behaviours can still be more easily described in terms of a wave nature. Again, the issue of photoelectricity only became apparent after Einstein’s publication on this subject in 1905, although aspects of the wave model can also be shown to be problematic, even within the confines of Maxwell’s equations.