The Propagation of Light
Analysis of the propagation of light today still uses Huygens’ principle, first published in 1678, which states that that “all points of a wave front of light in a vacuum or transparent medium may be regarded as new sources of wavelets that expand in every direction at a rate depending on their velocities”.
“Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light. The new wave-front is the tangential surface to all of these secondary wavelets.” (Wikipedia)
And later in 1816, Fresnel showed that Huygens’ principle, together with his own principle of interference could explain both the rectilinear propagation of light and also diffraction effects.
To obtain agreement with experimental results, he had to include additional arbitrary assumptions about the phase and amplitude of the secondary waves, and also an obliquity factor. These assumptions have no obvious physical foundation but led to predictions that agreed with many experimental observations. (Wikipedia)
There are two important points here as highlighted, firstly “These assumptions have no obvious physical foundation” in other words it is assumed that there is no material or physical medium that can be a vehicle for the propagation of such wavelets, and secondly “in a vacuum”, which accordingly has to assume that such wavelets of light can actually propagate in a vacuum.
“Every point on a wave-front may be considered a source of secondary spherical wavelets” in effect this means that these are points on a wave front in a vacuum that are the origin of secondary wavelets.
But if Huygens’ principle agrees “with many experimental observations” then there must be a “physical foundation” the propagation of these wavelets, but in terms of current atomic theory there can be no such foundations.
Below is a copy of a textbook diagram of Huygens’ principle.
This diagram below shows Huygens wavelets propagating through a ‘kinetic’ gas, and it is clear that there can be no interaction with kinetic atoms (moving at average velocities of up to 900 metres a second) to facilitate the transmission of light, while the propagation of waves through the inter-atomic vacuum/empty space is also not possible.
However if the images of atoms at the surfaces of solid matter (as discussed here ) demonstrate that atoms are continuous, and this continuity extends to the gaseous state, as in the diagram below, then it is quite clear that such an arrangement can allow the transmission of waves of light.
The outlines of atoms in this diagram, as depicted by the hexagonal outlines, are of course idealised as in reality a plane cross section of atoms would not form such a regular geometric arrangement, but it is quite clear that such a continuum of atomic matter could transmit spherical wavelets of light directly from atom to atom, and that each wavelet or combination of wavelets impacting on an atom can stimulate it to generate another wavelet onward.