So what is so special about glass?

Glass is an amorphous, artificial, non-crystalline substance made by fusing at high temperature some form of silica, usually sand, and an alkali such as potash and soda. It has been made in one form or another since 3,500 B.C. (it is very brittle, as we all know, yet when drawn out into the thinnest fibre it has the almost unbelievable strength of 2,000,000 lbs per square inch). None of its ingredients is transparent, so why is the finished product transparent?

The key lies in the molecular structure of the fused compound. I said at the beginning that glass and bricks are both solids. In fact this is an understandable popular misconception. Glass, along with tar and plastics, is not strictly speaking a solid. They are what chemists describe as pseudosolids or amorphous solids. They do not have a melting point like all true solids, instead they become more and more viscous as more heat is applied; nor do they have the the other criteria of a true solid, a regular crystalline structure. What they have instead is a large complex molecule as their structural unit.

Thus glass is in reality an amorphous solid with its macromolecules trapped at random in lattice positions which are similar to the ever changing molecules in air or water. But unlike air and water there are strong inter-molecular forces holding them in place.

Now, to explain transparency. What an incoming photon of light 'sees' on encountering glass is a stationary mesh, a lattice of holes; on the photon's time scale glass and water flow at the same rate, that is, they appear stationary. What happens next is described by Richard Feynman, one of the 20th century's foremost physicists, as "a complete mystery": out of every 100 photons that go straight down at 90°, 96 pass through the surface of glass and 4 are reflected. The mystery being how does an individual photon 'know' whether to pass through or bounce back?

The common term for the 4% of photons bouncing back is partial reflection and you have seen it many time when looking out of a window from a lit room. The image of the light in the window is that 4% striking your eye. We could assume that the photons bouncing back have hit minute spots of reflective glass, due to surface irregularities, and the other 96% have passed through holes.

In my original article of April 2001, in the next paragraph, I said that:

Isaac Newton brilliantly deduced that this wasn't the case from the fact that he could polish glass from being opaque to a transparent clarity. What we term polishing is in reality just finer and finer scratches caused by using finer and finer abrasive powders, the finer the scratches and the more of them the more light passes through the glass not less. From this Newton reasoned that a theory of surface irregularities as the cause of partial reflection is simply not tenable.

In April 2016 Michael A. Gottlieb, Caltech Visitor in Physics and editor of The Feynman Lectures on Physics, New Millennium Edition, kindly pointed out to me that this was not correct and I am grateful to him for taking the time to address this.

What Newton actually stated in his Second Book of Optics. Para 31 (following proposition VIII, was:

“Lastly, were the rays of Light reflected by impinging on the solid parts of Bodies, their reflexions from polished Bodies could not be so regular as they are. For in polishing Glass with Sand, Putty or Tripoly, it is not to be imagined that those substances can by grating and fretting the Glass bring all its least particles to an accurate polish; so that all their surfaces shall be truly plain or truly spherical, and look all the same way, so as together to compose one even surface. The smaller the particles of those substances are, the smaller will be the scratches by which they continually fret and wear away the Glass until it be polished, but be they never so small they can wear away the Glass no otherwise than by grating and scratching it, and breaking the protuberances, and therefore polish it no otherwise than by bringing its roughness to a very fine Grain, so that the scratches and frettings of the surface become too small to be visible. And therefore if Light were reflected by impinging upon the solid parts of the Glass, it would be scattered as much by the most polished Glass as by the roughest. So then it remains a Problem, how Glass polished by fretting substances can reflect Light so regularly as it does. And this Problem is scarce otherwise to be solved than by saying, that the reflexion of a ray is effected, not by a single point of the reflecting Body, but by some power of the Body which is evenly diffused all over its surface, and by which it acts upon the ray without immediate contact. For that the parts of Bodies do act upon Light at a distance shall be shewn hereafter.”

Michael continued: “So, what Newton thought is that if light behaved like particles that interact only locally with the part of its surface where they collide, then it should be reflected ("scattered") _the same_ amount from smoothly polished glass as it is from roughly polished glass; that is to say he reasoned that _the same_ amount of light should pass through smoothly polished glass as through roughly polished glass (if light interacted only locally with the surface of the glass), and he never wrote or implied that "the finer the scratches and the more of them the more light passes through the glass not less," which would be contrary to his reasoning (and hard to explain).”

Returning to the text of my article:

Newton thought that light consisted of small particles which he termed corpuscles and which we now know of as photons. But since light corpuscles could not explain the phenomenon of refraction and since Huygens wave theory of light appeared to account for it, Newton's corpuscule theory was dropped for the next two centuries. In 1905 Einstein made a bold hypothesis, since confirmed repeatedly by experiment, that light consisted of discreet bundles which he termed light quanta, which we now call photons. From this the theory arose of the dual wave/particle nature of light, which you will still see expounded in text books. Feynman, however, in his development of Quantum Electrodynamics has established beyond doubt that light consists solely of photons. As for our 4% of photons bounced back from the glass, we are still no nearer knowing why it happens.

Aside from this, of the 96 photons that finally emerge from the glass most will have originally entered the glass; but many will have struck atoms within the glass and be absorbed causing cascades of fresh photons and electrons to emerge and strike other atoms in a chain reaction, all at near the speed of light, until our 96 emerge precisely. But here there is another surface and another reflection.

At this point events are even more weird. We might expect that a further 4% of the passing light is reflected at this second surface, making a total of 8%, but this depends entirely on the thickness of the glass!

If we experiment with the thinnest possible layer of glass, i.e. one molecule thick, we find that 0% of light is reflected, but as we increase the thickness so the amount of reflected light increases. At about 5 millionths of an inch thickness 16% is reflected (16 photons out of every hundred). But then as we increase the thickness more the percentage of reflected light decreases until it reaches zero, then it increases again until it reaches a maximum again of 16%, and so it continues, 0%  1%  2%  3%  4%  5%  6%  7%  8%  9%  10%  11%  12%  13%  14%  15%  16%  15%  14%  13%  12%  11%  10%  9%  8%  7%  6%  5%  4%  3%  2%  1% to 0% to 16% to 0% to 16%... and so on.

With pure monochromatic laser light this sequence has been experimentally repeated with the cycle still going strong after 100 million repetitions, which equals a glass more than 50 metres thick! The average for all glass, including everyday panes of window glass of every conceivable thickness is 8% (the average of 0 to 16), but individual glasses vary between those limits. The viewing ports of deep ocean submersibles are generally made of truncated cones of clear acrylic plastic which take advantage of this, and are set at a thickness to give the clearest view.

We are not readily aware of this phenomenon because we don't meet with pure monochromatic light in our daily activities. Looking at a star through a telescope with ultra efficient lenses, compouded of different materials, reflective losses might only amount to 0.4%, or 4 photons in every 1,000. Of the photons emerging to strike our eye the majority of photons will have left the star billions of years ago but a small proportion (amounting to many millions) will have been created within the glass by photons striking electrons in chain reactions, all merging to match perfectly.

Although no-one fully understands how this happens we should be thankful that it does, for without glass and its mysterious interaction with light our knowledge of the universe would be abysmal.

Peter Ghiringhelli, B.A.(Hons), M.A.

For a much clearer exposition, lucidly written and without mathematics, of how light interacts with glass you should read QED - The Strange Theory of Light and Matter by Richard P. Feynman, first published in the USA in 1985 by Princeton University Press, and in the UK by Penguin Books in 1990.