Transparency versus opaqueness - why?

Factual Questions
1

A basic but very provocative concept about apparently solid matter is that the vast majority of its volume is composed of “empty” space. What we feel and see as solid, then, is simply a manifestation of electromagnetic fields/forces. Likewise, I aasume, an object’s transparency or opaqueness must also depend on those fields and forces. Is that right?

Indeed, I don’t understand why some forms of electromagnetic radiation penetrate the “empty” space of matter (i.e. the matter’s electromagnetic fields) with relative ease, while others cannot get through at all. Examples of the former, with respect to, say, a piece of paper, include x-rays (short wavelengths) and radio waves (long wavelengths). The prototype example of the latter is visible light which has a wavelength between X-rays and radio waves. Given this, it seems reasonable to conclude that the ability of an EM wave to pass through matter is not entirely dependent on its wavelength.

Question 1: What is it about photons of visible light that prevents them from penetrating the “empty” space of most ordinary matter?

On the other hand, some forms of matter do allow visible light to pass through, e.g. glass.

Question 2: What is it about glass and other transparent materials that allows visible light to pass through?

and, most generically, Question 3: Is there a relationship between the electromagnetic fields/forces that constitute the “empty” space of a given type of matter and the ability (or not) of a particular type of EM radiation to pass through it?

Thanks!

2

Here’s the relevant article by Mr. Adams to get you started:

http://www.straightdope.com/classics/a1_120.html

3

As a physics/EE guy, I have some problems with Cecil’s answer, as it is rather misleading as to the “why”. There are plenty of non-glass structure materials that are quite transparent, and glass isn’t transparent because of it’s glass solid state but instead because of the inherent electromagnetic characteristics of it as a particular material. Cecil’s column would lead one to believe that a diamond, for example, could not be transparent.

In fact, glass is only transparent for certain wavelengths, and is highly reflective for others. The boundary layer also plays a huge role (e.g. is the light going from air to glass to air, or from water to glass to water). All materials share the property of being transmissive for certain wavelengths and reflective for others, and combinations of both.

The underlying math is unfortunately quite hairy and requires diving into Maxwell’s Laws or Quantum Mechnics, but Wikipedia is a decent beginning place.

Physics of Reflection

Physics of Opacity

Short answer for “why” is that “because we observe it to behave so”. Then we build piles of mathematical models to reproduce the behavior.

4

…and to follow up more directly to your Question 3: Yes.

Whether something reflects or transmits depends on essentially 5 factors

  1. Property of medium the light is traveling in
  2. Property of medium the light is traveling to
  3. Angle of incidence between the light and the boundary layer
  4. The distance to the next boundary layer (i.e. how thick is the pane)
  5. Property of the light (wavelength and polarization)

It is a mathematical tug of war between all of these that determines the final result that we perceive.

There are additional “weirdnesses” that come into play with surfaces. For example, if the scale of roughnesses in the surface or the thickness of the medium (thin films like soap bubbles) approach the wavelengths of your light, then additional funny results occur.

5

FWIW a few comments:

Some materials are opaque and reflect light because they are conductors. Metals, generally, are working this way. They do the same thing with microwaves, which is why no microwaves come out of a microwave oven with a metal interior, and also why they can make a parabolic mirror to focus radio waves on a small antenna by making a metal dish (your satellite TV antenna, for instance).

Generally, photons don’t go through the empty space of any solids or liquids. Bear in mind that this is a somewhat contorted discussion, because on the size scale of our experience balls can be thrown through the empty space in a forest, but the language we developed to express situations in our experience is fairly inappropriate on the size scale of the empty space in matter. But, wavelengths of visible light are a few hundred nanometers in size, and the distance between atomic nuclei in a solid are a few hundred picometers. So, the empty spaces are something like 1000 times smaller than the wavelengths.

6

“It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.” – Neils Bohr

As for opacity and transparency, it can be easily and quantifiably explained in terms of quantum electrodynamics; however, only for very simplified and idealized scenarios. Real world constructs are so complicated that our explanation is limited to qualitative discussions, i.e. glass is transparent because the visible wavelengths of light can flit through with only a small part of it being absorbed or reflected.

Stranger

7

Thank you all for your comments.

Alas, but as predicted, the underlying mathematics, even as presented in the Wikipedia articles, is too formidible for an ignoramus such as myself to penetrate. On the other hand, I found Napier’s statement regarding the actual sizes involved very helpful. In fact, a lot of my confusion evaporated simply by knowing that the wavelengths of visible light are much greater than the internuclear distance. Given that, it’s no surprise they can’t “fit” through.

This ‘simple’ fact does raise another question, though: if the typical internuclear distance in glass is also so small compared to the wavelengths of visible light, then why is glass transparent?

8

Short answer: because it doesn’t reflect or absorb light.

Somewhat longer answer: Visible light mainly interacts with matter by excitation of electron energy levels in atoms. If an incident photon has an energy comparable to that of a permissible excitation state of an atom, it can be absorbed, causing a valence electron to transition to a higher energy level. Because these energy levels are different for every given electron configuration (atom/molecule), different materials absorb different wavelengths, giving rise to what we perceive as colour – if a material absorbs blue, green and yellow, it will appear red.

Reflection in a conductor (e.g. a metal) is caused by free electrons, since those free electrons interact with electromagnetic radiation in such a way that causes them (essentially, the E-field vector of the incident light wave causes the electrons to oscillate) to emit electromagnetic radiation themselves, in such a way that we obtain the familiar law of reflection (angle of incidence = angle of reflection).
Reflection also occurs at each discontinuity in the transmitting medium, because such a discontinuity forms a barrier to the wave propagation (the speed of the wave/wavelength will be different, leading to Snell’s law). However, in this case, generally only a partial reflection occurs.

Since glass is an insulator (a dielectric), it has no free electrons, and thus only reflects partially in dependence of the angle of incidence. The rest of the light is transmitted. Because glass doesn’t absorb that transmitted light, it is transparent.
However, this holds true for a great many of substances that still aren’t transparent. The answer to this lies in their crystalline structure: throughout the material, their exist ‘fault lines’, where the planes of the crystals don’t line up. This, however, constitutes a barrier to light wave propagation, and thus causes partial reflection. Across a given macroscopic width of material, there’ll usually be enough of those fault lines to render the material opaque. A single crystal of this material, then, would be transparent.

Glass’ molecular structure differs from these materials due to its amorphous nature – it’s solid in the sense that its molecules don’t move terribly much, and liquid in the sense that they’re not neatly ordered. This ‘not neatly ordered’-ness makes it such that there are essentially no barriers to impede the movement of the light, and thus, it gets transmitted.

9

Very clear (pardon the pun!) and very helpful. Thanks!