This question is not really new. I have heard before and could only vaguely recall the statements. Now she was asked by a friend whose daughter asked many years ago after it (the daughter was then 4 years, now she's pretty grown up, but Günter thanks for the repetition of the exciting question). When I tried to give an answer to the question now, I only became clear that what is essential for comprehension, in my own lecture on Solid state physics occurs. But the things I've never combined, as it is necessary to answer this question.
I would like to discuss the question of an orderly solid before I come to why this is true largely for glass, although glass is known to be an amorphous material. An ordered solid is a crystal. All atoms (or molecules) are arranged in rows perfectly regularly. This means that only certain local vibrations of the atoms and electrons of the specific suggestions are possible.
light turn is an electromagnetic vibration, which propagates in space. The word I would like to avoid radiation here as it is now occupied largely negative. But (coming from the sun) without light we could not live at all. The light consists of electric and magnetic fields, which alternate each other and produce. However, light is not material. Light can not directly collide with atoms. Light can excite the electrons of an atom to vibrate and when the frequency or the energy fit, the light can be absorbed. Here, a light portion, which is a quantum of light, destroys a photon and its energy absorbed by the atom. Energy conservation must be considered essential, which is one of the fundamental laws of physics (as long as a conversion of energy into mass or vice versa takes place - in line with the general relativity theory - but it is another form of energy and mass conservation law is the right again).
light is quantized and can only be given in portions or recorded. Since we do not get around it. Einstein has said in the explanation of the photoelectric effect and get the Nobel Prize (probably out of embarrassment - the special and general relativity were too controversial). Max Planck was the first not to take too seriously even though he himself used to calculate the black body radiation, a quantization of the light has accepted, but it has proven to be absolutely correct and heralded the beginning of quantum mechanics. The quantum mechanics to the theory of relativity was the second major paradigm shift in the physics of the twentieth century. With the paradigm shift is meant that the foundations on which rests the whole of physics (actually, it rests not at all) must be changed.
with the theory of relativity it was realized that there is no single length scale and not a single time in space. Everything depends on the state of motion of each observer. Space and time are therefore not given and there are objects in it but, but space and time determined only by the objects themselves. Quantum mechanics states that both matter and light (small) portions, ie Quantum is available. At the same time but both also wave character. This represents a contradiction to classical physics, because you would have said something either particle or wave. But according to the understanding of modern physics have things both properties and, depending on the type of observation (the experiment) occurs either one or the other property in appearance. Then not even the Greek philosophers have come, even though the have thought through almost all mentally. The emphasis is on "thought" not to know. In quantum physics is still to the fact that certain properties of objects with a given probability can be predicted and, in principle not with absolute certainty, but we do not want to go further.
Back to the light. It is not just so easily say, therefore, that light is absorbed or released only in certain portions can - no, there is the realization of many scholars and a half century behind it. And these portions, and quantization is the deeper reason why glass is transparent.
The electromagnetic radiation can have almost any wavelength. For the human eye, wavelengths are in a very small range between about 400 and 700 nanometers (abbreviated nm) are visible. The short wavelengths are to the eye as a blue light, the long visible as a red light. The other colors in between, or are mixtures of different wavelengths. If we call the transparent glass, we think that going through these visible wavelengths, but it says nothing about other wavelengths. In fact, glass for most of the other wavelengths is not transparent. Why is this so?
Because of quantum physics, light can only be taken in certain amounts of energy. But even in a solid all states are quantized in the interior, even here, the laws of quantum physics. Because of conservation of energy must now be the amount of energy of a photon (light quantum) is just the energy difference between two states in the solid state correspond to the absorption is possible. And herein lies the problem. In an ordered solid state because of the similarity of the atoms and the order of the energy states are uniform. In the whole object only certain transitions between these states are not uniform and arbitrary. This is the big difference from a disordered solid. In this account of the different atoms and molecules and due to the different degree of interaction because of the disordered arrangement of such a large inner chaos is present, found that virtually all possible energy states and all possible energy differences. It can virtually every photon is absorbed in accordance with the conservation of energy. The order makes the difference. Which is
internal states it is? There are both common, mechanical vibrations of the atoms and the other, the states of the electrons in the crystal. The joint vibrations of the atoms are so-called lattice vibrations, and since these are quantized Enter the name of phonons. These all have low energy and can absorb only small amounts of energy per absorption process. We have the size of the energy levels of light have not discussed in detail. The energy depends only on the wavelength of light. The shorter the wavelength, the greater the energy, and the longer the wavelength the smaller the energy. The small amounts of energy of the phonons therefore lead to an absorption of light in the long-wave infrared range, which is not perceived by the eye.
The electronic states have also due to the regular arrangement of atoms, only certain energy states that are described with the so-called energy bands. In the possible states can have only a single electron (if the spin is also observed in different conditions) can be accommodated, also again a consequence of quantum physics (keyword antisymmetry of electron states, and Pauli's exclusion principle). If it is completely full in a crystal such energy band, so the electrons can be excited only in states in an empty energy band with a lot more energy and this absorbed photon is in the short wavelength ultraviolet region, which is perceived by the eye no longer as well. For the visible light there is in ordered solids, but no other internal suggestions that could absorb the energy levels of visible light.
As mentioned earlier, glass is not a perfectly ordered crystal, but an amorphous solid. But glass is not there so much confusion as you might think would. Glass is very similar to quartz, a beautiful perfectly ordered crystal, namely rock crystal. However, in glass, other atom are added, so some confusion arises. About something greater distances across the atoms no longer lined up and it would at first sometimes the opportunities are lacking, so that all electrons can be accommodated in a chemical bond in the interior. But with the addition of an appropriate proportion of hydrogen, can the remaining electrons form a bond and there are no free electrons present, which can be excited at visible energies. The lattice vibrations are largely as in a perfect crystal. Because of the confusion still reigns in the glass, come in contrast to the perfect crystal electronic excitations to a small extent even at somewhat lower energies and lattice vibrations also to a small extent even at somewhat higher energies before, but these only go so minimal into the visible portion of the light spectrum that glass is virtually transparent . The transparency of glass
is ultimately the fact that there are no matching energy levels inside there that could absorb the energy of light. And this can only be understood in that both are to describe the energy of the light and the internal states of Glass by the laws of quantum physics.
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