The main distinguishing feature of glass is its transparency. And, probably, many people wondered: “Why does it have such a property?” Indeed, thanks to this quality, glass has become widespread and widely used in everyday life.

If you delve into this topic, then it may seem rather difficult and incomprehensible to most people, since many physical processes are affected in such areas as optics, quantum mechanics and chemistry. For general reference, it is better to use a lighter narrative language that will be understandable to many users.

So, it is known that all bodies consist of molecules, and molecules, in turn, are made of atoms, the structure of which is quite simple. At the center of an atom is a nucleus consisting of protons and neutrons, around which electrons revolve in their orbits. Light is also quite simple. It is only necessary to imagine it as a stream of photon balls flying out of a flashlight, to which our eyes react. If you put a concrete wall between the eyes and the flashlight, the light will become invisible. But if you shine a flashlight on this wall from the side of the observer, you can see how the rays of light are reflected from the concrete and again fall into the eyes. It is quite logical that the photon balls do not pass through the concrete barrier due to the fact that they hit the electrons, which move at such an incredible speed that the photon of light cannot penetrate the electron orbits to the nucleus and, as a result, is reflected from the electrons.

Also on the topic: Why does the foam turn yellow?

However, why does light penetrate glass barriers? After all, inside the glass there are also molecules and atoms. If we take a fairly thick glass, then a flying photon must necessarily collide with them, since there are simply an unmeasured number of atoms in each grain of glass. In this case, everything depends on how the collisions of electrons with photons occur. For example, when an electron rotating around a proton is hit by a photon, then all of its energy is transferred to the electron. The photon is absorbed by it and disappears. In turn, the electron receives additional energy (the one that the photon had) and with its help moves to a higher orbit, thus starting to rotate farther from the nucleus. Usually, distant orbits are less stable, so after a while the electron releases the taken particle and returns to its stable orbit. The emitted photon is sent in any arbitrary direction, after which it is absorbed by some neighboring atom. It will continue to wander in the substance until it radiates back or eventually goes, as in this particular case, to heat the concrete wall.

Also on the topic: Why does soap lather?

It is important that the electron orbits are not randomly located around the atomic nucleus. The atoms of each chemical element have a well-defined set of levels or orbits, that is, an electron is not able to go up or down. It has the ability to jump only a clear gap down or up. And all these levels differ in different energies. Therefore, it turns out that only a photon with a certain, precisely specified energy is able to direct an electron to a higher orbit.

It turns out that among the three flying photons with different energy charge indices, only one will dock with an atom whose energy will be exactly equal to the energy difference between the levels of a single specific atom. The rest will fly by and will not be able to give the electron a given portion of energy to enable it to move to another level.

The transparency of glass is explained by the fact that the electrons in its atoms are located in such orbits that their transition to a higher level requires energy, which is not enough for a photon of visible light. For this reason, the photon does not collide with atoms and passes through glass quite easily.

Also on the topic: How to enhance hydrolysis?

Let's say right away that the statement that the more powerful and brighter the light source, the more energy the photons will have is incorrect. Power depends on more of them. The energy of each individual particle of light is the same. How to find photons with different energy charges? To do this, we must remember that light is still not just a stream of particle balls, it is also a wave. Different photons differ from each other by different wavelengths. And the higher the frequency of oscillations, the more powerful the particle carries a charge of energy. Low frequency photons carry little energy, high frequency photons carry a lot. The former include radio waves and infrared light. The second is X-rays. The light visible to our eye is somewhere in the middle. At the same time, for example, the same concrete is transparent for radio waves, for gamma radiation and infrared radiation, but is opaque for ultraviolet, X-ray and visible light.

07.02.2017 15:49 850

Why is glass transparent?

Glass is a very important material that a person uses in different areas of life. Windows, dishes, mirrors, lenses for glasses, etc. are made from it ...

Just imagine: you are returning from school and find that there are no glasses in the windows of your apartment. Disappeared also from the house and all the glassware. You want to look at your surprised face in the mirror, but it was not in place either ... And we would not have many other useful things now if glass had not appeared at one time.

In our article, we will tell you the history of glass, how it entered our lives and why it is so transparent. Who invented this useful, fragile material? Oddly enough, no one. The fact is that glass was created by nature itself.

Once upon a time, many millions of years before the appearance of the first man on earth, glass already existed. And it was formed from first red-hot, and then cooled lava, which escaped to the surface from volcanoes. This natural glass is now called obsidian.

However, they could not glaze, for example, windows. And not only because there were no windows then, but also because natural glass has a dirty gray color, and absolutely nothing can be seen through it.

So how did glass suitable for consumption, that is, transparent, appear? Maybe people have learned to wash it? Alas, natural glass is dirty not from the outside, but from the inside, so even the most modern detergents will not help here ...

There are several legends about how people first made glass close to modern glass. All of them are very monotonous and their meaning boils down to the fact that travelers, not having stones at hand for the hearth, used pieces of natural soda instead.

Moreover, this happened in the desert or on the shore of a reservoir, where there was necessarily sand. And so, under the influence of fire, soda and sand melted and combined together, forming glass. People have believed in these legends for a long time. But quite recently it turned out that all this is not true, because the heat emanating from the fire is not enough for such an alloy.

People began to produce glass with their own hands more than 5 thousand years ago, it was in Egypt. True, even then it was not transparent, but due to the fact that foreign impurities came across in the sand, it had a green or blue tint. But gradually in the East they learned to get rid of these impurities. Judging by the excavations, the first glass items were beads.

A little later, glass began to cover the dishes. And it took another 2,000 years to learn how to make it entirely out of glass. The secret of glass production was so valuable in those days that the government of Venice at the beginning of the 13th century sent special people to the east to find out. As a result, the Venetians got this secret.

They set up their own production and were able to make the glass even more transparent, guessing to add a little lead to its composition. At first, glass was made in Venice itself. The local authorities were very afraid that someone would find out the secret of production, so the area where these workshops were located was always cordoned off by soldiers.

None of the workers employed in the production of glass had the right to leave the city. For any attempt to do this, not only the glassmaker himself, but also his entire family was sentenced to death. In the end, it was decided to move the workshops to the island of Murano. It was more difficult to escape from there, and it was also difficult to get there.

In 1271, Venetian grinders learned how to make glass lenses, which at first were not in great demand. But in 1281 they guessed to insert them into specially designed frames. This is how the first glasses appeared. At first they cost so much that they were a wonderful gift even for kings and emperors.

At the end of the 15th century, when they learned how to make dishes from glass in Venice, Murano (named after the island where they were made) products became so popular all over the world that additional ships had to be built to deliver them.

But the improvement of glass continued later. The time has come, and people came up with the idea of ​​covering it with a special composition - an amalgam, so mirrors appeared.

In Russia, glass production began a thousand years ago, in small workshops. And in 1634, the first glass factory was built near Moscow.

The optical properties of glasses are associated with the characteristic features of the interaction of light rays with glass. It is the optical properties that determine the beauty and originality of the decorative processing of glass products.

Refraction and dispersion characterize the laws of propagation of light in a substance, depending on its structure. Refraction of light is a change in the direction of propagation of light when it passes from one medium to another, which differs from the first in the value of the propagation velocity.

On fig. 6 shows the path of the beam as it passes through a plane-parallel glass plate. The incident beam forms angles with the normal to the media interface at the point of incidence. If the beam goes from air to glass, then i is the angle of incidence, r is the angle of refraction (in the figure i> r, because the speed of propagation of light waves in air is greater than in glass, in this case, air is a medium optically less dense than glass).

The refraction of light is characterized by a relative refractive index - the ratio of the speed of light in the medium from which light falls on the interface to the speed of light in the second medium. The refractive index is determined from the ratio n=sin i/sin r . The relative refractive index has no dimension, and for transparent media, air - glass is always greater than one. For example, relative refractive indices (with respect to air): water - 1.33, crystal glass - 1.6, - 2.47.

Rice. 7. Prismatic (dispersive) spectrum a - decomposition of a light beam by a prism; b - color ranges of the visible part

Light dispersion is the dependence of the refractive index on the frequency of light (wavelength). Normal dispersion is characterized by an increase in the refractive index with increasing frequency or with decreasing wavelength.

Due to dispersion, a beam of light passing through a glass prism forms an iridescent band on a screen installed behind the prism - the prismatic (dispersive) spectrum (Fig. 7, a). In the spectrum, colors are arranged in a certain sequence, starting from purple and ending with red (Fig. 7.6).

The reason for the decomposition of light (dispersion) is the dependence of the refractive index on the frequency of light (wavelength): the higher the frequency of light (shorter wavelength), the higher the refractive index. In the prismatic spectrum, violet rays have the highest frequency and shortest wavelength, and red rays have the lowest frequency and longest wavelength, therefore, violet rays are refracted more than red ones.

The refractive index and dispersion depend on the composition of the glass, and the refractive index also depends on the density. The higher the density, the higher the refractive index. Oxides CaO, Sb 2 O 3 , PbO, BaO, ZnO and alkali increase the refractive index, the addition of SiO 2 reduces it. The dispersion increases with the introduction of Sb 2 O 3 and PbO. CaO and BaO have a stronger effect on the refractive index than on the dispersion. Glass containing up to 30% PbO is mainly used for the production of highly artistic products, high-quality tableware, which are subjected to grinding, since PbO significantly increases the refractive index and dispersion.

reflection of light- a phenomenon observed when light falls on the interface of two optically dissimilar media and consists in the formation of a reflected wave propagating from the interface into the same medium from which the incident wave comes. Reflection is characterized by a reflection coefficient, which is equal to the ratio of the reflected light flux to the incident light.

About 4% of the light is reflected from the glass surface. The reflection effect is enhanced by the presence of numerous polished surfaces (diamond carving, faceting).

If the interface irregularities are small compared to the wavelength of the incident light, then specular reflection occurs; if the irregularities are larger than the wavelength, diffuse reflection occurs, in which light is scattered by the surface in all possible directions. Reflection is called selective if the reflection coefficient is not the same for light with different wavelengths. Selective reflection explains the coloring of opaque bodies.

light scattering- a phenomenon observed during the propagation of light waves in a medium with randomly distributed inhomogeneities and consisting in the formation of secondary waves that propagate in all possible directions.

In ordinary transparent glass, light scattering practically does not occur. If the glass surface is uneven (frosted glass) or inhomogeneities (crystals, inclusions) are evenly distributed in the thickness of the glass, then light waves cannot pass through the glass without scattering, and therefore such glass is opaque.

Transmission and absorption of light is explained as follows. When a light beam of intensity I 0 passes through a transparent medium (substance), the intensity of the initial flux is weakened and the light beam leaving the medium will have intensity I< I 0 . Ослабление светового потока связано частично с явлениями отражения и рассеяния света, что главным образом происходит за счет поглощения световой энергии, обусловленного взаимодействием света с частицами среды.

Absorption reduces the overall translucency of the glass, which is approximately 93% for colorless soda lime silicate glass. The absorption of light is different for different wavelengths, so tinted glasses have different colors. The color of glass (Table 2), which is perceived by the eye, is due to the color of that part of the incident light beam that passed through the glass unabsorbed.

Transmission (absorption) indicators in the visible region of the spectrum are important for assessing the color of high-quality, signal and other colored glasses, in the infrared region - for technological processes of glass melting and molding of products (thermal transparency of glasses), in the ultraviolet region - for the performance properties of glasses (uviol glass products). should pass ultraviolet rays, and tare should delay).

double refraction- bifurcation of a beam of light when passing through an optically anisotropic medium, i.e. a medium with different properties in different directions (for example, most crystals). This phenomenon occurs because the refractive index depends on the direction of the electric vector of the light wave. A beam of light entering a crystal is decomposed into two beams - ordinary and extraordinary. The propagation speeds of these rays are different. Birefringence is measured by the difference in the path of the rays, nm / cm.

With uneven cooling or heating of glass, internal stresses arise in it, causing birefringence, i.e., glass is likened to a birefringent crystal, such as quartz, mica, gypsum. This phenomenon is used to control the quality of glass heat treatment, mainly annealing and tempering.

As you know, all bodies are made up of molecules, and molecules are made up of atoms. Atoms are also not complicated (in our simple description on the fingers). At the center of each atom is a nucleus, consisting of a proton, or a group of protons and neutrons, and around, in a circle, electrons rotate in their electronic orbits / orbitals.

The light is also simple. Forget (who remembered) about wave-particle duality and Maxwell's equations, let the light be a stream of photon balls flying from a flashlight straight into our eyes.

Now, if we put a concrete wall between the flashlight and the eye, we will no longer see the light. And if we shine a flashlight on this wall from our side, we will see, on the contrary, because the beam of light will be reflected from the concrete and hit our eyes. But light will not go through concrete.

It is logical to assume that the photon balls are reflected and do not pass through the concrete wall because they hit the atoms of the substance, i.e. concrete. More precisely, they hit the electrons, because the electrons rotate so fast that the photon does not penetrate the electron orbital to the nucleus, but bounces and is reflected from the electron.

Why does light pass through a glass wall? Indeed, there are also molecules and atoms inside the glass, and if we take a sufficiently thick glass, any photon must sooner or later collide with one of them, because there are trillions of atoms in each grain of glass! It's all about how electrons collide with photons. Let's take the simplest case, one electron revolves around one proton (this is a hydrogen atom) and imagine that a photon hit this electron.

All the energy of the photon was transferred to the electron. The photon is said to be absorbed by the electron and disappear. And the electron received additional energy (which the photon carried with it) and from this additional energy it jumped to a higher orbit and began to fly farther from the nucleus.

Most often, higher orbits are less stable, and after some time, the electron will emit this photon, i.e. “let him go free,” and he himself will return to his low stable orbit. The emitted photon will fly in a completely random direction, then it will be absorbed by another, neighboring atom, and will remain wandering in the substance until it accidentally radiates back, or eventually goes to heat the concrete wall.

And now the most interesting. Electron orbits cannot be anywhere around the nucleus of an atom. Each atom of each chemical element has a well-defined and finite set of levels or orbits. An electron cannot rise a little higher or fall a little lower. It can jump only quite a clear gap up or down, and since these levels differ in energy, this means that only a photon with a certain and very precisely given energy can push the electron to a higher orbit.

It turns out that if we have three photons flying with different energies, and only for one it is exactly equal to the energy difference between the levels of a particular atom, only this photon will “collide” with the atom, the rest will fly by, literally “through the atom” , because they will not be able to inform the electron of a clearly defined portion of energy for the transition to another level.

And how can we find photons with different energies?

It seems that the greater the speed, the higher the energy, everyone knows this, but after all, all photons fly at the same speed - the speed of light!

Maybe the brighter and more powerful the light source (for example, if you take an army searchlight instead of a flashlight), the more energy the photons will have? No. In a powerful and bright beam of a spotlight, there are simply more pieces of photons themselves, but the energy of each individual photon is exactly the same as that of those that fly out of a dead flashlight.

And here we still have to remember that light is not only a stream of particle balls, but also a wave. Different photons have different wavelengths, i.e. different frequencies of natural oscillations. And the higher the oscillation frequency, the more powerful charge of energy the photon carries.

Low frequency photons (infrared light or radio waves) carry little energy, high frequency photons (ultraviolet light or X-rays) carry a lot. Visible light is somewhere in the middle. Here lies the key to the transparency of glass! All atoms in glass have electrons in such orbits that they need a boost of energy to move to a higher one, which photons of visible light do not have enough. Therefore, it passes through the glass, practically without colliding with its atoms.

But ultraviolet photons carry the energy necessary for electrons to move from orbit to orbit, so in ultraviolet light ordinary window glass is completely black and opaque.

And what is interesting. Too much energy is also bad. The energy of a photon must be exactly equal to the energy of the transition between orbits, from which any substance is transparent for some wavelengths (and frequencies) of electromagnetic waves, and not transparent for others, because all substances consist of different atoms and their configurations.

For example, concrete is transparent to radio waves and infrared radiation, opaque to visible light and ultraviolet, not transparent to x-rays, but again transparent (to some extent) to gamma radiation.

That is why it is correct to say that glass is transparent to visible light. And for radio waves. And for gamma radiation. But opaque to ultraviolet light. And almost opaque to infrared light.

And if we also remember that visible light is also not all white, but consists of different wavelengths (i.e. colors) of waves from red to dark blue, it will become approximately clear why objects have different colors and shades, why roses are red, and violets are blue.

Why are gases transparent and solids not?

Temperature plays a decisive role in whether a given substance is solid, liquid or gaseous. At normal pressure on the surface of the earth at a temperature of 0 degrees Celsius and below, water is a solid body. At temperatures between 0 and 100 degrees Celsius, water is a liquid. At temperatures above 100 degrees Celsius, water is a gas. The steam from the pot spreads evenly in all directions throughout the kitchen. Based on the foregoing, let us assume that it is possible to see through gases, but it is impossible to see through solids. But some solids, such as glass, are as transparent as air. How does it work? Most solids absorb light that falls on them. Part of the absorbed light energy goes to heat the body. Most of the incident light is reflected. Therefore, we see a solid body, but cannot see through it.

conclusions

A substance looks transparent when light quanta (photons) pass through it without being absorbed. But photons have different energies, and each chemical compound absorbs only those photons that have the right energy for it. Visible light, from red to violet, has a very small range of photon energies. And just this range is "not interested" in silicon dioxide, the main component of glass. Therefore, photons of visible light pass through the glass almost unhindered.

The question is not why glass is transparent, but why other objects are not transparent. It's all about the energy levels at which the electrons are in the atom. You can imagine them as different rows in the stadium. The electron has a specific place on one of the rows. However, if he has enough energy, he can jump to another row. In some cases, the absorption of one of the photons passing through the atom will provide the necessary energy. But here's the catch. To transfer an electron from row to row, a photon must have a strictly defined amount of energy, otherwise it will fly by. This is what happens with glass. The rows are so far apart that the energy of a visible light photon is simply not enough to move electrons between them.

And the photons of the ultraviolet spectrum have enough energy, so they are absorbed, and here, no matter how hard you try, hiding behind the glass, you won’t get a tan. In the century that has passed since the production of glass, people have fully appreciated its unique property of being both solid and transparent. From windows that let in daylight and protect from the elements, to devices that allow you to look far into space, or observe microscopic worlds.

Deprive modern civilization of glass, and what will be left of it? Oddly enough, we rarely think about how important it is. Probably, this happens because, being transparent, the glass remains invisible, and we forget that it is.

As a child, I once asked my father, "Why does glass let light through?" By that time I had learned that light is a stream of particles called photons, and it seemed to me amazing how such a small particle could fly through thick glass. The father replied: "Because it is transparent." I kept silent, because I understood that "transparent" is just a synonym for the expression "transmits light", and the father does not really know the answer. There was no answer in school textbooks either, but I would like to know. Why does glass let light through?

Answer

Physicists call light not only visible light, but also invisible infrared radiation, ultraviolet radiation, x-rays, gamma radiation, radio waves. Materials that are transparent to one part of the spectrum (for example, to green light) may be opaque to other parts of the spectrum (red glass, for example, does not transmit green rays). Ordinary glass does not transmit ultraviolet radiation, and quartz glass is transparent to ultraviolet radiation. Materials that are transparent to X-rays are materials that do not transmit visible light at all. Etc.

Light is made up of particles called photons. Photons of different "color" (frequency) carry different portions of energy.

Photons can be absorbed by matter, transferring energy to it and heating it (well known to everyone who sunbathed on the beach). Light can be reflected from matter, falling into our eyes after that, so we see objects around us, and in complete darkness, where there are no light sources, we see nothing. And light can pass through a substance - and then we say that this substance is transparent.

Different materials absorb, reflect and transmit light in different proportions and therefore differ in their optical properties (darker and lighter, different colors, gloss, transparency): soot absorbs 95% of the light falling on it, and a polished silver mirror reflects 98% of the light. Created a material based on carbon nanotubes, which reflects only 45 thousandths of a percent of the incident light.

Questions arise: when is a photon absorbed by matter, when is it reflected, and when does it pass through matter? We are now only interested in the third question, but in passing we will answer the first.

The interaction of light and matter is the interaction of photons with electrons. An electron can absorb a photon and can emit a photon. There is no reflection of photons. Reflection of photons is a two-step process: the absorption of a photon and the subsequent emission of exactly the same photon.

Electrons in an atom can occupy only certain orbits, each of which has its own energy level. The atom of each chemical element is characterized by its own set of energy levels, i.e., the allowed orbits of electrons (the same applies to molecules, crystals, the condensed state of matter: soot and diamond have the same carbon atoms, but the optical properties of substances are different; metals, fine reflecting light, are transparent and even change color (green gold) if thin films are made from them; amorphous glass does not transmit ultraviolet, and from the same silicon oxide molecules, crystalline glass is transparent to ultraviolet).

Having absorbed a photon of a certain energy (color), the electron moves to a higher orbit. On the contrary, having emitted a photon, the electron moves to a lower orbit. Electrons can absorb and emit not any photons, but only those whose energy (color) correspond to the difference in the energy levels of this particular atom.

Thus, how light behaves when it encounters matter (reflects, absorbs, passes through) depends on what are the allowed energy levels of a given substance and what energy photons have (i.e., what color is the light falling on the substance).

In order for a photon to be absorbed by one of the electrons in an atom, it must have a strictly defined energy corresponding to the energy difference of any two energy levels of the atom, otherwise it will fly past. In glass, the distance between individual energy levels is large, and not a single photon of visible light has the corresponding energy, which would be enough for an electron, having absorbed a photon, to be able to jump to a higher energy level. Therefore, glass transmits photons of visible light. But the photons of ultraviolet light have sufficient energy, so the electrons absorb these photons and the glass retains the ultraviolet. In quartz glass, the distance between the allowed energy levels (energy gap) is even greater, and therefore photons of not only visible, but also ultraviolet light do not have sufficient energy for electrons to absorb them and go to the upper allowed levels.

So, photons of visible light pass through glass because they don't have the appropriate energy to move electrons to a higher energy level, and the glass therefore appears transparent.

By adding impurities to glass that have a different energy spectrum, it can be made colored - the glass will absorb photons of certain energies and transmit the remaining photons of visible light.