Why does glass let light through? Why is glass transparent? Add your price to the database Comment Why glass is transparent and metal is not

Look out the window. If you wear glasses, wear them. Take binoculars and don't forget a magnifying glass. What do you see? No matter what you look at, numerous layers of glass will not interfere with your vision. But how is it that such a solid substance is practically invisible?

To understand this, you need to know the structure of glass and the nature of its origin.

Everything starts with earth's crust composed mostly of silicon and oxygen. These elements form silicon dioxide in the reaction, the molecules of which line up in the correct crystal lattice quartz. In particular, the sand used for making glass is rich in crystalline quartz. You probably know that glass is solid and does not at all consist of small pieces of quartz, and this is no accident.

First, the rough edges of sand grains and microdefects in the crystal structure reflect and scatter the light incident on them. But if you heat quartz to high temperatures, the molecules will begin to vibrate more strongly, which will lead to a break in the bond between them. And the crystal itself will turn into a liquid, just as ice turns into water. True, with the only difference: when cooling back into the crystal, the quartz molecules will no longer assemble. On the contrary, as the molecules lose energy, the probability of ordering only decreases. The result is an amorphous body. A solid with the properties of a liquid, which is characterized by the absence of intercrystalline boundaries. Thanks to this, at the microscopic level, the glass acquires homogeneity. Now the light passes through the material almost unhindered.

But this does not explain why glass transmits light, and does not absorb it, like other solids. The answer lies on the smallest scale, intra-atomic. Although many people know that an atom consists of a nucleus and electrons revolving around, how many people know that the atom is almost a perfect void? If an atom were the size of a football stadium, then the nucleus would be the size of a pea in the center of the field, and the electrons would be tiny grains of sand somewhere in the back rows. Thus, there is more than enough space for the free passage of light.

The question is not why glass is transparent, but why other objects are not transparent. It's all about energy levels where the electrons are located in the atom. You can imagine them as different rows in our 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 tan. In the century that has passed since the receipt of glass, people have fully appreciated it unique property be both solid and transparent at the same time. 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.

Keywords: the structure of glass, the origin of glass, Science on the Experiment portal, scientific articles

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 eye. 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 by more high orbit and began to fly farther from the core.

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 there is a well-determined and finite set of levels or orbits. An electron cannot rise a little higher or fall a little lower. It can jump only a very clear interval 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 searchlight, 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 absolutely 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 lengths (and frequencies) electromagnetic waves, and is not transparent to others, because all substances are made up 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.

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 - solid. 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? Majority solids absorbs the light falling 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.

findings

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 appropriate energy. 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.

In the article, I try to explain why some substances are transparent to visible light, while others are not. This topic is completely complex and goes into the very jungle of physical processes, affecting optics, chemistry, quantum mechanics and many more related disciplines and includes eye-catching formulas and a furious machine. I will consciously make very broad assumptions, omitting 9/10x of what happens in matter. in fact .

My goal is to tell in such a way that it becomes clear to a schoolboy who has not even begun to study physics, i.e. Literally a 5th grader.

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

Light is also quite simple. Let's forget (who remembered) about corpuscular-wave dualism and Maxwell's equations, let the light be a stream of photon balls flying from a flashlight directly into our eyes.

It is logical to assume that 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 electrons, because electrons spinning so fast that the photon does not penetrate through the electron orbital to the nucleus, but bounces off and is already reflected from the electron.

It's all about as 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 core.

Absorption of a photon by an electron and the transition of the latter to a higher orbit

Most often, higher orbits are less stable, and after some time, the electron will emit this photon, i.e. "let him go free", and will return to its 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 a very clear interval 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 higher the speed, the higher the energy, everyone knows this, but after all, all photons fly at the same speed - the speed of light!

And here we still have to remember that light is not only a stream of balls-particles, 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 photon energy must be precisely is equal to the transition energy between orbits, from which any substance is transparent for some lengths (and frequencies) of electromagnetic waves, and not transparent for others, because all substances consist of different atoms and their configurations, i.e. molecules.

That is why it is correct to say that glass is transparent. for 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. But, this is already a topic for another post explaining complex physical phenomena plain language analogies on fingers™.

The optical properties of glasses are related to characteristic features 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 more than one. For example, relative refractive indices (with respect to air): water - 1.33, crystal glass - 1.6, - 2.47.

Rice. 6. Scheme of beam passage through a plane-parallel glass plate

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. Glasses containing up to 30% PbO are mainly used for the production of highly artistic products, high-quality glassware, 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 reflected light flux to the incident.

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 roughness of the interface is small compared to the wavelength of the incident light, then specular reflection occurs, if the roughness is larger than the wavelength - diffuse reflection, 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 with 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 varietal, signal and other colored glasses, in the infrared region - for technological processes of glass melting and molding 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.