Why does glass let light through. Does glass transmit ultraviolet light?

There were times when tanned skin was considered a sign of low birth, and noble ladies tried to protect their faces and hands from the sun's rays in order to maintain an aristocratic pallor. Later, the attitude towards tanning has changed - it has become an indispensable attribute of a healthy and successful person. Today, despite the ongoing debate about the benefits and harms of insolation, the bronze skin tone is still at the peak of popularity. But not everyone has the opportunity to visit the beach or the solarium, and in this regard, many are interested in whether it is possible to sunbathe through the window glass, sitting, for example, on a glazed loggia or attic heated by the sun.

Probably, every professional driver or just a person who spends a long time behind the wheel of a car noticed that his hands and face become covered with a light tan over time. The same applies to office workers who are forced to sit at an uncurtained window for the entire shift. On their faces, you can often find traces of sunburn even in winter. And if a person is not a frequenter of solariums and does not make a daily promenade through the parks, then this phenomenon cannot be explained otherwise than by tanning through glass. So does glass transmit ultraviolet light and is it possible to tan through a window? Let's figure it out.

The nature of tanning

In order to answer the question of whether it is possible to get a tan through ordinary window glass in a car or on a loggia, you need to understand exactly how the process of darkening of the skin takes place and what factors influence it. First of all, it should be noted that sunburn is nothing more than a protective reaction of the skin to solar radiation. Under the influence of ultraviolet light, the cells of the epidermis (melanocytes) begin to produce the substance melanin (dark pigment), due to which the skin acquires a bronze tint. The higher the concentration of melanin in the upper layers of the dermis, the more intense the tan. However, not all UV rays cause such a reaction, but only those in a very narrow wavelength range. Ultraviolet rays are conventionally divided into three types:

• A-rays (long wavelength)- are practically not delayed by the atmosphere and freely reach the earth's surface. Such radiation is considered the safest for the human body, since it does not activate the synthesis of melanin. All it can do is cause a slight darkening of the skin, and then only with prolonged exposure. However, with excessive insolation with long-wave rays, collagen fibers are destroyed and the skin becomes dehydrated, as a result of which it begins to age faster. And some people are allergic to the sun because of A-rays. Long-wave radiation easily overcomes the thickness of window glass and leads to gradual fading of wallpaper, the surface of furniture and carpets, but it is impossible to get a full tan with its help.
• B-rays (medium wave)- linger in the atmosphere and reach the Earth's surface only partially. This type of radiation has a direct effect on the synthesis of melanin in skin cells and contributes to the appearance of a quick tan. And with its intense exposure to the skin, burns of varying degrees occur. B-rays cannot penetrate through ordinary window glass.
• C-rays (shortwave)- represent a great danger to all living organisms, but, fortunately, they are almost completely neutralized by the atmosphere, before reaching the surface of the Earth. You can only encounter such radiation high in the mountains, but even there its effect is extremely weakened.

Physicists distinguish another type of ultraviolet radiation - extreme, for which the term "vacuum" is often used due to the fact that waves of this range are completely absorbed by the Earth's atmosphere and do not fall on the earth's surface.

Can you tan through glass?

Whether it is possible to get a tan through window glass or not depends on what properties it has. The fact is that glasses come in different types, each of which is affected by UV rays in different ways. So, organic glass has a high transmission capacity, which allows the passage of the entire spectrum of solar radiation. The same applies to quartz glass, which is used in tanning lamps and room disinfection devices. Ordinary glass, used in residential premises and cars, transmits only long-wave rays of type A, and it is impossible to sunbathe through it. Another thing, if you replace it with plexiglass. Then it will be possible to sunbathe and enjoy a beautiful tan almost all year round.

Although sometimes there are cases when a person spends some time under the sun's rays passing through the window, and then finds a light tan on the exposed skin. Of course, he is in full confidence that he got tanned precisely by insolation through the glass. But it is not so. There is a very simple explanation for this phenomenon: the change in shade in this case occurs as a result of the activation of a small amount of the residual pigment (melanin) produced under the influence of type B ultraviolet radiation, located in the skin cells. As a rule, such a “tan” is temporary, that is, it quickly disappears. In a word, in order to get a full-fledged tan, you must either visit a solarium or regularly take sunbaths, and it will not work to change the natural skin tone towards a darker one through ordinary window or car glass.

Is it necessary to defend?

Worrying about whether it is possible to get a tan through glass is only for those people who have very sensitive skin and a predisposition to the appearance of age spots. They are advised to use special means with a minimum degree of protection (SPF) at all times. Apply such cosmetics should be mainly on the face, neck and décolleté. However, it is still not worth protecting yourself too actively from ultraviolet, especially long-wave, because the sun's rays in moderation are very useful and even necessary for the normal functioning of the human body.

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. 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. Glass containing up to 30% PbO is 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 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 operational 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.

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.

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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.

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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.

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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.