Beneficial and harmful properties of infrared radiation on humans. Infrared radiation: influence on the human body, the effect of rays, their properties, benefits and harms, possible consequences

Infrared radiation is actively used in medicine, and it beneficial features were noticed long before the appearance modern research. Even in antiquity, the heat of coals, heated salt, metal and other materials was used to treat wounds, bruises, frostbite, tuberculosis and many other diseases.

Research of the XX-XXI centuries has proven that infrared radiation has a certain effect on the external integument and internal organs, which allows it to be used for therapeutic and prophylactic purposes.

Impact of infrared radiation on the body

Infrared rays not only heat, but only a few people know about it. Since the discovery of infrared radiation by Herschel in 1800, scientists and physicians have identified the following types of effects on the human body:

• activation of metabolism;
• dilation of blood vessels, including capillaries;
• activation of capillary blood circulation;
• antispasmodic effect;
• analgesic effect;
• anti-inflammatory effect;
• activation of reactions inside the cell.

When used in doses, exposure to infrared rays has a general health effect. Already today, many devices have been developed that are used in physiotherapy rooms.

Naturally, exposure should be carried out in doses to avoid overheating, burns and other negative reactions.

Methods of using infrared rays

Since infrared rays dilate blood vessels and accelerate blood flow, they are used to improve and stimulate blood circulation. When long-wave infrared rays are directed at the skin, its receptors are irritated, which causes a reaction in the hypothalamus, sending a signal to “relax” the smooth muscles of the blood vessels. As a result, capillaries, veins and arteries expand, and blood flow accelerates.

Not only the walls of blood vessels react to infrared radiation, but at the cellular level there is an acceleration of metabolism, as well as an improvement in the course of neuroregulatory processes.

Exposure to infrared rays plays an invaluable role in improving immunity. Thanks to the increased production of macrophagocytes, phagocytosis is accelerated, and a person’s immunity is strengthened at the fluid and cellular level. In parallel, there is stimulation of amino acid synthesis, as well as increased production of enzymes and nutrients.

A disinfecting effect has also been noted; infrared rays kill a number of bacteria in the human body and neutralize the effects of some harmful substances.

Medical problems that can be solved using IR radiation

Infrared therapy is used as part of the treatment, as it allows you to solve the following effects:

• the severity of pain decreases;
• the pain syndrome goes away;
• water-salt balance is restored;
• memory improves;
• there is a lymphatic drainage effect;
• blood circulation (including cerebral) and blood supply to tissues are normalized;
• blood pressure normalizes;
• toxins and heavy metal salts are eliminated faster;
• the production of endorphin and melatonin increases;
• hormone production is normalized;
• pathogenic organisms and fungi are destroyed;
• the growth of cancer cells is suppressed;
• there is an antinuclear effect;
• a deodorizing effect is manifested;
• the immune system is restored;
• Hypertonicity and increased muscle tension are relieved;
• emotional stress goes away;
• Fatigue accumulates less;
• sleep is normalized;
• the functions of internal organs return to normal.

Diseases that are treated with infrared radiation

Naturally, such a large-scale positive effect is actively used to treat a whole range of diseases:

• bronchial asthma;
• flu;
• pneumonia;
• oncological diseases;
• formation of adhesions;
• adenoma;
• peptic ulcer;
• parotitis;
• gangrene;
• obesity;
• phlebeurysm;
• salt deposits;
• spurs, corns, calluses;
• skin diseases;
• vascular diseases;
• poorly healing wounds;
• burns, frostbite;
• diseases of the peripheral nervous system;
• paralysis;
• bedsores.

Due to the fact that metabolism is activated and blood flow is normalized, including in capillaries, organs and tissues are restored much faster and return to normal operation.

With regular exposure to infrared rays on the body, inflammatory processes reverse, tissue regeneration, anti-infective protection and local resistance increase.

When emitting devices are used together with medicines and physiotherapeutic procedures, it is possible to achieve positive dynamics 1.5-2 times faster. Recovery is faster and the likelihood of relapse is reduced.

A separate topic is the use of infrared ray therapy in obese patients. Here the main effect is achieved by normalizing metabolism, including cellular metabolism. Also, heating the body surface contributes to more quick deliverance from accumulated fat mass. IR radiation is used in conjunction with diet and drug treatment.

Infrared radiation in sports medicine

Research into effective injury recovery treatments has shown that infrared rays speed up the healing of injuries. Practical results quite impressive, the athletes showed such positive changes.

There are different sources of infrared radiation. They are currently in household appliances, automation and security systems, and are also used for drying industrial products. Infrared light sources, when used correctly, do not affect the human body, which is why the products are very popular.

History of discovery

For many centuries, outstanding minds have been studying the nature and action of light.

Infrared light was discovered in the early 19th century through the research of astronomer W. Herschel. Its essence was to study the heating abilities of various solar areas. The scientist brought a thermometer to them and monitored the increase in temperature. This process was observed when the device touched the red border. V. Herschel concluded that there is a certain radiation that cannot be seen visually, but can be determined using a thermometer.

Infrared rays: application

They are widespread in human life and have found their application in various fields:

• Warfare. Modern rockets and warheads capable of independently aiming at a target, equipped with which are the result of the use of infrared radiation.
• Thermography. Infrared radiation is used to study overheated or supercooled areas. Infrared images are also used in astronomy to detect celestial bodies.
• Life The operation of which is aimed at heating interior items and walls has gained great popularity. They then release heat into the space.
• Remote control. All existing remote controls for TV, furnaces, air conditioners, etc. equipped with infrared rays.
• In medicine, infrared rays are used for treatment and prevention. various diseases.

Let's look at where these elements are used.

Infrared gas burners

An infrared burner is used to heat various rooms.

At first it was used for greenhouses, garages (i.e. non-residential premises). However modern technologies allowed to use it even in apartments. Popularly, such a burner is called a solar device, since when turned on, the working surface of the equipment resembles sunlight. Over time, such devices replaced oil heaters and convectors.

Main features

An infrared burner differs from other devices in its heating method. Heat is transferred through means that are not noticeable to humans. This feature allows heat to penetrate not only into the air, but also onto interior items, which subsequently also increase the temperature in the room. The infrared emitter does not dry out the air, because the rays are primarily directed at interior items and walls. In the future, heat will be transferred from walls or objects directly to the space of the room, and the process occurs in a few minutes.

Positive sides

The main advantage of such devices is quick and easy heating of the room. For example, it will take 20 minutes to heat a cold room to a temperature of +24ºС. During the process, there is no air movement, which contributes to the formation of dust and large contaminants. Therefore, an infrared emitter is installed indoors by those people who have allergies.

In addition, infrared rays, when hitting a surface with dust, do not cause it to burn, and, as a result, there is no smell of burnt dust. The quality of heating and durability of the device depends on the heating element. Such devices use a ceramic type.

Price

The price of such devices is quite low and accessible to all segments of the population. For example, a gas burner costs from 800 rubles. A whole stove can be purchased for 4,000 rubles.

Sauna

What is an infrared cabin? This is a special room that is built from natural types of wood (for example, cedar). Infrared emitters are installed in it, acting on the tree.

During heating, phytoncides are released - useful components, which prevent the development or appearance of fungi and bacteria.

Such an infrared cabin is popularly called a sauna. The air temperature inside the room reaches 45ºС, so it is quite comfortable to be in it. This temperature allows you to warm up human body evenly and deeply. Therefore, heat does not affect the cardiovascular system. During the procedure, accumulated toxins and waste are removed, metabolism in the body is accelerated (due to the rapid movement of blood), and tissues are also enriched with oxygen. However, sweating is not the main feature of an infrared sauna. It is aimed at improving well-being.

Impact on humans

Such premises have a beneficial effect on the human body. During the procedure, all muscles, tissues and bones are warmed up. Accelerating blood circulation affects metabolism, which helps saturate muscles and tissues with oxygen. In addition, the infrared cabin is visited to prevent various diseases. Most people leave only positive reviews.

Negative effects of infrared radiation

Sources of infrared radiation can cause not only positive effects on the body, but also cause harm to it.

With prolonged exposure to rays, the capillaries expand, which leads to redness or burns. Sources of infrared radiation cause particular harm to the organs of vision - this is the formation of cataracts. In some cases, a person experiences seizures.

Short rays affect the human body, causing a deterioration in the brain's temperature by several degrees: darkening of the eyes, dizziness, nausea. A further increase in temperature can lead to the formation of meningitis.

Deterioration or improvement of the condition occurs due to the intensity of the electromagnetic field. It is characterized by temperature and distance to the source of thermal energy radiation.

Long waves of infrared radiation play a special role in various life processes. Short ones have a greater effect on the human body.

How to prevent the harmful effects of infrared rays?

As mentioned earlier, short-term thermal radiation has a negative effect on the human body. Let's look at examples in which IR radiation is dangerous.

Today, infrared heaters that emit temperatures above 100ºC can be harmful to health. Among them are the following:

• Industrial equipment emitting radiant energy. To prevent negative impact, special clothing and heat-protective elements should be used, as well as preventive measures should be taken among working personnel.
• Infrared device. The most famous heater is the stove. However, it has long gone out of use. More and more often in apartments, country houses and dachas began to use electric infrared heaters. Its design includes a heating element (in the form of a spiral), which is protected by a special heat-insulating material. Such exposure to rays does not harm to the human body. The air in the heated zone is not dried. You can heat the room in 30 minutes. First, infrared radiation heats objects, and then they heat the entire apartment.

Infrared radiation is widely used in various fields, from industrial to medicine.

However, they should be handled with care, as the rays can have a negative effect on humans. It all depends on the wavelength and distance to the heating device.

So, we found out what sources of infrared radiation exist.

Infrared radiation- electromagnetic radiation, occupying the spectral region between the red end of visible light (with a wavelength λ = 0.74 μm and a frequency of 430 THz) and microwave radio radiation (λ ~ 1-2 mm, frequency 300 GHz).

The entire range of infrared radiation is conventionally divided into three areas:

The long-wavelength edge of this range is sometimes separated into a separate range of electromagnetic waves - terahertz radiation (submillimeter radiation).

Infrared radiation is also called “thermal radiation”, since infrared radiation from heated objects is perceived by the human skin as a sensation of heat. In this case, the wavelengths emitted by the body depend on the heating temperature: the higher the temperature, the shorter the wavelength and the higher the radiation intensity. The radiation spectrum of an absolute black body at relatively low (up to several thousand Kelvin) temperatures lies mainly in this range. Infrared radiation is emitted by excited atoms or ions.

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Discovery history and general characteristics

Infrared radiation was discovered in 1800 by the English astronomer W. Herschel. While studying the Sun, Herschel was looking for a way to reduce the heating of the instrument with which the observations were made. Using thermometers to determine the effects of different parts of the visible spectrum, Herschel discovered that the “maximum of heat” lies behind the saturated red color and, possibly, “beyond visible refraction.” This study marked the beginning of the study of infrared radiation.

Earlier laboratory sources Infrared radiation was produced exclusively by hot bodies or electrical discharges in gases. Nowadays, modern sources of infrared radiation with adjustable or fixed frequency have been created based on solid-state and molecular gas lasers. To record radiation in the near-infrared region (up to ~1.3 μm), special photographic plates are used. Photoelectric detectors and photoresistors have a wider sensitivity range (up to approximately 25 microns). Radiation in the far infrared region is recorded by bolometers - detectors that are sensitive to heating by infrared radiation.

IR equipment is widely used in both military equipment(for example, for missile guidance), and in civilian (for example, in fiber-optic communication systems). IR spectrometers use either lenses and prisms or diffraction gratings and mirrors as optical elements. To eliminate the absorption of radiation in air, spectrometers for the far-IR region are manufactured in a vacuum version.

Since infrared spectra are associated with rotational and vibrational movements in the molecule, as well as with electronic transitions in atoms and molecules, IR spectroscopy makes it possible to obtain important information about the structure of atoms and molecules, as well as the band structure of crystals.

Infrared radiation ranges

Objects typically emit infrared radiation across the entire spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors typically only collect radiation within a certain bandwidth. Thus, the infrared range is often subdivided into smaller bands.

Conventional division scheme

Most often, division into smaller ranges is done as follows:

CIE scheme

International Illumination Commission International Commission on Illumination ) recommends dividing infrared radiation into the following three groups:

• IR-A: 700 nm – 1400 nm (0.7 µm – 1.4 µm)
• IR-B: 1400 nm – 3000 nm (1.4 µm – 3 µm)
• IR-C: 3000 nm – 1 mm (3 µm – 1000 µm)

ISO 20473 diagram

Thermal radiation

Thermal radiation or radiation is the transfer of energy from one body to another in the form of electromagnetic waves emitted by bodies due to their internal energy. Thermal radiation mainly falls in the infrared region of the spectrum from 0.74 microns to 1000 microns. Distinctive feature radiant heat exchange is that it can be carried out between bodies located not only in any medium, but also in a vacuum. An example of thermal radiation is light from an incandescent lamp. The power of thermal radiation of an object that meets the criteria of an absolute black body is described by the Stefan-Boltzmann law. The relationship between the emissive and absorptive abilities of bodies is described by Kirchhoff's radiation law. Thermal radiation is one of the three elementary types of thermal energy transfer (in addition to thermal conductivity and convection). Equilibrium radiation is thermal radiation that is in thermodynamic equilibrium with matter.

Application

Night-vision device

There are several ways to visualize an invisible infrared image:

• Modern semiconductor video cameras are sensitive in the near infrared. To avoid color rendering errors, ordinary household video cameras are equipped with a special filter that cuts off the IR image. Cameras for security systems, as a rule, do not have such a filter. However, in dark time no day natural sources near-IR, so without artificial illumination (for example, infrared LEDs), such cameras will not show anything.
• Electron-optical converter is a vacuum photoelectronic device that amplifies light in the visible spectrum and near-IR. It has high sensitivity and is capable of producing images in very low light conditions. They are historically the first night vision devices and are still widely used today in cheap night vision devices. Since they work only in near-IR, they, like semiconductor video cameras, require lighting.
• Bolometer - thermal sensor. Bolometers for technical vision systems and night vision devices are sensitive in the wavelength range 3..14 microns (mid-IR), which corresponds to radiation from bodies heated from 500 to −50 degrees Celsius. Thus, bolometric devices do not require external lighting, registering the radiation of the objects themselves and creating a picture of the temperature difference.

Thermography

Infrared thermography, thermal imaging or thermal video is a scientific method of obtaining a thermogram - an image in infrared rays showing a pattern of distribution of temperature fields. Thermographic cameras or thermal imagers detect radiation in infrared range electromagnetic spectrum (approximately 900-14000 nanometers or 0.9-14 µm) and based on this radiation they create images that make it possible to determine overheated or supercooled areas. Since infrared radiation is emitted by all objects that have a temperature, according to Planck's formula for black-body radiation, thermography allows one to "see" the environment with or without visible light. The amount of radiation emitted by an object increases as its temperature increases, so thermography allows us to see differences in temperature. When we look through a thermal imager, warm objects are visible better than those cooled to ambient temperature; people and warm-blooded animals are more easily visible in environment, both day and night. As a result, the advancement of thermography use can be attributed to the military and security services.

Infrared homing

Infrared homing head - a homing head that works on the principle of capturing infrared waves emitted by the target being captured. It is an optical-electronic device designed to identify a target against the surrounding background and issue a locking signal to an automatic aiming device (ADU), as well as to measure and issue a line of sight angular velocity signal to the autopilot.

Infrared heater

Data transfer

The spread of infrared LEDs, lasers and photodiodes has made it possible to create a wireless optical method of data transmission based on them. In computer technology, it is usually used to connect computers with peripheral devices (IrDA interface). Unlike the radio channel, the infrared channel is insensitive to electromagnetic interference, and this allows it to be used in industrial environments. The disadvantages of the infrared channel include the need for optical windows on the equipment, correct relative orientation of devices, low speeds transmission (usually does not exceed 5-10 Mbit/s, but when using infrared lasers significantly more is possible high speeds). In addition, the confidentiality of information transfer is not ensured. Under direct visibility conditions, the infrared channel can provide communication over distances of several kilometers, but it is most convenient for connecting computers located in the same room, where reflections from the walls of the room provide stable and reliable communication. The most natural type of topology here is a “bus” (that is, the transmitted signal is simultaneously received by all subscribers). The infrared channel could not become widespread; it was supplanted by the radio channel.

Thermal radiation is also used to receive warning signals.

Remote control

Infrared diodes and photodiodes are widely used in remote control panels, automation systems, security systems, and some mobile phones(infrared port), etc. Infrared rays do not distract a person’s attention due to their invisibility.

Interestingly, the infrared radiation of a household remote control is easily recorded using a digital camera.

Medicine

Infrared radiation is most widely used in medicine in various sensors blood flow (PPG).

Widely used heart rate (HR - Heart Rate) and blood oxygen saturation (Sp02) meters use green (for pulse) and red and infrared (for SpO2) LEDs.

Infrared laser radiation is used in the DLS (Digital Light Scattering) technique to determine heart rate and blood flow characteristics.

Infrared rays are used in physiotherapy.

Effect of long-wave infrared radiation:

• Stimulation and improvement of blood circulation. When exposed to long-wave infrared radiation on the skin, skin receptors are irritated and, due to the reaction of the hypothalamus, the smooth muscles of the blood vessels relax, as a result of which the vessels dilate.
• Improving metabolic processes. When exposed to heat, infrared radiation stimulates activity at the cellular level, improving the processes of neuroregulation and metabolism.

Food Sterilization

Sterilizes using infrared radiation food products for the purpose of disinfection.

Food industry

The peculiarity of the use of IR radiation in Food Industry is the possibility of penetration of an electromagnetic wave into capillary-porous products such as grain, cereals, flour, etc. to a depth of 7 mm. This value depends on the nature of the surface, structure, material properties and frequency characteristics of the radiation. An electromagnetic wave of a certain frequency range has not only a thermal, but also a biological effect on the product, helping to accelerate biochemical transformations in biological polymers (

William Herschel first noticed that behind the red edge of the prism-derived spectrum of the Sun there was invisible radiation that caused the thermometer to heat up. This radiation was later called thermal or infrared.

Near-infrared radiation is very similar to visible light and is detected by the same instruments. Mid- and far-IR uses bolometers to detect changes.

The entire planet Earth and all objects on it, even ice, shine in the mid-IR range. Due to this, the Earth does not overheat solar heat. But not all infrared radiation passes through the atmosphere. There are only a few windows of transparency, the rest of the radiation is absorbed carbon dioxide, water vapor, methane, ozone and other greenhouse gases that prevent the rapid cooling of the Earth.

Due to atmospheric absorption and thermal radiation from objects, mid- and far-IR telescopes are taken into space and cooled to the temperature of liquid nitrogen or even helium.

The infrared range is one of the most interesting for astronomers. It contains cosmic dust, important for the formation of stars and the evolution of galaxies. IR radiation passes through clouds of cosmic dust better than visible radiation and allows one to see objects that are inaccessible to observation in other parts of the spectrum.

Sources

A fragment of one of the so-called Hubble Deep Fields. In 1995, a space telescope collected light coming from one part of the sky for 10 days. This made it possible to see extremely faint galaxies up to 13 billion light years away (less than one billion years from the Big Bang). Visible light from such distant objects undergoes a significant red shift and becomes infrared.

The observations were carried out in a region far from the galactic plane, where relatively few stars are visible. That's why most of registered objects are galaxies at different stages of evolution.

The giant spiral galaxy, also designated M104, is located in a cluster of galaxies in the constellation Virgo and is visible to us almost edge-on. It has a huge central bulge (a spherical thickening in the center of the galaxy) and contains about 800 billion stars - 2-3 times more than the Milky Way.

At the center of the galaxy is a supermassive black hole with a mass of about a billion solar masses. This is determined by the speed of movement of stars near the center of the galaxy. In the infrared, a ring of gas and dust is clearly visible in the galaxy, in which stars are actively being born.

Receivers

Main mirror diameter 85 cm made of beryllium and cooled to a temperature of 5.5 TO to reduce the mirror's own infrared radiation.

The telescope was launched in August 2003 under the program NASA's four great observatories, including:

• Compton Gamma-ray Observatory (1991–2000, 20 keV-30 GeV), see Sky at 100 MeV gamma rays,
• Chandra X-ray Observatory (1999, 100 eV-10 keV),
• Hubble Space Telescope (1990, 100–2100 nm),
• Spitzer infrared telescope (2003, 3–180 µm).

The Spitzer telescope is expected to have a lifespan of about 5 years. The telescope received its name in honor of astrophysicist Lyman Spitzer (1914–97), who in 1946, long before the launch of the first satellite, published the article “Advantages for Astronomy of an Extraterrestrial Observatory,” and 30 years later convinced NASA and the American Congress to begin developing a space telescope. Hubble."

Sky Reviews

Near-infrared sky 1–4 µm and in the mid-infrared range 25 µm(COBE/DIRBE)

In the near-infrared range, the Galaxy is visible even more clearly than in the visible.

But in the mid-IR range the Galaxy is barely visible. Observations are greatly hampered by dust in solar system. It is located along the ecliptic plane, which is inclined to the galactic plane at an angle of about 50 degrees.

Both surveys were obtained by the DIRBE (Diffuse Infrared Background Experiment) instrument on board the COBE (Cosmic Background Explorer) satellite. This experiment, which began in 1989, produced full maps infrared sky brightness in the range from 1.25 to 240 µm.

Terrestrial Application

The device is based on an electron-optical converter (EOC), which allows one to significantly (from 100 to 50 thousand times) amplify weak visible or infrared light.

The lens creates an image on the photocathode, from which, as in the case of a PMT, electrons are knocked out. Then they are accelerated by high voltage (10–20 kV), are focused by electron optics (an electromagnetic field of a specially selected configuration) and fall onto a fluorescent screen similar to a television. On it, the image is viewed through eyepieces.

Acceleration of photoelectrons makes it possible in low light conditions to use literally every quantum of light to obtain an image, but in complete darkness a backlight is required. In order not to reveal the presence of an observer, they use a near-infrared illuminator (760–3000 nm).

There are also devices that detect objects’ own thermal radiation in the mid-IR range (8–14 µm). Such devices are called thermal imagers; they allow you to notice a person, animal or heated engine due to their thermal contrast with the surrounding background.

All the energy consumed by an electric heater ultimately turns into heat. A significant part of the heat is carried away by air, which comes into contact with the hot surface, expands and rises, so that mainly the ceiling is heated.

To avoid this, heaters are equipped with fans that direct warm air, for example, on a person’s feet and contribute to mixing the air in the room. But there is another way to transfer heat to surrounding objects: infrared radiation from a heater. The hotter the surface and the larger its area, the stronger it is.

To increase the area, radiators are made flat. However, the surface temperature cannot be high. Other heater models use a spiral heated to several hundred degrees (red heat) and a concave metal reflector that creates a directed stream of infrared radiation.

In 1800, scientist William Herschel announced his discovery at a meeting of the Royal Society of London. He measured temperatures outside the spectrum and discovered invisible rays with great heating power. He carried out the experiment using telescope filters. He noticed that they absorb light and heat to varying degrees sun rays.

After 30 years, the existence of invisible rays located beyond the red part of the visible solar spectrum was indisputably proven. The French Becquerel called this radiation infrared.

Properties of IR radiation

The spectrum of infrared radiation consists of individual lines and bands. But it can also be continuous. It all depends on the source of the IR rays. In other words, what matters is the kinetic energy or temperature of an atom or molecule. Any element of the periodic table in the conditions different temperatures It has various characteristics.

For example, the infrared spectra of excited atoms, due to the relative state of rest of the nucleus bundle, will have strictly line IR spectra. And excited molecules are striped and randomly located. Everything depends not only on the mechanism of superposition of the own linear spectra of each atom. But also from the interaction of these atoms with each other.

As the temperature rises, the spectral characteristics of the body change. Thus, heated solids and liquids emit a continuous infrared spectrum. At temperatures below 300°C, the radiation of heated solid entirely located in the infrared region. Both the study of IR waves and the application of their most important properties depend on the temperature range.

The main properties of IR rays are absorption and further heating of bodies. The principle of heat transfer by infrared heaters differs from the principles of convection or conduction. Being in a flow of hot gases, an object loses some amount of heat as long as its temperature is lower than the temperature of the heated gas.

And vice versa: if infrared emitters irradiate an object, this does not mean that its surface absorbs this radiation. It can also reflect, absorb or transmit rays without loss. Almost always, the irradiated object absorbs part of this radiation, reflects part and transmits part.

Not all luminous objects or heated bodies emit infrared waves. For example, fluorescent lamps or flame gas stove do not have such radiation. The operating principle of fluorescent lamps is based on glow (photoluminescence). Its spectrum is closest to the spectrum of daylight, white light. Therefore, there is almost no IR radiation in it. And the highest radiation intensity of a gas stove flame falls on the wavelength blue color. The IR radiation of the listed heated bodies is very weak.

There are also substances that are transparent to visible light, but are not capable of transmitting infrared rays. For example, a layer of water several centimeters thick will not transmit infrared radiation with a wavelength greater than 1 micron. In this case, a person can distinguish objects located at the bottom with the naked eye.