You have probably heard about special electromagnetic waves perceived by the human eye, occupying a certain part of the spectrum and having a wavelength from 380 nm (for violet) to 780 nm (for red). This is visible radiation. We can find out its properties, usually indicated in terahertz, by measuring the frequency range of such waves (we obtain a gap of 400–790 THz). Visible radiation is also called visible light. To put it even more simply, this is the light to which we are so accustomed and whose physical side we don’t even think about. Our eye is most susceptible to an indicator of 555 nm (this is the green part of the spectrum, 540 THz).
An interesting fact: the spectrum does not contain some of the colors that we perceive. For example, there is no pink color (and its shades). It is formed when other colors are mixed.
Have you ever wondered why the sky seems blue? It's simple: a part of the spectrum with a short wavelength is scattered by air more than with the long, corresponding to the red side of the spectrum. Due to this, the sky has a blue color, especially in the afternoon. Visible radiation enters the so-called “optical window”. The terrestrial atmosphere does not absorb this region of the spectrum.
Some animals see radiation inaccessible to human vision (it simply does not fall into the range we see). Many insects (such as bees) perceive the ultraviolet range, so they find nectar in flowers so quickly. Ultraviolet radiation can also see birds. Some species of birds even have marks that are noticeable only in this type of radiation, according to which the male and female accurately identify each other.
The use of man has also found visible radiation. Its most common form is thermal, based on the movement of atoms: the more the body heats up, the faster the movement of atoms that emit light during a collision. All the light sources familiar to us - the Sun, an incandescent lamp, a candle (flame) - are thermal sources of visible radiation.
The next example is electroluminescence. Here, the energy that causes the emission of light, atoms get from non-thermal sources. When discharges in a gas by an electric field, a certain kinetic energy is transmitted, electrons hit the atoms (the term “collide” is used), and the energy of excited atoms is already observed in the form of a glow (electroluminescence).
Electroluminescence is now used in advertising luminous labels. By the way, the northern lights also apply to the manifestation of electroluminescence: charged particles are emitted by the sun in streams, are attracted by the magnetic field of our planet, are excited at its poles and glow. But the cathode ray tubes in the TV are shining due to cathodoluminescence: the emission here causes electron bombardment.
Chemiluminescence is another example of visible radiation. Here, the result of the glow is the energy released during chemical reactions. The temperature of the light source is equal to the temperature of the medium in which it is located. An example would be a firefly with a small luminous green “flashlight”. Other organisms also glow: some fish, bacteria, and insects. Sometimes you can see the glow of rotting trees.
Some bodies begin to glow after being in the light: absorbing energy, they themselves become sources of light. This is photoluminescence, the properties of which are actively used in paints, clothing, road signs, etc.
The color components of visible light are used by psychologists in practice: green, as you know, calms, red - excites, violet, black and blue - on the contrary, inhibit and even oppress, and with a lack of white in a person, depression occurs at all. Visible radiation is used in medicine (chromotherapy): for example, yellow light is used in the treatment of certain eye diseases. Some diseases and pathologies of the gonads are treated with red light: infantility, infertility, menstrual irregularities in women, insufficient seed quantity in men, and extinction of sexual desire. Blue light eliminates itching, treats exudative diathesis and various kinds of hemorrhages.