REFRACTION INDEX(refractive index) - optical. characteristic of the environment associated with refraction of light at the interface between two transparent optically homogeneous and isotropic media during its transition from one medium to another and due to the difference in the phase velocities of light propagation in the media. The value of P. p. is equal to the ratio of these speeds. relative

P. p. of these environments. If light falls on the second or first medium from (where is the speed of light With), then the quantities absolute pp of these averages. In this case, a the law of refraction can be written in the form where and are the angles of incidence and refraction.

The magnitude of the absolute power factor depends on the nature and structure of the substance, its state of aggregation, temperature, pressure, etc. At high intensities, the power factor depends on the intensity of light (see. Nonlinear optics). In a number of substances, P. changes under the influence of external influences. electric fields ( Kerr effect- in liquids and gases; electro-optical Pockels effect- in crystals).

For a given medium, the absorption band depends on the light wavelength l, and in the region of absorption bands this dependence is anomalous (see Fig. Light dispersion).In X-ray. region, the power factor for almost all media is close to 1, in the visible region for liquids and solids it is about 1.5; in the IR region for a number of transparent media 4.0 (for Ge).

They are characterized by two PPs: ordinary (similar to isotropic media) and extraordinary, the magnitude of which depends on the angle of incidence of the beam and, therefore, the direction of propagation of light in the medium (see. Crystal optics For media with absorption (in particular, for metals), the absorption coefficient is a complex value and can be represented in the form where ha is the usual absorption coefficient, and is the absorption index (see Light absorption, Metal optics).

P. p. is macroscopic. characteristics of the environment and is associated with it dielectric constant n mag. permeability Classic electron theory (see Light dispersion) allows us to relate the value of P. p. with microscopic. characteristics of the environment - electronic polarizability atom (or molecule) depending on the nature of the atoms and the frequency of light and medium: where N- number of atoms per unit volume. Electricity acting on an atom (molecule). The field of the light wave causes a displacement of the optical wave. electron from the equilibrium position; the atom acquires inducers. the dipole moment varies in time with the frequency of the incident light, and is the source of secondary coherent waves, which. interfering with a wave incident on the medium, they form a resulting light wave propagating in the medium with phase velocity and therefore

The intensity of conventional (non-laser) light sources is relatively low, the electrical intensity. The field of a light wave acting on an atom is much less than the intra-atomic electric power. fields, and an electron in an atom can be considered as harmonic. oscillator. In this approximation, the value and P. p.

They are constant quantities (at a given frequency), independent of light intensity. In intense light fluxes created by powerful lasers, the electrical value. The field of a light wave can be commensurate with intra-atomic electric power. fields and the harmony oscillator model turns out to be unacceptable. Taking into account the anharmonicity of forces in the electron-atom system leads to the dependence of the polarizability of the atom, and therefore the polarizability of the particle, on the intensity of light. The relationship between and turns out to be nonlinear; P. p. can be represented in the form

Where - P. p. at low light intensities; (usually accepted designation) - nonlinear addition to the P. p., or coefficient. nonlinearity. P. p. depends on the nature of the environment, for example. for silicate glasses

P. p. is also affected by high intensity as a result of the effect electrostriction, changing the density of the medium, high-frequency for anisotropic molecules (in liquid), and also as a result of an increase in temperature caused by absorption

Processes that are associated with light are an important component of physics and surround us everywhere in our everyday lives. The most important in this situation are the laws of reflection and refraction of light, on which modern optics is based. The refraction of light is an important part of modern science.

Distortion effect

This article will tell you what the phenomenon of light refraction is, as well as what the law of refraction looks like and what follows from it.

Basics of a physical phenomenon

When a beam falls on a surface that is separated by two transparent substances that have different optical densities (for example, different glasses or in water), some of the rays will be reflected, and some will penetrate into the second structure (for example, they will propagate in water or glass). When moving from one medium to another, a ray typically changes its direction. This is the phenomenon of light refraction.
The reflection and refraction of light is especially visible in water.

Distortion effect in water

Looking at things in water, they appear distorted. This is especially noticeable at the boundary between air and water. Visually, underwater objects appear to be slightly deflected. The described physical phenomenon is precisely the reason why all objects appear distorted in water. When the rays hit the glass, this effect is less noticeable.
Refraction of light is a physical phenomenon that is characterized by a change in the direction of movement of a solar ray at the moment it moves from one medium (structure) to another.
To improve our understanding of this process, consider an example of a beam hitting water from air (similarly for glass). By drawing a perpendicular line along the interface, the angle of refraction and return of the light beam can be measured. This index (angle of refraction) will change as the flow penetrates the water (inside the glass).
Note! This parameter is understood as the angle formed by a perpendicular drawn to the separation of two substances when a beam penetrates from the first structure to the second.

Beam Passage

The same indicator is typical for other environments. It has been established that this indicator depends on the density of the substance. If the beam falls from a less dense to a denser structure, then the angle of distortion created will be greater. And if it’s the other way around, then it’s less.
At the same time, a change in the slope of the decline will also affect this indicator. But the relationship between them does not remain constant. At the same time, the ratio of their sines will remain a constant value, which is reflected by the following formula: sinα / sinγ = n, where:

• n is a constant value that is described for each specific substance (air, glass, water, etc.). Therefore, what this value will be can be determined using special tables;
• α – angle of incidence;
• γ – angle of refraction.

To determine this physical phenomenon, the law of refraction was created.

Physical law

The law of refraction of light fluxes allows us to determine the characteristics of transparent substances. The law itself consists of two provisions:

• First part. The beam (incident, modified) and the perpendicular, which was restored at the point of incidence on the boundary, for example, of air and water (glass, etc.), will be located in the same plane;
• The second part. The ratio of the sine of the angle of incidence to the sine of the same angle formed when crossing the boundary will be a constant value.

Description of the law

In this case, at the moment the beam exits the second structure into the first (for example, when the light flux passes from the air, through the glass and back into the air), a distortion effect will also occur.

An important parameter for different objects

The main indicator in this situation is the ratio of the sine of the angle of incidence to a similar parameter, but for distortion. As follows from the law described above, this indicator is a constant value.
Moreover, when the value of the decline slope changes, the same situation will be typical for a similar indicator. This parameter is of great importance because it is an integral characteristic of transparent substances.

Indicators for different objects

Thanks to this parameter, you can quite effectively distinguish between types of glass, as well as various precious stones. It is also important for determining the speed of light in various environments.

Note! The highest speed of light flow is in a vacuum.

When moving from one substance to another, its speed will decrease. For example, in diamond, which has the highest refractive index, the speed of photon propagation will be 2.42 times higher than that of air. In water, they will spread 1.33 times slower. For different types of glass, this parameter ranges from 1.4 to 2.2.

Note! Some glasses have a refractive index of 2.2, which is very close to diamond (2.4). Therefore, it is not always possible to distinguish a piece of glass from a real diamond.

Optical density of substances

Light can penetrate through different substances, which are characterized by different optical densities. As we said earlier, using this law you can determine the density characteristic of the medium (structure). The denser it is, the slower the speed at which light will propagate through it. For example, glass or water will be more optically dense than air.
In addition to the fact that this parameter is a constant value, it also reflects the ratio of the speed of light in two substances. The physical meaning can be displayed as the following formula:

This indicator tells how the speed of propagation of photons changes when moving from one substance to another.

Another important indicator

When a light flux moves through transparent objects, its polarization is possible. It is observed during the passage of a light flux from dielectric isotropic media. Polarization occurs when photons pass through glass.

Polarization effect

Partial polarization is observed when the angle of incidence of the light flux at the boundary of two dielectrics differs from zero. The degree of polarization depends on what the angles of incidence were (Brewster's law).

Full internal reflection

Concluding our short excursion, it is still necessary to consider such an effect as full internal reflection.

The phenomenon of full display

For this effect to appear, it is necessary to increase the angle of incidence of the light flux at the moment of its transition from a more dense to a less dense medium at the interface between substances. In a situation where this parameter exceeds a certain limiting value, then photons incident on the boundary of this section will be completely reflected. Actually, this will be our desired phenomenon. Without it, it was impossible to make fiber optics.

Conclusion

The practical application of the behavior of light flux has given a lot, creating a variety of technical devices to improve our lives. At the same time, light has not yet revealed all its possibilities to humanity and its practical potential has not yet been fully realized.

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The law of light refraction. Absolute and relative refractive indices (coefficients). Total internal reflection

Law of light refraction was established experimentally in the 17th century. As light passes from one transparent medium to another, the direction of the light may change. The change in the direction of light at the boundary of different media is called refraction of light. As a result of refraction, an apparent change in the shape of the object occurs. (example: spoon in a glass of water). Law of light refraction: At the boundary of two media, the refracted ray lies in the plane of incidence and forms, with the normal to the interface restored at the point of incidence, an angle of refraction such that: =n 1-incidence, 2-reflection, n-refractive index (f. Snelius) - relative indicator The refractive index of a ray incident on a medium from airless space is called its absolute refractive index. The angle of incidence at which the refracted beam begins to slide along the interface between two media without moving into an optically denser medium – limiting angle of total internal reflection. Total internal reflection- internal reflection, provided that the angle of incidence exceeds a certain critical angle. In this case, the incident wave is completely reflected, and the value of the reflection coefficient exceeds its highest values ​​for polished surfaces. The reflectance of total internal reflection is independent of wavelength. In optics, this phenomenon is observed for a wide range of electromagnetic radiation, including the X-ray range. In geometric optics, the phenomenon is explained within the framework of Snell's law. Considering that the angle of refraction cannot exceed 90°, we find that at an angle of incidence whose sine is greater than the ratio of the smaller refractive index to the larger index, the electromagnetic wave must be completely reflected into the first medium. Example: The bright shine of many natural crystals, and especially cut precious and semi-precious stones, is explained by total internal reflection, as a result of which each ray entering the crystal forms a large number of fairly bright rays that emerge, colored as a result of dispersion.

Refraction or refraction is a phenomenon in which the direction of a ray of light or other waves changes when they cross the boundary separating two media, both transparent (transmitting these waves) and inside the medium in which the properties continuously change.

We encounter the phenomenon of refraction quite often and perceive it as an everyday phenomenon: we can see that a stick located in a transparent glass with a colored liquid is “broken” at the point of separation of air and water (Fig. 1). When light is refracted and reflected during rain, we rejoice when we see a rainbow (Fig. 2).

The refractive index is an important characteristic of a substance associated with its physicochemical properties. It depends on the temperature values, as well as on the wavelength of light at which the determination is carried out. According to quality control data in a solution, the refractive index is affected by the concentration of the substance dissolved in it, as well as the nature of the solvent. In particular, the refractive index of blood serum is affected by the amount of protein contained in it. This is due to the fact that with different speeds of propagation of light rays in media having different densities, their direction changes at the interface between the two media. If we divide the speed of light in a vacuum by the speed of light in the substance under study, we get the absolute refractive index (refractive index). In practice, the relative refractive index (n) is determined, which is the ratio of the speed of light in air to the speed of light in the substance under study.

The refractive index is determined quantitatively using a special device - a refractometer.

Refractometry is one of the easiest methods of physical analysis and can be used in quality control laboratories in the production of chemical, food, biologically active food additives, cosmetics and other types of products with minimal time and the number of samples being tested.

The design of the refractometer is based on the fact that light rays are completely reflected when they pass through the boundary of two media (one of them is a glass prism, the other is the test solution) (Fig. 3).

Rice. 3. Refractometer diagram

From the source (1), a light beam falls on the mirror surface (2), then, being reflected, passes into the upper lighting prism (3), then into the lower measuring prism (4), which is made of glass with a high refractive index. 1–2 drops of sample are applied between prisms (3) and (4) using a capillary. To avoid causing mechanical damage to the prism, it is necessary not to touch its surface with the capillary.

Through the eyepiece (9) a field with crossed lines is seen to establish the interface. When moving the eyepiece, the point of intersection of the fields must be aligned with the interface (Fig. 4). The plane of the prism (4) plays the role of the interface, on the surface of which the light beam is refracted. Since the rays are scattered, the border between light and shadow turns out to be blurry, iridescent. This phenomenon is eliminated by the dispersion compensator (5). The beam is then passed through the lens (6) and the prism (7). The plate (8) has sighting lines (two straight lines crossed crosswise), as well as a scale with refractive indices, which is observed through the eyepiece (9). The refractive index is calculated from it.

The dividing line between the field boundaries will correspond to the angle of internal total reflection, which depends on the refractive index of the sample.

Refractometry is used to determine the purity and authenticity of a substance. This method is also used to determine the concentration of substances in solutions during quality control, which is calculated using a calibration graph (a graph showing the dependence of the refractive index of a sample on its concentration).

At KorolevPharm, the refractive index is determined in accordance with approved regulatory documentation during incoming inspection of raw materials, in extracts of our own production, as well as during the release of finished products. The determination is made by qualified employees of an accredited physical and chemical laboratory using an IRF-454 B2M refractometer.

If, based on the results of the incoming inspection of raw materials, the refractive index does not meet the necessary requirements, the quality control department issues a Certificate of Non-Conformity, on the basis of which this batch of raw materials is returned to the supplier.

Method of determination

1. Before starting measurements, the cleanliness of the surfaces of the prisms in contact with each other is checked.

2. Checking the zero point. Apply 2÷3 drops of distilled water to the surface of the measuring prism and carefully cover it with the lighting prism. We open the lighting window and, using a mirror, install the light source in the most intense direction. By rotating the screws of the eyepiece, we obtain a clear, sharp distinction between the dark and light fields in its field of view. We rotate the screw and direct the line of shadow and light so that it coincides with the point where the lines intersect in the upper window of the eyepiece. On the vertical line in the lower window of the eyepiece we see the desired result - the refractive index of distilled water at 20 ° C (1.333). If the readings are different, use the screw to set the refractive index to 1.333, and using a key (remove the adjustment screw) bring the boundary of shadow and light to the point where the lines intersect.

3. Determine the refractive index. We lift the chamber of the lighting prism and remove the water with filter paper or a gauze napkin. Next, apply 1-2 drops of the test solution to the surface of the measuring prism and close the chamber. Rotate the screws until the boundaries of the shadow and light coincide with the point of intersection of the lines. On the vertical line in the lower window of the eyepiece we see the desired result - the refractive index of the test sample. We calculate the refractive index using the scale in the lower window of the eyepiece.

4. Using a calibration graph, we establish the relationship between the concentration of the solution and the refractive index. To construct a graph, it is necessary to prepare standard solutions of several concentrations using preparations of chemically pure substances, measure their refractive indices and plot the obtained values ​​on the ordinate axis, and the corresponding concentrations of solutions on the abscissa axis. It is necessary to select concentration intervals at which a linear relationship is observed between concentration and refractive index. We measure the refractive index of the sample under study and use a graph to determine its concentration.

Light refraction- a phenomenon in which a ray of light, passing from one medium to another, changes direction at the boundary of these media.

Refraction of light occurs according to the following law:
The incident and refracted rays and the perpendicular drawn to the interface between the two media at the point of incidence of the ray lie in the same plane. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for two media:
,
Where α - angle of incidence,
β - refraction angle,
n - a constant value independent of the angle of incidence.

When the angle of incidence changes, the angle of refraction also changes. The greater the angle of incidence, the greater the angle of refraction.
If light comes from an optically less dense medium to a more dense medium, then the angle of refraction is always less than the angle of incidence: β < α.
A ray of light directed perpendicular to the interface between two media passes from one medium to another without refraction.

absolute refractive index of a substance- a value equal to the ratio of the phase speeds of light (electromagnetic waves) in vacuum and in a given environment n=c/v
The quantity n included in the law of refraction is called the relative refractive index for a pair of media.

The value n is the relative refractive index of medium B with respect to medium A, and n" = 1/n is the relative refractive index of medium A with respect to medium B.
This value, other things being equal, is greater than unity when the beam passes from a denser medium to a less dense medium, and less than unity when the beam passes from a less dense medium to a denser medium (for example, from a gas or from a vacuum to a liquid or solid). There are exceptions to this rule, and therefore it is customary to call a medium optically more or less dense than another.
A ray falling from airless space onto the surface of some medium B is refracted more strongly than when falling on it from another medium A; The refractive index of a ray incident on a medium from airless space is called its absolute refractive index.

(Absolute - relative to vacuum.
Relative - relative to any other substance (the same air, for example).
The relative indicator of two substances is the ratio of their absolute indicators.)

Total internal reflection- internal reflection, provided that the angle of incidence exceeds a certain critical angle. In this case, the incident wave is completely reflected, and the value of the reflection coefficient exceeds its highest values ​​for polished surfaces. The reflectance of total internal reflection is independent of wavelength.

In optics, this phenomenon is observed for a wide range of electromagnetic radiation, including the X-ray range.

In geometric optics, the phenomenon is explained within the framework of Snell's law. Considering that the angle of refraction cannot exceed 90°, we find that at an angle of incidence whose sine is greater than the ratio of the lower refractive index to the larger index, the electromagnetic wave must be completely reflected into the first medium.

In accordance with the wave theory of the phenomenon, the electromagnetic wave still penetrates into the second medium - the so-called “non-uniform wave” propagates there, which decays exponentially and does not carry energy with it. The characteristic depth of penetration of an inhomogeneous wave into the second medium is of the order of the wavelength.

Laws of light refraction.

From all that has been said we conclude:
1 . At the interface between two media of different optical densities, a light ray changes its direction when passing from one medium to another.
2. When a light beam passes into a medium with a higher optical density, the angle of refraction is less than the angle of incidence; When a light ray passes from an optically denser medium to a less dense medium, the angle of refraction is greater than the angle of incidence.
The refraction of light is accompanied by reflection, and with an increase in the angle of incidence, the brightness of the reflected beam increases, and the refracted beam weakens. This can be seen by conducting the experiment shown in the figure. Consequently, the reflected beam carries with it more light energy, the greater the angle of incidence.

Let MN- the interface between two transparent media, for example, air and water, JSC- incident ray, OB- refracted ray, - angle of incidence, - angle of refraction, - speed of light propagation in the first medium, - speed of light propagation in the second medium.