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

Law of refraction of light was established empirically in the 17th century. When light passes from one transparent medium to another, the direction of light can change. Changing the direction of light at the boundary of different media is called light refraction. The omniscience of refraction is an apparent change in the shape of an object. (example: a spoon in a glass of water). The law of refraction of light: At the boundary of two media, the refracted beam 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-fall, 2 reflections, n-refractive index (f. Snelius) - relative indicator The refractive index of a beam incident on a medium from airless space is called its absolute index of refraction. The angle of incidence at which the refracted beam begins to slide along the interface between two media without transition to 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 reflection coefficient for total internal reflection does not depend on the wavelength. In optics, this phenomenon is observed for a wide spectrum of electromagnetic radiation, including the X-ray range. In geometric optics, the phenomenon is explained in terms of Snell's law. Considering that the angle of refraction cannot exceed 90°, we obtain that at an angle of incidence whose sine is greater than the ratio of the smaller refractive index to the larger one, the electromagnetic wave should be completely reflected into the first medium. Example: The bright brilliance of many natural crystals, and especially faceted precious and semiprecious stones, is explained by total internal reflection, as a result of which each ray that enters the crystal forms a large number of sufficiently bright rays that come out, colored as a result of dispersion.

TO LECTURE №24

"INSTRUMENTAL METHODS OF ANALYSIS"

REFRACTOMETRY.

Literature:

1. V.D. Ponomarev "Analytical Chemistry" 1983 246-251

2. A.A. Ishchenko "Analytical Chemistry" 2004 pp 181-184

REFRACTOMETRY.

Refractometry is one of the simplest physical methods of analysis, requiring a minimum amount of analyte and is carried out in a very short time.

Refractometry- a method based on the phenomenon of refraction or refraction i.e. change in the direction of light propagation when passing from one medium to another.

Refraction, as well as the absorption of light, is a consequence of its interaction with the medium. The word refractometry means dimension refraction of light, which is estimated by the value of the refractive index.

Refractive index value n depends

1) on the composition of substances and systems,

2) from at what concentration and what molecules the light beam meets on its way, because Under the action of light, the molecules of different substances are polarized in different ways. It is on this dependence that the refractometric method is based.

This method has a number of advantages, as a result of which it has found wide application both in chemical research and in the control of technological processes.

1) The measurement of refractive indices is a very simple process that is carried out accurately and with a minimum investment of time and amount of substance.

2) Typically, refractometers provide up to 10% accuracy in determining the refractive index of light and the content of the analyte

The refractometry method is used to control authenticity and purity, to identify individual substances, to determine the structure of organic and inorganic compounds in the study of solutions. Refractometry is used to determine the composition of two-component solutions and for ternary systems.

Physical basis of the method

REFRACTIVE INDICATOR.

The deviation of a light beam from its original direction when it passes from one medium to another is the greater, the greater the difference in the speeds of light propagation in two

these environments.

Consider the refraction of a light beam at the boundary of any two transparent media I and II (See Fig.). Let us agree that medium II has a greater refractive power and, therefore, n 1 and n 2- shows the refraction of the corresponding media. If medium I is neither vacuum nor air, then the ratio sin of the angle of incidence of the light beam to sin of the angle of refraction will give the value of the relative refractive index n rel. The value of n rel. can also be defined as the ratio of the refractive indices of the media under consideration.

n rel. = ----- = ---

The value of the refractive index depends on

1) the nature of substances

The nature of a substance in this case is determined by the degree of deformability of its molecules under the action of light - the degree of polarizability. The more intense the polarizability, the stronger the refraction of light.

2)incident light wavelength

The measurement of the refractive index is carried out at a light wavelength of 589.3 nm (line D of the sodium spectrum).

The dependence of the refractive index on the wavelength of light is called dispersion. The shorter the wavelength, the greater the refraction. Therefore, rays of different wavelengths are refracted differently.

3)temperature at which the measurement is taken. A prerequisite for determining the refractive index is compliance with the temperature regime. Usually the determination is performed at 20±0.3 0 С.

As the temperature rises, the refractive index decreases, and as the temperature decreases, it increases..

The temperature correction is calculated using the following formula:

n t \u003d n 20 + (20-t) 0.0002, where

n t - Bye refractive index at a given temperature,

n 20 - refractive index at 20 0 С

The influence of temperature on the values ​​of the refractive indices of gases and liquids is related to the values ​​of their coefficients of volumetric expansion. The volume of all gases and liquids increases when heated, the density decreases and, consequently, the indicator decreases

The refractive index, measured at 20 0 C and a light wavelength of 589.3 nm, is indicated by the index n D 20

The dependence of the refractive index of a homogeneous two-component system on its state is established experimentally by determining the refractive index for a number of standard systems (for example, solutions), the content of components in which is known.

4) the concentration of a substance in a solution.

For many aqueous solutions of substances, the refractive indices at various concentrations and temperatures have been reliably measured, and in these cases reference data can be used. refractometric tables. Practice shows that when the content of the dissolved substance does not exceed 10-20%, along with the graphical method, in very many cases it is possible to use linear equation like:

n=n o +FC,

n- refractive index of the solution,

no is the refractive index of the pure solvent,

C- concentration of the dissolved substance,%

F-empirical coefficient, the value of which is found

by determining the refractive indices of solutions of known concentration.

REFRACTOMETERS.

Refractometers are devices used to measure the refractive index. There are 2 types of these instruments: Abbe type refractometer and Pulfrich type. Both in those and in others, the measurements are based on determining the magnitude of the limiting angle of refraction. In practice, refractometers of various systems are used: laboratory-RL, universal RLU, etc.

The refractive index of distilled water n 0 \u003d 1.33299, in practice, this indicator takes as reference as n 0 =1,333.

The principle of operation on refractometers is based on the determination of the refractive index by the limiting angle method (the angle of total reflection of light).

Hand refractometer

Refractometer Abbe

The processes that are associated with light are an important component of physics and surround us everywhere in our everyday life. 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.

Fundamentals 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, it will propagate in water or glass). When passing from one medium to another, the beam is characterized by a change in its direction. This is the phenomenon of light refraction.
Reflection and refraction of light can be seen especially well in water.

water distortion effect

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

Beam passage

The same indicator is typical for other environments. It is established that this indicator depends on the density of the substance. If the beam is incident from a less dense to a denser structure, then the angle of distortion created will be larger. And if vice versa, then less.
At the same time, a change in the slope of the fall 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 constant, which is displayed 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 from special tables;
• α is the angle of incidence;
• γ is the angle of refraction.

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

physical law

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

• First part. The beam (incident, changed) and the perpendicular, which was restored at the point of incidence at the boundary, for example, air and water (glass, etc.), will be located in the same plane;
• second part. The indicator of 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.
At the same time, when the value of the slope of the fall changes, the same situation will be typical for a similar indicator. This parameter is of great importance, since 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 a variety of precious stones. It is also important for determining the speed of light in various media.

Note! The highest speed of the light flux is in vacuum.

When moving from one substance to another, its speed will decrease. For example, diamond, which has the highest refractive index, will have a photon propagation speed 2.42 times faster than 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 density. As we said earlier, using this law, you can determine the characteristic of the density of the medium (structure). The denser it is, the slower the speed of light will propagate in 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 passing from one substance to another.

Another important indicator

When moving the light flux 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 digression, it is still necessary to consider such an effect as a full-fledged internal reflection.

Full Display Phenomenon

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 denser to a less dense medium at the interface between substances. In a situation where this parameter exceeds a certain limit value, then the 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 features of the behavior of the light flux gave a lot, creating a variety of technical devices to improve our lives. At the same time, light has not opened all its possibilities to mankind, and its practical potential has not yet been fully realized.

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

The refraction of light occurs according to the following law:
The incident and refracted rays and the perpendicular drawn to the interface between two media at the point of incidence of the beam 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 - angle of refraction
n - a constant value independent of the angle of incidence.

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

absolute refractive index of a substance- a value equal to the ratio of the phase velocities of light (electromagnetic waves) in vacuum and in a given medium n=c/v
The value 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, ceteris paribus, 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 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 beam 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 index of two substances is the ratio of their absolute indices.)

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 reflection coefficient for total internal reflection does not depend on the wavelength.

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

In geometric optics, the phenomenon is explained in terms of Snell's law. Considering that the angle of refraction cannot exceed 90°, we obtain 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 should be completely reflected into the first medium.

In accordance with the wave theory of the phenomenon, the electromagnetic wave nevertheless penetrates into the second medium - the so-called "non-uniform wave" propagates there, which decays exponentially and does not carry away 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 refraction of light.

From all that has been said, we conclude:
1 . At the interface between two media of different optical density, a beam of light 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 beam 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, while the refracted one weakens. This can be seen by conducting the experiment shown in the figure. Consequently, the reflected beam carries away with it the more light energy, the greater the angle of incidence.

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

Light, by its nature, propagates in different media at different speeds. The denser the medium, the lower the speed of propagation of light in it. An appropriate measure has been established relating both to the density of a material and to the speed of propagation of light in that material. This measure is called the index of refraction. For any material, the refractive index is measured relative to the speed of light in a vacuum (vacuum is often referred to as free space). The following formula describes this relationship.

The higher the refractive index of a material, the denser it is. When a beam of light passes from one material to another (with a different refractive index), the angle of refraction will be different from the angle of incidence. A beam of light penetrating a medium with a lower refractive index will exit at an angle greater than the angle of incidence. A beam of light penetrating a medium with a high refractive index will exit at an angle smaller than the angle of incidence. This is shown in fig. 3.5.

Rice. 3.5.a. A beam passing from a medium with high N 1 to a medium with low N 2

Rice. 3.5.b. A beam passing from a medium with low N 1 to a medium with high N 2

In this case, θ 1 is the angle of incidence and θ 2 is the angle of refraction. Some typical refractive indices are listed below.

It is curious to note that for x-rays the refractive index of glass is always less than for air, therefore, when passing from air into glass, they deviate away from the perpendicular, and not towards the perpendicular, like light rays.