Calculate the refractive index. Absolute refractive index and its relation to relative refractive index

Fields of application of refractometry.

The device and principle of operation of the IRF-22 refractometer.

The concept of the refractive index.

Plan

Refractometry. Characteristics and essence of the method.

To identify substances and check their purity, use

refractor.

Refractive index of a substance- a value equal to the ratio of the phase velocities of light (electromagnetic waves) in vacuum and the seen medium.

The refractive index depends on the properties of the substance and the wavelength

electromagnetic radiation. The ratio of the sine of the angle of incidence relative to

the normal drawn to the plane of refraction (α) of the beam to the sine of the angle of refraction

refraction (β) during the transition of the beam from medium A to medium B is called the relative refractive index for this pair of media.

The value n is the relative refractive index of the medium B according to

in relation to environment A, and

The relative refractive index of the medium A with respect to

The refractive index of a beam incident on a medium from an airless

th space is called its absolute refractive index or

simply the refractive index of a given medium (Table 1).

Table 1 - Refractive indices of various media

Liquids have a refractive index in the range of 1.2-1.9. Solid

substances 1.3-4.0. Some minerals do not have an exact value of the indicator

for refraction. Its value is in a certain "fork" and determines

due to the presence of impurities in the crystal structure, which determines the color

crystal.

Identification of the mineral by "color" is difficult. So, the mineral corundum exists in the form of ruby, sapphire, leucosapphire, differing in

refractive index and color. Red corundums are called rubies

(chromium admixture), colorless blue, light blue, pink, yellow, green,

violet - sapphires (impurities of cobalt, titanium, etc.). Light-colored

nye sapphires or colorless corundum is called leucosapphire (widely

used in optics as a light filter). The refractive index of these crystals

stall lies in the range of 1.757-1.778 and is the basis for identifying

Figure 3.1 - Ruby Figure 3.2 - Sapphire blue

Organic and inorganic liquids also have characteristic refractive index values ​​that characterize them as chemical

nye compounds and the quality of their synthesis (table 2):

Table 2 - Refractive indices of some liquids at 20 °C

4.2. Refractometry: concept, principle.

Method for the study of substances based on the determination of the indicator

(coefficient) of refraction (refraction) is called refractometry (from

lat. refractus - refracted and Greek. metreo - I measure). Refractometry

(refractometric method) is used to identify chemical

compounds, quantitative and structural analysis, determination of physico-

chemical parameters of substances. Refractometry principle implemented

in Abbe refractometers, illustrated by Figure 1.

Figure 1 - The principle of refractometry

The Abbe prism block consists of two rectangular prisms: illuminating

body and measuring, folded by hypotenuse faces. Illuminator-

prism has a rough (matte) hypotenuse face and is intended

chena for illuminating a liquid sample placed between the prisms.

Scattered light passes through a plane-parallel layer of the investigated liquid and, being refracted in the liquid, falls on the measuring prism. The measuring prism is made of optically dense glass (heavy flint) and has a refractive index greater than 1.7. For this reason, the Abbe refractometer measures n values ​​less than 1.7. An increase in the measuring range of the refractive index can only be achieved by changing the measuring prism.

The test sample is poured onto the hypotenuse face of the measuring prism and pressed against the illuminating prism. In this case, a gap of 0.1-0.2 mm remains between the prisms in which the sample is located, and through

which passes by refracting light. To measure the refractive index

use the phenomenon of total internal reflection. It consists in

next.

If rays 1, 2, 3 fall on the interface between two media, then depending on

the angle of incidence when observing them in a refractive medium will be

the presence of a transition of areas of different illumination is observed. It's connected

with the incidence of some part of the light on the boundary of refraction at an angle of approx.

kim to 90° with respect to the normal (beam 3). (Figure 2).

Figure 2 - Image of refracted rays

This part of the rays is not reflected and therefore forms a lighter object.

refraction. Rays with smaller angles experience and reflect

and refraction. Therefore, an area of ​​less illumination is formed. In volume

the boundary line of total internal reflection is visible on the lens, the position

which depends on the refractive properties of the sample.

The elimination of the dispersion phenomenon (coloring of the interface between two areas of illumination in the colors of the rainbow due to the use of complex white light in Abbe refractometers) is achieved by using two Amici prisms in the compensator, which are mounted in the telescope. At the same time, a scale is projected into the lens (Figure 3). 0.05 ml of liquid is sufficient for analysis.

Figure 3 - View through the eyepiece of the refractometer. (The right scale reflects

concentration of the measured component in ppm)

In addition to the analysis of single-component samples, there are widely analyzed

two-component systems (aqueous solutions, solutions of substances in which

or solvent). In ideal two-component systems (forming-

without changing the volume and polarizability of the components), the dependence is shown

refractive index on the composition is close to linear if the composition is expressed in terms of

volume fractions (percentage)

where: n, n1, n2 - refractive indices of the mixture and components,

V1 and V2 are the volume fractions of the components (V1 + V2 = 1).

The effect of temperature on the refractive index is determined by two

factors: a change in the number of liquid particles per unit volume and

dependence of the polarizability of molecules on temperature. The second factor became

becomes significant only at very large temperature changes.

The temperature coefficient of the refractive index is proportional to the temperature coefficient of the density. Since all liquids expand when heated, their refractive indices decrease as the temperature rises. The temperature coefficient depends on the temperature of the liquid, but in small temperature intervals it can be considered constant. For this reason, most refractometers do not have temperature control, however, some designs provide

water temperature control.

Linear extrapolation of the refractive index with temperature changes is acceptable for small temperature differences (10 - 20°C).

The exact determination of the refractive index in wide temperature ranges is carried out according to empirical formulas of the form:

nt=n0+at+bt2+…

For solution refractometry over wide concentration ranges

use tables or empirical formulas. Display dependency-

refractive index of aqueous solutions of certain substances on concentration

is close to linear and makes it possible to determine the concentrations of these substances in

water in a wide range of concentrations (Figure 4) using refraction

tometers.

Figure 4 - Refractive index of some aqueous solutions

Usually, n liquid and solid bodies are determined by refractometers with precision

up to 0.0001. The most common are Abbe refractometers (Figure 5) with prism blocks and dispersion compensators, which make it possible to determine nD in "white" light on a scale or digital indicator.

Figure 5 - Abbe refractometer (IRF-454; IRF-22)

Refraction is called a certain abstract number that characterizes the refractive power of any transparent medium. It is customary to designate it n. There are absolute refractive index and relative coefficient.

The first is calculated using one of two formulas:

n = sin α / sin β = const (where sin α is the sine of the angle of incidence, and sin β is the sine of the light beam entering the medium under consideration from the void)

n = c / υ λ (where c is the speed of light in a vacuum, υ λ is the speed of light in the medium under study).

Here, the calculation shows how many times light changes its speed of propagation at the moment of transition from vacuum to a transparent medium. In this way, the refractive index (absolute) is determined. In order to find out the relative, use the formula:

That is, the absolute refractive indices of substances of different densities, such as air and glass, are considered.

Generally speaking, the absolute coefficients of any bodies, whether gaseous, liquid or solid, are always greater than 1. Basically, their values ​​range from 1 to 2. This value can be above 2 only in exceptional cases. The value of this parameter for some environments:

This value, when applied to the hardest natural substance on the planet, diamond, is 2.42. Very often, when conducting scientific research, etc., it is required to know the refractive index of water. This parameter is 1.334.

Since the wavelength is an indicator, of course, not constant, an index is assigned to the letter n. Its value helps to understand which wave of the spectrum this coefficient refers to. When considering the same substance, but with increasing wavelength of light, the refractive index will decrease. This circumstance caused the decomposition of light into a spectrum when passing through a lens, prism, etc.

By the value of the refractive index, you can determine, for example, how much of one substance is dissolved in another. This is useful, for example, in brewing or when you need to know the concentration of sugar, fruit or berries in the juice. This indicator is also important in determining the quality of petroleum products, and in jewelry, when it is necessary to prove the authenticity of a stone, etc.

Without the use of any substance, the scale visible in the eyepiece of the instrument will be completely blue. If you drop ordinary distilled water on a prism, with the correct calibration of the instrument, the border of blue and white colors will pass strictly along the zero mark. When examining another substance, it will shift along the scale according to what refractive index it has.

The refractive index of a medium relative to vacuum, i.e., for the case of the transition of light rays from vacuum to a medium, is called absolute and is determined by formula (27.10): n=c/v.

In calculations, the absolute refractive indices are taken from the tables, since their value is determined quite accurately using experiments. Since c is greater than v, then the absolute refractive index is always greater than unity.

If light radiation passes from vacuum to a medium, then the formula for the second law of refraction is written as:

sin i/sin β = n. (29.6)

Formula (29.6) is also often used in practice when rays pass from air to a medium, since the speed of light propagation in air differs very little from c. This can be seen from the fact that the absolute refractive index of air is 1.0029.

When the beam goes from the medium to vacuum (to air), then the formula for the second law of refraction takes the form:

sin i/sin β = 1/n. (29.7)

In this case, the rays, when leaving the medium, necessarily move away from the perpendicular to the interface between the medium and the vacuum.

Let's find out how you can find the relative refractive index n21 from the absolute refractive indices. Let the light pass from the medium with the absolute index n1 to the medium with the absolute index n2. Then n1 = c/V1 andn2 = s/v2, from where:

n2/n1=v1/v2=n21. (29.8)

The formula for the second law of refraction for such a case is often written as follows:

sini/sinβ = n2/n1. (29.9)

Let us remember that by Maxwell's theory absolute exponent refraction can be found from the relation: n = √(με). Since for substances transparent to light radiation, μ is practically equal to unity, we can assume that:

n = √ε. (29.10)

Since the frequency of oscillations in light radiation is of the order of 10 14 Hz, neither dipoles nor ions in a dielectric, which have a relatively large mass, have time to change their position with such a frequency, and the dielectric properties of a substance under these conditions are determined only by the electronic polarization of its atoms. This explains the difference between the value ε=n 2 from (29.10) and ε st in electrostatics. So, for water ε \u003d n 2 \u003d 1.77, and ε st \u003d 81; the ionic solid dielectric NaCl ε=2.25, and ε st =5.6. When a substance consists of homogeneous atoms or non-polar molecules, i.e., it has neither ions nor natural dipoles, then its polarization can only be electronic. For similar substances, ε from (29.10) and ε st coincide. An example of such a substance is diamond, which consists of only carbon atoms.

Note that the value of the absolute refractive index, in addition to the type of substance, also depends on the oscillation frequency, or on the radiation wavelength . As the wavelength decreases, as a rule, the refractive index increases.

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.

Refractive index

Refractive index substances - a value equal to the ratio of the phase velocities of light (electromagnetic waves) in vacuum and in a given medium. Also, the refractive index is sometimes spoken of for any other waves, for example, sound, although in cases such as the latter, the definition, of course, has to be somehow modified.

The refractive index depends on the properties of the substance and the wavelength of the radiation, for some substances the refractive index changes quite strongly when the frequency of electromagnetic waves changes from low frequencies to optical and beyond, and can also change even more sharply in certain areas of the frequency scale. The default is usually the optical range, or the range determined by the context.

Links

• RefractiveIndex.INFO refractive index database

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See what "Refraction index" is in other dictionaries:

Relative to two media n21, dimensionless ratio of optical radiation propagation velocities (c veta a) in the first (c1) and second (c2) media: n21=c1/c2. At the same time refers. P. p. is the ratio of the sines of the g and fall of j and at g l ... ... Physical Encyclopedia

See Refractive Index...

See index of refraction. * * * REFRACTIVE INDEX REFRACTIVE INDEX, see Refractive Index (see REFRACTIVE INDEX) … encyclopedic Dictionary- REFRACTIVE INDEX, a value that characterizes the medium and is equal to the ratio of the speed of light in vacuum to the speed of light in the medium (absolute refractive index). The refractive index n depends on the dielectric e and magnetic permeability m ... ... Illustrated Encyclopedic Dictionary

- (see REFRACTIVE INDICATOR). Physical Encyclopedic Dictionary. Moscow: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1983... Physical Encyclopedia

See refractive index... Great Soviet Encyclopedia

The ratio of the speed of light in vacuum to the speed of light in a medium (absolute refractive index). The relative refractive index of 2 media is 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 ... ... Big Encyclopedic Dictionary