Optics is one of the old branches of physics. Since the times of ancient Greece, many philosophers have been interested in the laws of movement and propagation of light in various transparent materials, such as water, glass, diamond and air. This article examines the phenomenon of light refraction, focusing on the refractive index of air.

Light beam refraction effect

Everyone in their life has encountered hundreds of times the manifestation of this effect when they looked at the bottom of a reservoir or at a glass of water with some object placed in it. At the same time, the pond did not seem as deep as it actually was, and the objects in the glass of water looked deformed or broken.

The phenomenon of refraction consists of a break in its straight path when it intersects the interface of two transparent materials. Summarizing a large amount of experimental data, at the beginning of the 17th century, the Dutchman Willebrord Snell obtained a mathematical expression that accurately described this phenomenon. This expression is usually written in the following form:

n 1 *sin(θ 1) = n 2 *sin(θ 2) = const.

Here n 1, n 2 are the absolute refractive indices of light in the corresponding material, θ 1 and θ 2 are the angles between the incident and refracted rays and the perpendicular to the interface plane, which is drawn through the intersection point of the ray and this plane.

This formula is called Snell's or Snell-Descartes' law (it was the Frenchman who wrote it down in the presented form, while the Dutchman used units of length rather than sines).

In addition to this formula, the phenomenon of refraction is described by another law, which is geometric in nature. It consists in the fact that the marked perpendicular to the plane and two rays (refracted and incident) lie in the same plane.

Absolute refractive index

This quantity is included in the Snell formula, and its value plays an important role. Mathematically, the refractive index n corresponds to the formula:

The symbol c is the speed of electromagnetic waves in a vacuum. It is approximately 3*10 8 m/s. The value v is the speed of light moving through the medium. Thus, the refractive index reflects the amount of retardation of light in a medium relative to airless space.

Two important conclusions follow from the formula above:

• the value of n is always greater than 1 (for vacuum it is equal to unity);
• it is a dimensionless quantity.

For example, the refractive index of air is 1.00029, while for water it is 1.33.

The refractive index is not a constant value for a particular medium. It depends on the temperature. Moreover, for each frequency of an electromagnetic wave it has its own meaning. Thus, the above figures correspond to a temperature of 20 o C and the yellow part of the visible spectrum (wavelength - about 580-590 nm).

The dependence of n on the frequency of light is manifested in the decomposition of white light by a prism into a number of colors, as well as in the formation of a rainbow in the sky during heavy rain.

Refractive index of light in air

Its value has already been given above (1.00029). Since the refractive index of air differs only in the fourth decimal place from zero, for solving practical problems it can be considered equal to one. A slight difference between n for air and unity indicates that light is practically not slowed down by air molecules, which is due to its relatively low density. Thus, the average air density is 1.225 kg/m 3, that is, it is more than 800 times lighter than fresh water.

Air is an optically weak medium. The process of slowing down the speed of light in a material is of a quantum nature and is associated with the acts of absorption and emission of photons by atoms of the substance.

Changes in the composition of air (for example, an increase in the content of water vapor in it) and changes in temperature lead to significant changes in the refractive index. A striking example is the mirage effect in the desert, which occurs due to differences in the refractive indices of air layers with different temperatures.

Glass-air interface

Glass is a much denser medium than air. Its absolute refractive index ranges from 1.5 to 1.66, depending on the type of glass. If we take the average value of 1.55, then the refraction of the beam at the air-glass interface can be calculated using the formula:

sin(θ 1)/sin(θ 2) = n 2 /n 1 = n 21 = 1.55.

The value n 21 is called the relative refractive index of air - glass. If the beam comes out of the glass into the air, then the following formula should be used:

sin(θ 1)/sin(θ 2) = n 2 /n 1 = n 21 = 1/1.55 ​​= 0.645.

If the angle of the refracted ray in the latter case is equal to 90 o, then the corresponding one is called critical. For the glass-air boundary it is equal to:

θ 1 = arcsin(0.645) = 40.17 o.

If the beam falls on the glass-air boundary with larger angles than 40.17 o, then it will be reflected completely back into the glass. This phenomenon is called “total internal reflection”.

The critical angle exists only when the beam moves from a dense medium (from glass to air, but not vice versa).

Ticket 75.

Law of Light Reflection: the incident and reflected rays, as well as the perpendicular to the interface between the two media, reconstructed at the point of incidence of the ray, lie in the same plane (plane of incidence). The angle of reflection γ is equal to the angle of incidence α.

Law of light refraction: the incident and refracted rays, as well as the perpendicular to the interface between the two media, reconstructed at the point of incidence of the ray, lie in the same plane. The ratio of the sine of the angle of incidence α to the sine of the angle of refraction β is a constant value for two given media:

The laws of reflection and refraction are explained in wave physics. According to wave concepts, refraction is a consequence of changes in the speed of propagation of waves when passing from one medium to another. Physical meaning of the refractive index is the ratio of the speed of propagation of waves in the first medium υ 1 to the speed of their propagation in the second medium υ 2:

Figure 3.1.1 illustrates the laws of reflection and refraction of light.

A medium with a lower absolute refractive index is called optically less dense.

When light passes from an optically denser medium to an optically less dense medium n 2< n 1 (например, из стекла в воздух) можно наблюдать total reflection phenomenon, that is, the disappearance of the refracted ray. This phenomenon is observed at angles of incidence exceeding a certain critical angle α pr, which is called limiting angle of total internal reflection(see Fig. 3.1.2).

For the angle of incidence α = α pr sin β = 1; value sin α pr = n 2 / n 1< 1.

If the second medium is air (n 2 ≈ 1), then it is convenient to rewrite the formula in the form

The phenomenon of total internal reflection is used in many optical devices. The most interesting and practically important application is the creation of optical fibers, which are thin (from several micrometers to millimeters) arbitrarily curved threads made of optically transparent material (glass, quartz). Light falling on the end of the light guide can propagate along it over long distances due to total internal reflection from the side surfaces (Figure 3.1.3). The scientific and technical direction involved in the development and application of optical light guides is called fiber optics.

Dispersion of light (decomposition of light)- this is a phenomenon caused by the dependence of the absolute refractive index of a substance on the frequency (or wavelength) of light (frequency dispersion), or, the same thing, the dependence of the phase speed of light in a substance on the wavelength (or frequency). It was discovered experimentally by Newton around 1672, although theoretically quite well explained much later.

Spatial dispersion is called the dependence of the dielectric constant tensor of the medium on the wave vector. This dependence causes a number of phenomena called spatial polarization effects.

One of the most clear examples of dispersion - white light decomposition when passing through a prism (Newton's experiment). The essence of the dispersion phenomenon is the difference in the speed of propagation of light rays of different wavelengths in a transparent substance - an optical medium (while in a vacuum the speed of light is always the same, regardless of the wavelength and therefore color). Typically, the higher the frequency of a light wave, the higher the refractive index of the medium for it and the lower the speed of the wave in the medium:

Newton's experiments Experiment on the decomposition of white light into a spectrum: Newton directed a beam of sunlight through a small hole onto a glass prism. When hitting the prism, the beam was refracted and on the opposite wall gave an elongated image with a rainbow alternation of colors - a spectrum. Experiment on the passage of monochromatic light through a prism: Newton placed red glass in the path of the sun's ray, behind which he received monochromatic light (red), then a prism and observed on the screen only the red spot from the light ray. Experience in the synthesis (production) of white light: First, Newton directed a ray of sunlight onto a prism. Then, having collected the colored rays emerging from the prism using a collecting lens, Newton received a white image of a hole on a white wall instead of a colored stripe. Newton's conclusions:- a prism does not change light, but only decomposes it into its components - light rays that differ in color differ in the degree of refraction; Violet rays refract most strongly, red ones less strongly - red light, which refracts less, has the highest speed, and violet has the least, which is why the prism decomposes the light. The dependence of the refractive index of light on its color is called dispersion.

Conclusions:- a prism decomposes light - white light is complex (composite) - violet rays are refracted more strongly than red ones. The color of a light beam is determined by its vibration frequency. When moving from one medium to another, the speed of light and wavelength change, but the frequency that determines the color remains constant. The boundaries of the ranges of white light and its components are usually characterized by their wavelengths in vacuum. White light is a collection of waves with lengths from 380 to 760 nm.

Ticket 77.

Absorption of light. Bouguer's law

The absorption of light in a substance is associated with the conversion of the energy of the electromagnetic field of the wave into the thermal energy of the substance (or into the energy of secondary photoluminescent radiation). The law of light absorption (Bouguer's law) has the form:

I=I 0 exp(- x),(1)

Where I 0 , I-light intensity at the input (x=0) and leaving the layer of medium thickness X, - absorption coefficient, it depends on .

For dielectrics  =10 -1  10 -5 m -1 , for metals =10 5  10 7 m -1 , Therefore, metals are opaque to light.

Dependency  ( ) explains the color of absorbing bodies. For example, glass that absorbs red light poorly will appear red when illuminated with white light.

Scattering of light. Rayleigh's law

Diffraction of light can occur in an optically inhomogeneous medium, for example in a turbid environment (smoke, fog, dusty air, etc.). By diffracting on inhomogeneities of the medium, light waves create a diffraction pattern characterized by a fairly uniform distribution of intensity in all directions.

This diffraction by small inhomogeneities is called scattering of light.

This phenomenon is observed when a narrow beam of sunlight passes through dusty air, scatters on dust particles and becomes visible.

If the sizes of inhomogeneities are small compared to the wavelength (no more than 0,1  ), then the intensity of the scattered light turns out to be inversely proportional to the fourth power of the wavelength, i.e.

I diss ~ 1/  4 , (2)

this dependence is called Rayleigh's law.

Light scattering is also observed in clean media that do not contain foreign particles. For example, it can occur on fluctuations (random deviations) of density, anisotropy or concentration. This type of scattering is called molecular scattering. It explains, for example, the blue color of the sky. Indeed, according to (2), blue and blue rays are scattered more strongly than red and yellow ones, because have a shorter wavelength, thereby causing the blue color of the sky.

Ticket 78.

Polarization of light- a set of wave optics phenomena in which the transverse nature of electromagnetic light waves is manifested. Transverse wave- particles of the medium oscillate in directions perpendicular to the direction of wave propagation ( Fig.1).

Fig.1 Transverse wave

Electromagnetic light wave plane polarized(linear polarization), if the directions of oscillation of vectors E and B are strictly fixed and lie in certain planes ( Fig.1). A plane polarized light wave is called plane polarized(linearly polarized) light. Unpolarized(natural) wave - an electromagnetic light wave in which the directions of oscillation of the vectors E and B in this wave can lie in any planes perpendicular to the velocity vector v. Unpolarized light- light waves in which the directions of oscillations of the vectors E and B change chaotically so that all directions of oscillations in planes perpendicular to the ray of wave propagation are equally probable ( Fig.2).

Fig.2 Unpolarized light

Polarized waves- in which the directions of the vectors E and B remain unchanged in space or change according to a certain law. Radiation in which the direction of vector E changes chaotically - unpolarized. An example of such radiation is thermal radiation (chaotically distributed atoms and electrons). Plane of polarization- this is a plane perpendicular to the direction of oscillations of the vector E. The main mechanism for the occurrence of polarized radiation is the scattering of radiation by electrons, atoms, molecules, and dust particles.

1.2. Types of polarization There are three types of polarization. Let's give them definitions. 1. Linear Occurs if the electric vector E maintains its position in space. It seems to highlight the plane in which vector E oscillates. 2. Circular This is polarization that occurs when the electric vector E rotates around the direction of propagation of the wave with an angular velocity equal to the angular frequency of the wave, while maintaining its absolute value. This polarization characterizes the direction of rotation of the vector E in a plane perpendicular to the line of sight. An example is cyclotron radiation (a system of electrons rotating in a magnetic field). 3. Elliptical It occurs when the magnitude of the electric vector E changes so that it describes an ellipse (rotation of the vector E). Elliptical and circular polarization can be right-handed (vector E rotates clockwise when looking towards the propagating wave) and left-handed (vector E rotates counter-clockwise when looking towards the propagating wave).

In reality, it occurs most often partial polarization (partially polarized electromagnetic waves). Quantitatively, it is characterized by a certain quantity called degree of polarization R, which is defined as: P = (Imax - Imin) / (Imax + Imin) Where Imax,Immin- the highest and lowest density of electromagnetic energy flux through the analyzer (Polaroid, Nicolas prism...). In practice, radiation polarization is often described by Stokes parameters (they determine radiation fluxes with a given polarization direction).

Ticket 79.

If natural light falls on the interface between two dielectrics (for example, air and glass), then part of it is reflected, and part of it is refracted and spreads in the second medium. By installing an analyzer (for example, tourmaline) in the path of the reflected and refracted rays, we make sure that the reflected and refracted rays are partially polarized: when the analyzer is rotated around the rays, the light intensity periodically increases and weakens (complete quenching is not observed!). Further studies showed that in the reflected beam, vibrations perpendicular to the plane of incidence predominate (they are indicated by dots in Fig. 275), while in the refracted beam, vibrations parallel to the plane of incidence (depicted by arrows) predominate.

The degree of polarization (the degree of separation of light waves with a certain orientation of the electric (and magnetic) vector) depends on the angle of incidence of the rays and the refractive index. Scottish physicist D. Brewster(1781-1868) installed law, according to which at the angle of incidence i B (Brewster angle), determined by the relation

(n 21 - refractive index of the second medium relative to the first), the reflected beam is plane polarized(contains only vibrations perpendicular to the plane of incidence) (Fig. 276). The refracted ray at the angle of incidencei B polarized to the maximum, but not completely.

If light strikes an interface at the Brewster angle, then the reflected and refracted rays mutually perpendicular(tg i B = sin i B/cos i B, n 21 = sin i B / sin i 2 (i 2 - angle of refraction), whence cos i B=sin i 2). Hence, i B + i 2 =  /2, but i’ B= i B (law of reflection), therefore i’ B+ i 2 =  /2.

The degree of polarization of reflected and refracted light at different angles of incidence can be calculated from Maxwell’s equations, if we take into account the boundary conditions for the electromagnetic field at the interface between two isotropic dielectrics (the so-called Fresnel formulas).

The degree of polarization of refracted light can be significantly increased (by multiple refraction, provided that the light is incident each time on the interface at the Brewster angle). If, for example, for glass ( n= 1.53) the degree of polarization of the refracted beam is 15%, then after refraction into 8-10 glass plates superimposed on each other, the light emerging from such a system will be almost completely polarized. Such a collection of plates is called foot. The foot can be used to analyze polarized light both during its reflection and during its refraction.

Ticket 79 (for Spur)

As experience shows, during the refraction and reflection of light, the refracted and reflected light turns out to be polarized, and the reflection. light can be completely polarized at a certain angle of incidence, but incidentally. light is always partially polarized. Based on Frinell's formulas, it can be shown that reflection. Light is polarized in a plane perpendicular to the plane of incidence and refracted. the light is polarized in a plane parallel to the plane of incidence.

The angle of incidence at which the reflection the light is completely polarized is called the Brewster angle. The Brewster angle is determined from Brewster's law: - Brewster's law. In this case, the angle between the reflections. and refraction. rays will be equal. For an air-glass system, the Brewster angle is equal. To obtain good polarization, i.e. , when refracting light, many edible surfaces are used, which are called Stoletov’s Stop.

Ticket 80.

Experience shows that when light interacts with matter, the main effect (physiological, photochemical, photoelectric, etc.) is caused by oscillations of the vector, which in this regard is sometimes called the light vector. Therefore, to describe the patterns of light polarization, the behavior of the vector is monitored.

The plane formed by the vectors and is called the plane of polarization.

If vector oscillations occur in one fixed plane, then such light (ray) is called linearly polarized. It is conventionally designated as follows. If the beam is polarized in a perpendicular plane (in the plane xoz, see fig. 2 in the second lecture), then it is designated.

Natural light (from ordinary sources, the sun) consists of waves that have different, chaotically distributed planes of polarization (see Fig. 3).

Natural light is sometimes conventionally designated as such. It is also called non-polarized.

If, as the wave propagates, the vector rotates and the end of the vector describes a circle, then such light is called circularly polarized, and the polarization is called circular or circular (right or left). There is also elliptical polarization.

There are optical devices (films, plates, etc.) - polarizers, which extract linearly polarized light or partially polarized light from natural light.

Polarizers used to analyze the polarization of light are called analyzers.

The plane of the polarizer (or analyzer) is the plane of polarization of the light transmitted by the polarizer (or analyzer).

Let linearly polarized light with amplitude fall on a polarizer (or analyzer) E 0 . The amplitude of the transmitted light will be equal to E=E 0 cos j, and intensity I=I 0 cos 2 j.

This formula expresses Malus's law:

The intensity of linearly polarized light passing through the analyzer is proportional to the square of the cosine of the angle j between the plane of oscillation of the incident light and the plane of the analyzer.

Ticket 80 (for spur)

Polarizers are devices that make it possible to obtain polarized light. Analyzers are devices that can be used to analyze whether light is polarized or not. Structurally, a polarizer and an analyzer are one and the same. Zn Malus. Let intensity light fall on the polarizer, if the light is natural -th then all directions of the vector E are equally probable. Each vector can be decomposed into two mutually perpendicular components: one of which is parallel to the plane of polarization of the polarizer, and the other is perpendicular to it.

Obviously, the intensity of the light emerging from the polarizer will be equal. Let us denote the intensity of the light emerging from the polarizer by (). If an analyzer is placed on the path of the polarized light, the main plane of which makes an angle with the main plane of the polarizer, then the intensity of the light emerging from the analyzer is determined by the law.

Ticket 81.

While studying the glow of a solution of uranium salts under the influence of radium rays, the Soviet physicist P. A. Cherenkov drew attention to the fact that the water itself also glows, in which there are no uranium salts. It turned out that when rays (see Gamma radiation) are passed through pure liquids, they all begin to glow. S. I. Vavilov, under whose leadership P. A. Cherenkov worked, hypothesized that the glow was associated with the movement of electrons knocked out of atoms by radium quanta. Indeed, the glow strongly depended on the direction of the magnetic field in the liquid (this suggested that it was caused by the movement of electrons).

But why do electrons moving in a liquid emit light? The correct answer to this question was given in 1937 by Soviet physicists I. E. Tamm and I. M. Frank.

An electron, moving in a substance, interacts with the atoms surrounding it. Under the influence of its electric field, atomic electrons and nuclei are shifted in opposite directions - the medium is polarized. Polarized and then returning to their original state, the atoms of the medium located along the electron trajectory emit electromagnetic light waves. If the speed of the electron v is less than the speed of light in the medium (the refractive index), then the electromagnetic field will overtake the electron, and the substance will have time to polarize in space ahead of the electron. The polarization of the medium in front of the electron and behind it is opposite in direction, and the radiation of oppositely polarized atoms, “added”, “quenches” each other. When atoms that have not yet been reached by an electron do not have time to polarize, and radiation appears directed along a narrow conical layer with an apex coinciding with the moving electron and an angle at the apex c. The appearance of the light "cone" and the radiation condition can be obtained from the general principles of wave propagation.

Rice. 1. Mechanism of wavefront formation

Let the electron move along the axis OE (see Fig. 1) of a very narrow empty channel in a homogeneous transparent substance with a refractive index (the empty channel is needed so that collisions of the electron with atoms are not taken into account in the theoretical consideration). Any point on the OE line successively occupied by an electron will be the center of light emission. Waves emanating from successive points O, D, E interfere with each other and are amplified if the phase difference between them is zero (see Interference). This condition is satisfied for a direction that makes an angle of 0 with the trajectory of the electron. Angle 0 is determined by the relation:.

Indeed, let us consider two waves emitted in a direction at an angle of 0 to the electron velocity from two points of the trajectory - point O and point D, separated by a distance. At point B, lying on line BE, perpendicular to OB, the first wave at - after time To point F, lying on line BE, a wave emitted from the point will arrive at the moment of time after the wave is emitted from point O. These two waves will be in phase, i.e. the straight line will be a wave front if these times are equal:. That gives the condition of equality of times. In all directions for which, the light will be extinguished due to the interference of waves emitted from sections of the trajectory separated by a distance D. The value of D is determined by the obvious equation, where T is the period of light oscillations. This equation always has a solution if.

If , then the direction in which the emitted waves, when interfering, are amplified, does not exist and cannot be greater than 1.

Rice. 2. Distribution of sound waves and the formation of a shock wave during body movement

Radiation is observed only if .

Experimentally, electrons fly in a finite solid angle, with some spread in speed, and as a result, radiation propagates in a conical layer near the main direction determined by the angle.

In our consideration, we neglected the electron slowdown. This is quite acceptable, since the losses due to Vavilov-Cerenkov radiation are small and, to a first approximation, we can assume that the energy lost by the electron does not affect its speed and it moves uniformly. This is the fundamental difference and unusualness of the Vavilov-Cherenkov radiation. Typically, charges emit while experiencing significant acceleration.

An electron outpacing its light is similar to an airplane flying at a speed greater than the speed of sound. In this case, a conical shock sound wave also propagates in front of the aircraft (see Fig. 2).

FOR LECTURE No. 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 using 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. changing the direction of light propagation when passing from one medium to another.

Refraction, as well as absorption of light, is a consequence of its interaction with the medium. The word refractometry means measurement 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 the fact in what concentration and what molecules the light beam encounters on its path, because Under the influence of light, molecules of different substances are polarized differently. 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) Measuring refractive indexes is a very simple process that is carried out accurately and with minimal time and amount of material.

2) Typically, refractometers provide an accuracy of up to 10% 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, and to determine the structure of organic and inorganic compounds when studying solutions. Refractometry is used to determine the composition of two-component solutions and for ternary systems.

Physical basis of the method

REFRACTIVE INDEX.

The greater the difference in the speed of light propagation in the two, the greater the deviation of a light ray from its original direction when it passes from one medium to another.

these environments.

Let us 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 not a vacuum or air, then the ratio of the sin angle of incidence of the light beam to the sin angle of refraction will give the value of the relative refractive index n rel. Value 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) nature of substances

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

2)wavelength of incident light

The refractive index measurement 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 carried out. A prerequisite for determining the refractive index is compliance with the temperature regime. Usually the determination is performed at 20±0.3 0 C.

As the temperature increases, the refractive index decreases; as the temperature decreases, it increases..

The correction for temperature effects is calculated using the following formula:

n t =n 20 + (20-t) 0.0002, where

n t – Bye refractive index at a given temperature,

n 20 - refractive index at 20 0 C

The influence of temperature on the values ​​of the refractive indices of gases and liquids is associated with the values ​​of their volumetric expansion coefficients. 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 designated 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) concentration of the substance in solution.

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

n=n o +FC,

n- refractive index of the solution,

no- refractive index of a pure solvent,

C- solute concentration,%

F-empirical coefficient, the value of which is found

by determining the refractive index of solutions of known concentration.

REFRACTOMETERS.

Refractometers are instruments used to measure the refractive index. There are 2 types of these devices: Abbe type and Pulfrich type refractometer. In both cases, measurements are based on determining the maximum refraction angle. In practice, refractometers of various systems are used: laboratory-RL, universal RL, etc.

The refractive index of distilled water is n 0 = 1.33299, but practically this indicator is taken as reference as n 0 =1,333.

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

Handheld refractometer

Abbe refractometer

Refraction of light- 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.

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.