Absolute refractive index in water. Refractive index
In the 8th grade physics course, you got acquainted with the phenomenon of light refraction. Now you know that light is electromagnetic waves of a certain frequency range. Based on knowledge about the nature of light, you will be able to understand the physical cause of refraction and explain many other light phenomena associated with it.
Rice. 141. Passing from one medium to another, the beam is refracted, i.e., changes the direction of propagation
According to the law of light refraction (Fig. 141):
- rays incident, refracted and 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 these two media
where n 21 is the relative refractive index of the second medium relative to the first.
If the beam passes into any medium from a vacuum, then
where n is the absolute refractive index (or simply refractive index) of the second medium. In this case, the first "environment" is vacuum, the absolute index of which is taken as one.
The law of light refraction was discovered empirically by the Dutch scientist Willebord Snellius in 1621. The law was formulated in a treatise on optics, which was found in the scientist's papers after his death.
After the discovery of Snell, several scientists put forward a hypothesis that the refraction of light is due to a change in its speed when it passes through the boundary of two media. The validity of this hypothesis was confirmed by theoretical proofs carried out independently by the French mathematician Pierre Fermat (in 1662) and the Dutch physicist Christian Huygens (in 1690). By different paths they arrived at the same result, proving that
- the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for these two media, equal to the ratio of the speeds of light in these media:
(3)
From equation (3) it follows that if the angle of refraction β is less than the angle of incidence a, then the light of a given frequency in the second medium propagates more slowly than in the first, i.e. V 2 The relationship of the quantities included in equation (3) served as a good reason for the appearance of another formulation of the definition of the relative refractive index: n 21 \u003d v 1 / v 2 (4) Let a beam of light pass from vacuum to some medium. Replacing v1 in equation (4) with the speed of light in vacuum c, and v 2 with the speed of light in a medium v, we obtain equation (5), which is the definition of the absolute refractive index: According to equations (4) and (5), n 21 shows how many times the speed of light changes when it passes from one medium to another, and n - when it passes from vacuum to a medium. This is the physical meaning of the refractive indices. The value of the absolute refractive index n of any substance is greater than unity (this is confirmed by the data contained in the tables of physical reference books). Then, according to equation (5), c/v > 1 and c > v, i.e., the speed of light in any substance is less than the speed of light in vacuum. Without giving rigorous justifications (they are complex and cumbersome), we note that the reason for the decrease in the speed of light during its transition from vacuum to matter is the interaction of a light wave with atoms and molecules of matter. The greater the optical density of the substance, the stronger this interaction, the lower the speed of light and the greater the refractive index. Thus, the speed of light in a medium and the absolute refractive index are determined by the properties of this medium. According to the numerical values of the refractive indices of substances, one can compare their optical densities. For example, the refractive indices of various types of glass range from 1.470 to 2.040, while the refractive index of water is 1.333. This means that glass is an optically denser medium than water. Let us turn to Figure 142, with the help of which we can explain why, at the boundary of two media, with a change in speed, the direction of propagation of a light wave also changes. Rice. 142. When light waves pass from air to water, the speed of light decreases, the front of the wave, and with it its speed, change direction The figure shows a light wave passing from air into water and incident on the interface between these media at an angle a. In air, light propagates at a speed v 1 , and in water at a slower speed v 2 . Point A of the wave reaches the boundary first. Over a period of time Δt, point B, moving in the air at the same speed v 1, will reach point B. "During the same time, point A, moving in water at a lower speed v 2, will cover a shorter distance, reaching only point A". In this case, the so-called wave front A "B" in the water will be rotated at a certain angle with respect to the front of the AB wave in the air. And the velocity vector (which is always perpendicular to the wave front and coincides with the direction of its propagation) rotates, approaching the straight line OO", perpendicular to the interface between the media. In this case, the angle of refraction β turns out to be less than the angle of incidence α. This is how the refraction of light occurs. It can also be seen from the figure that upon transition to another medium and rotation of the wave front, the wavelength also changes: upon transition to an optically denser medium, the velocity decreases, the wavelength also decreases (λ 2< λ 1). Это согласуется и с известной вам формулой λ = V/v, из которой следует, что при неизменной частоте v (которая не зависит от плотности среды и поэтому не меняется при переходе луча из одной среды в другую) уменьшение скорости распространения волны сопровождается пропорциональным уменьшением длины волны. 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 phenomenon of dispersion (colouring 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) 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. 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. 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. 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. To determine this physical phenomenon, the law of refraction was created. The law of refraction of light fluxes allows you to determine the characteristics of transparent substances. The law itself consists of two provisions: 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. 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. 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. 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. This indicator tells how the speed of propagation of photons changes when passing from one substance to another. 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). Concluding our short digression, it is still necessary to consider such an effect as a full-fledged internal reflection. Full Display Phenomenon For the appearance of this effect, 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 will exceed 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. 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.
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. Wikimedia Foundation. 2010 . 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 characterizing the medium and 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 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. 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.
Questions
An exercise
Fundamentals of a physical phenomenon
Reflection and refraction of light can be seen especially well in water.
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.
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:physical law
An important parameter for different objects
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.
Optical density of substances
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:Another important indicator
Full internal reflection
Conclusion
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See what "Refraction index" is in other dictionaries:
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