In this work, we use an Abbe refractometer, the operation of which is based on measuring the limiting refraction angle. The optical diagram of the refractometer is shown in Fig. 4. The test solution is placed between the planes of two prisms - the lighting 3 and measuring 4 , made of glass with a high refractive index ( n = 1.9 ). The large refractive index of the measuring prism allows maintaining the condition n p < n st for a wide range of densities of measured liquids. The instrument scale is calibrated to the value n p =1.7 .From the source 1 a beam of light is directed by a condenser 2 to the entrance face of the lighting prism. Passing the lighting prism 3, light falls on the matte hypotenuse face AB of this prism , bordering a thin layer of the test liquid. The matte surface has irregularities, the dimensions of which are several wavelengths. Light is scattered by these irregularities over the entire surface and, having passed through a thin layer of solution, falls on the “solution-glass” interface at all possible angles of incidence, i.e. the angle of incidence varies from 0 0 before 90 0 .
On the mirror hypotenuse face CD measuring prism 4 light is refracted (the size of the irregularities on this face is less than the wavelength). Due to the fact that n p < n st , the angle of refraction varies from zero to γ etc . At angles γ > γ etc no radiation is observed. Thus, at a refraction angle equal to γ etc , a light-shadow boundary appears. Magnitude n p is determined from the relation sinγ etc = n p / n st , where is the value n st known.
The path of light rays as they exit the measuring prism is easily taken into account when calibrating the device, since light refraction occurs at the glass-air boundary. , Moreover, the refractive indices of both media are known. The angle of light refraction at this boundary does not affect the measurement accuracy n p .
Due to the illumination of the entire layer of solution, the boundary between light and shadow is observed quite sharply. Therefore, when setting up the device for operation, the light from the illuminator must be directed onto the prism so that it evenly illuminates the entire surface of the face. AB diverging prism. To determine the angle at which the rays emerge from the measuring prism, a telescope formed by the lens is used 6 and eyepiece 9, light enters through a system of direct vision prisms 5 . In this case, the property of the telescope is used that the rays coming to it parallel to its axis are collected at the rear focus, where a transparent plate is placed 7 with a grid cross mark on it. The crosshair is exactly in focus.
Rice. 4. The path of rays in a refractometer when measuring the refractive index using the grazing beam method.
Optical design of the device: 1-light source, 2-condenser, 3-illuminating prism, 4-measurement prism, 5-direct vision prism, 6-spotting scope lens, 7-reticle with crosshair, 8-scale, 9-spotting scope eyepiece , 10-field of view of the eyepiece.
Direct vision prisms and the telescope are rigidly connected to each other and can be rotated relative to the measuring prism. The rotation angle is measured using a fixed scale 8, located in the common focal plane of the lens and eyepiece. The scale is graduated in the values of the refractive index of the solution under study based on formula (6). By rotating the telescope, you can set its axis parallel to the rays refracted at the edge CD at extreme angle γ etc. In this case, light and dark areas will be observed in the field of view of the eyepiece, the boundary between which will coincide with the crosshair. The light area is formed by rays refracted at the edge CD at angles less than the limit, and the dark area arises due to the absence of rays traveling at angles greater than the limit. The position of the boundary of light and shadow formed by rays refracted at the maximum angle will indicate on the scale 8 the desired value of the refractive index of the solution.
Light source 1 is not monochromatic. Therefore, due to the dispersion of both the substance under study and the material of the measuring prism (the dependence of their refractive indices on the wavelength of light), the boundary of light and shadow observed through the telescope turns out to be blurred and colored. To eliminate this effect, direct vision prisms are used 5 , forming dispersion compensator. The prisms are designed so that rays with a wavelength λ D= 589.3 nm (average sodium wavelength) did not deviate when passing through them. When one prism is rotated relative to another, their total dispersion changes, which makes it possible to compensate for the difference in the exit angles of rays with different wavelengths from the measuring prism and direct them into the telescope parallel to rays with wavelengths λ D. The boundary between light and shadow is sharp, uncolored and gives the value of the refractive index of the solution under study n D at wavelength λ D .
Refractometry, performed using refractometers, is one of the most common methods for identifying chemical compounds, quantitative and structural analysis, determination of physical and chemical parameters of substances.
What is a refractometer?
Refractometer - optical instrument, which measures the refractive index of light in a medium. Refractometry, performed using refractometers, is one of the most common methods for identifying chemical compounds, quantitative and structural analysis, and determining the physical and chemical parameters of substances.
The operation of a refractometer is based on measuring the refractive indices of light in various media. If the density of a substance increases, its refractive index increases proportionally (for example, when sugar is dissolved in water). A refractometer reads the relative "weight" of a sample compared to distilled water.
Point the refractometer towards natural daylight and look through the eyepiece. You will see a circular area (box) centered at the bottom. (Figure 1 shows the scale without calibration fluid or any other fluid.) |
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Tighten the calibration screw until the boundary between the upper blue area and the lower white area meet exactly at the zero mark. |
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Picture 1 This drawing illustrates what you can see in the eyepiece without any reference. Notice that the entire scale is colored blue. When viewing, make sure you use natural daylight. You should not take readings in the presence of fluorescent light. |
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Figure 2 This is what you see after the refractometer has been calibrated. Please note that for proper calibration, the border of the blue and white scale should be exactly at the zero mark when using distilled water as a sample. |
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Figure 3 This example shows a scale for measuring grape juice. You can see that the meter shows 23% Brix, the most the right time for making wine! After finishing the measurement, make sure you clean and dry the refractometer. |
Refractometer maintenance
Accurate measurement depends on careful and correct calibration. As a reminder, differences between ambient temperature and sample temperature will reduce the accuracy of the reading. Be sure to wait approximately 30 seconds before taking a reading.
Do not immerse the instrument in water or allow water to get inside.
Do not measure harsh or corrosive chemicals with this instrument because they may damage the prism coating.
Clean the instrument between each measurement using a soft cloth.
A refractometer is an optical instrument. It requires careful handling and storage. When used carefully and proper storage This tool will provide reliable performance for many years.
The air temperature during calibration should be 20° C. However, many modern models come with ATC (automatic temperature compensation), so you don't have to worry about calibration air temperature or sample temperature.
Sources: www.grapestompers.com, www.patech.ru
A refractometer is an optical instrument designed to measure the concentration of solutions, which is based on the phenomenon of light refraction.
Refractometry is a method for studying substances based on determining the index (coefficient) of refraction (refraction) and some of its functions. Refractometry ( refractometric method) is used for identification of chemical compounds, quantitative and structural analysis, determination of physical and chemical parameters of substances.
Classification:
1. Industrial
2. Laboratory
3. Portable digital
4. Portable handheld
Industrial and laboratory refractometers intended for the study of substances in scientific laboratories and control technological processes in production. They have high accuracy dimensions but also relatively large sizes.
Portable refractometersdesigned for operational control of substances in the laboratory, production or field conditions. In turn, portable refractometers can be classified into digital and manual.
Portable digital refractometers usually have a liquid crystal display that displays the measured results. Most often they also have additional options, such as simultaneous measurement of the density and refractive index of a solution, converting results into various units of measurement, maintaining the temperature of the sample, etc.
Hand-held (non-digital) refractometers are usually more compact in size and do not have any electronic circuits or batteries (with the exception of some backlit models), which makes them easy to use for measurements not only in production, but also at home. Today, such refractometers are very popular due to their accuracy, ease of use, portability and low affordable price.
What is the principle of operation of a refractometer based on?
Operating principle of a refractometeris based on the use of the phenomenon of refraction (refraction) of light flux. When a ray of light passes from one substance to another, it deviates from the rectilinear direction by a certain angle. The ratio of the angle of entry of a light beam into a substance and its angle of refraction at the interface between two media is called the refractive index.
The structure of a typical refractometer is shown schematically in the figure below. The main optical element of a refractometer is a prism onto which the test substance is applied. The prism is made of a material with a high refractive index.
Due to this, the incident light, passing through the substance and the prism, is refracted at a sufficiently large angle. Next, through a system of optical lenses, the light falls on the refractometer scale (graduated circle). Depending on the angle of refraction, the light beam appears higher or lower on the scale of the device. The illuminated part of the scale will be light; the part that the light beam does not hit will be dark. The angle of refraction of light depends on the composition of the solution and its concentration. Thus, by the position of the interface between light and shadow, the refractive index or optical density of the solution under study can be unambiguously determined.
However, it must be borne in mind that the refractive index of a substance also depends on temperature. Some models of hand-held refractometers take into account the influence of temperature using the ATC (Automatic Temperature Compensation System) function. Inside their body there is a bimetallic plate. It contracts or expands depending on temperature changes. The bimetallic plate is connected to the optical system of the refractometer, moving it smoothly as the temperature changes. The magnitude of the shifts is calculated so that the effect of temperature on the refractive index of the substance is completely compensated. When purchasing a refractometer, be sure to pay attention to the presence of an ATC function. If it is absent, it is necessary to use special tables to recalculate the obtained values depending on the ambient temperature.
Taking measurements
The handheld refractometer must be calibrated before taking measurements. Most refractometers use distilled water to calibrate. A few drops of water are applied to the main prism using a pipette, then the protective glass is closed. In this case, you need to make sure that the water under protective glass evenly coats the surface of the prism without leaving any air bubbles. Next, using the calibration screw (or in the case of more simple models calibration screw) the device scale is set to 0.0. After calibration, the prism must be carefully wiped with a soft cloth (it is advisable to use a cloth for glasses lenses, its material will not damage the refractometer lens). The refractometer is now ready for measurements. If the refractometer scale does not start from zero (0 is distilled water), then the refractometer is calibrated using a special oil.
To carry out measurements, the same steps are performed as during calibration, but instead of distilled water, the test solution is applied to the prism of the device. The calibration screw remains in its original position. After applying the solution, you must wait 30 seconds for the temperature of the solution to equal the temperature of the device. The refractometer is then pointed at a light source (daylight or incandescent) and readings are taken.
After taking measurements, the prism must be wiped again with a soft cloth. Handheld refractometer cannot be placed in water; This may cause water to enter the instrument and cloud the scale. Do not use the refractometer on hard or corrosive substances as they may damage the prism coating. Also, do not measure very hot solutions, as the main lens may come off. For most refractometers, the temperature limit is 50C.
Application of refractometers
Refractometers widely used in various areas human activity. Some of the most common applications of refractometers are listed below:
IN Food Industry:
1. quality control ;
2. determination of the mass fraction of soluble dry substances in processed products of fruits and vegetables;
In medicine:
1. determination of protein in blood serum;
2. determination of the density of urine, subretinal fluid of the eye;
The refractometer (Fig. 2a) is designed to measure the refractive index of solutions various substances. The principle of operation of a refractometer when measuring the refractive index of transparent solutions is to measure the limiting angle of refraction at the interface of the liquid under study and a glass prism with a known refractive index. The refractometer consists of two prisms: an auxiliary folding prism (1) with a matte finish; edge (2) and measuring prism (3). Between them there is a thin gap 0.1 mm thick, into which a few drops of the test liquid are placed (4). The limiting angle of refraction at the interface between the liquid and the measuring prism is measured. The compensator built into the device allows you to make the light-shadow boundary black and white when illuminated with white light. Readings are made by the eye (7).
The refractometer works as follows. The light beam passes through the auxiliary folding prism (1) and is scattered on the lower edge (2). In this case, the scattered rays propagate in all directions, including parallel to the surface of the measuring prism (3) (Fig. 26).
Next, these rays are refracted at the boundary between the liquid (4) and the measuring prism (3), and, having passed through this prism (3), enter the device (5). If the light-shadow boundary turns out to be colored and blurred, you need to use the compensator (6) to achieve a sharp black-and-white white boundary. The design of the reading device allows, when turning a special lever, to combine the light-shadow boundary with the marker of the reading device. In this case, the marker shows the refractive index values directly on the built-in scale .
Rice. 2.a, b - block diagram of the refractometer: 1 - auxiliary folding prism with a matte bottom edge (2); 3 - measuring prism; 4 - test liquid; 5 - reading device; 6 - compensator; 7 - eye; b - diagram of light scattering by the matte lower edge (2) of the folding prism
Design and purpose of an endoscope
Endoscopy-- a medical method for examining the cavity organs of the body (for example, the bladder, esophagus, stomach) by directly examining them by introducing them into them special tools- so-called endoscopes. An endoscope is actually a microscope with low magnification, adapted for insertion into a cavity, that is, having a small diameter at long length tube.
Rice. 3. Beam propagation in a light guide
Currently, flexible endoscopes are widely used, in which not a lens system is used to transmit images, but light guides - glass threads with a diameter of 10-50 microns.
Flexible light guide devices are based on the phenomenon of total internal reflection of light. The glass filament in the light guide is surrounded by a shell of another substance with a lower refractive index (Fig. 3a). As a result, rays incident on the interface between two media at an angle a > a and propagate along the core of the fiber without going beyond it (Fig. 36). Thus, the light guide allows light to be transmitted to significant distances, both along a straight and curved path.
Using a separate light guide with a diameter of 5-20 microns is convenient to illuminate cavities, but it is inconvenient to image objects. Therefore, as a rule, the image of objects is transferred using a fiberglass bundle made up of individual fibers.
