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Each of us in our daily life has more than once encountered and is faced with ordinary, on the one hand, but at the same time, amazing phenomena on the other hand, without thinking at all about what wonderful physical phenomena we are dealing with.
In the future, I would like to connect my life with such a science as physics, so I am already interested in any questions on this subject and have chosen one of the optical effects as the topic of my research.
To date, there are works devoted to optical effects, in particular, the Tyndall effect. However, I decided to explore this topic by conducting an experiment on my own experience.
Why do we observe different results when we pass light of different spectral colors through cloudy glass, smoky air or starch solution? Why thick fog or cumulus clouds appear white to us, and the haze from forest fires is bluish-purple. Let's try to explain these phenomena.
Objective of the project:
detect colloids using the Tyndall effect;
to study the influence of factors that determine the passage of a light beam through a colloidal solution.
Research objectives:
study of the influence of the wavelength on the implementation of the Tyndall effect;
study of the influence of particle size on the implementation of the Tyndall effect;
study of the influence of particle concentration on the implementation of the Tyndall effect;
search for additional information on the Tyndall effect;
generalization of the acquired knowledge.
Tyndall effect
Light refraction, reflection, dispersion, interference, diffraction and more: optical effects are all around us. One of them is the Tyndall effect, discovered by the English physicist John Tyndall.
John Tyndall is a surveyor, fellow of Faraday, director of the Royal Institution in London, glaciologist and optician, acoustician and specialist in magnetism. His surname gave its name to a crater on the Moon, a glacier in Chile, and an interesting optical effect.
The Tyndall effect is the glow of an optically inhomogeneous medium due to the scattering of light passing through it. This phenomenon is due to the diffraction of light on individual particles or elements of the inhomogeneity of the medium, the size of which is much smaller than the wavelength of the scattered light.
What is a heterogeneous environment? An inhomogeneous medium is a medium characterized by a change in the refractive index. Those. n ≠ const.
What is the characteristic feature of this effect? The Tyndall effect is typical for colloidal systems (systems in which one substance in the form of particles of various sizes is distributed in another. For example, hydrosols, tobacco smoke, fog, gel, etc.) with a low concentration of particles having a refractive index different from the index refraction of the medium. It is usually observed as a light cone on a dark background (Tyndall's cone) when a focused light beam is passed from the side through a glass vessel with plane-parallel walls filled with a colloidal solution. (Colloid solutions are highly dispersed two-phase systems consisting of a dispersion medium and a dispersed phase, with the linear particle sizes of the latter ranging from 1 to 100 nm).
The Tyndall effect is essentially the same as opalescence (a sharp increase in light scattering). But traditionally, the first term refers to the intense scattering of light in a limited space along the path of the beam, and the second term refers to the weak scattering of light by the entire volume of the observed object.
Experimental work
Using a simple technique, we will see how the Tyndall effect can be used to detect colloidal systems in liquids.
Materials: 2 glass containers with lids, directional light source (eg laser pointer), table salt, surfactant solution (eg liquid detergent), 1 chicken egg, dilute hydrochloric acid solution.
Conducting an experiment:
Pour water into a glass container, completely dissolve a little table salt in it.
We illuminate the side of the glass with the resulting solution with a narrow beam of light (laser pointer beam). Since the salt has completely dissolved, no noticeable effect is observed.
Experiment with biological material:
Dissolve chicken protein in about 300 ml of 1% salt solution.
We illuminate the resulting solution with a narrow beam of light. If you look at the glass from the side, a bright luminous strip is visible in the path of the beam - the appearance of the Tyndall effect.
Then add a dilute hydrochloric acid solution to the protein solution. The protein will coagulate (denature) to form a whitish precipitate. At the top of the glass, the beam of light will no longer be visible.
Experiment results: If you direct a beam of light from the side to a glass beaker with a salt solution, the beam will be invisible in the solution. If a beam of light is passed through a beaker containing a colloidal solution (surfactant solution), it will be visible because the light is scattered by the colloidal particles.
Influence of Wavelength, Particle Size, and Concentration on the Realization of the Tyndall Effect
Wavelength. Since blue wavelengths have the shortest length in the visible spectrum, it is these wavelengths that are reflected from particles during the Tyndall effect, and longer red ones scatter worse.
Particle size. If the particle size increases, then they can affect the scattering of light of any wavelength, and the "split" rainbow folds back, getting completely white light.
Particle concentration. The scattered light intensity is directly proportional to the particle concentration in the colloidal solution.
Applying the Tyndall effect
Based on the Tyndall effect, methods for detecting, determining the size and concentration of colloidal particles are widely used in scientific research and industrial practice (for example, in ultramicroscopes).
An ultramicroscope is an optical instrument for detecting the smallest (colloidal) particles whose dimensions are smaller than the resolution limit of conventional light microscopes. The possibility of detecting such particles using an ultramicroscope is due to the diffraction of light on them by the Tyndall effect. With strong side illumination, each particle in the ultramicroscope is marked by the observer as a bright point (luminous diffraction spot) against a dark background. Due to diffraction on the smallest particles, there is very little light, therefore, as a rule, strong light sources are used in an ultramicroscope.
Depending on the intensity of illumination, the wavelength of light, the difference between the refractive indices of the particle and the medium, particles ranging in size from 20-50 nm to 1-5 μm can be detected. It is impossible to determine the true size, shape and structure of particles from diffraction spots. The ultramicroscope does not provide optical images of the objects under study. However, using an ultramicroscope, it is possible to determine the presence and number concentration of particles, study their movement, and also calculate the average size of particles if their weight concentration and density are known.
Ultramicroscopes are used in the study of dispersed systems, to control the purity of atmospheric air. Water, the degree of contamination of optically transparent media with foreign inclusions.
Conclusion
During my research, I learned a lot about optical effects, in particular the Tyndall effect. This work helped me to take a fresh look at both some branches of physics and our wonderful world in general.
In addition to the aspects considered in this paper, in my opinion, it would be interesting to explore the possibilities of a wider practical application of the Tyndall effect.
As for the purpose of the study, it can be useful and interesting for school students who are fond of optics, as well as for everyone who is interested in physics and various kinds of experiments.
Bibliography
Gavronskaya Yu.Yu. Colloid Chemistry: Textbook. SPb.: Publishing house of the Russian State Pedagogical University im. A. I. Herzen, 2007. - 267 p.
New Polytechnic Dictionary. - M.: Great Russian Encyclopedia, 2000. - .20 p. , 231 p. , 460 p.
Guidelines for performing experiments for "NanoSchoolBox". NanoBioNet e.V/Scince Park Translated by INT.
https://indicator.ru/article/2016/12/04/istoriya-nauki-chelovek-rasseyanie.
http://kf.info.urfu.ru/fileadmin/user_upload/site_62_6389/pdf/FiHNS_proceedings.pdf
http://www.ngpedia.ru/id623274p1.html
ELECTROKINETIC PROPERTIES OF COLLOIDS
Electrokinetic phenomena are divided into two groups: direct and reverse. The direct ones include those electrokinetic phenomena that occur under the action of an external electric field (electrophoresis and electroosmosis). The reverse is called electrokinetic phenomena, in which, during the mechanical movement of one phase relative to another, an electric potential arises (the flow potential and the sedimentation potential).
Electrophoresis and electroosmosis were discovered by F. Reiss (1808). He discovered that if two glass tubes are immersed in wet clay, filled with water and electrodes are placed in them, then when a direct current is passed, clay particles move towards one of the electrodes.
This phenomenon of movement of particles of the dispersed phase in a constant electric field was called electrophoresis.
In another experiment, the middle part of a U-shaped tube containing water was filled with crushed quartz, an electrode was placed in each elbow of the tube, and a direct current was passed through. After some time, in the knee, where the negative electrode was located, a rise in the water level was observed, in the other - a drop. After turning off the electric current, the water levels in the elbows of the tube were equalized.
This phenomenon of movement of a dispersion medium relative to a stationary dispersed phase in a constant electric field is called electroosmosis.
Later, Quincke (1859) discovered a phenomenon inverse to electroosmosis, called the percolation potential. It consists in the fact that when a fluid flows under pressure through a porous diaphragm, a potential difference arises. Clay, sand, wood, and graphite were tested as diaphragm materials.
The phenomenon, the reverse of electrophoresis, and called the sedimentation potential, was discovered by Dorn (1878). When particles of the quartz suspension settled under the action of gravity, a potential difference arose between the levels of different heights in the vessel.
All electrokinetic phenomena are based on the presence of a double electric layer at the boundary of the solid and liquid phases.
http://junk.wen.ru/o_6de5f3db9bd506fc.html
18. Special optical properties of colloidal solutions due to their main features: dispersion and heterogeneity. The optical properties of dispersed systems are largely affected by the size and shape of the particles. The passage of light through a colloidal solution is accompanied by such phenomena as absorption, reflection, refraction and scattering of light. The predominance of any of these phenomena is determined by the ratio between the particle size of the dispersed phase and the wavelength of the incident light. AT coarse systems mainly the reflection of light from the surface of the particles is observed. AT colloidal solutions particle sizes are comparable to the wavelength of visible light, which determines the scattering of light due to the diffraction of light waves.
Light scattering in colloidal solutions manifests itself in the form opalescence– a matte glow (usually of bluish hues), which is clearly visible against a dark background with side illumination of the sol. The cause of opalescence is the scattering of light on colloidal particles due to diffraction. Opalescence is associated with a phenomenon characteristic of colloidal systems - Tyndall effect: when a beam of light is passed through a colloidal solution from directions perpendicular to the beam, the formation of a luminous cone in the solution is observed.
Tyndall effect, Tyndall scattering is an optical effect, the scattering of light when a light beam passes through an optically inhomogeneous medium. It is usually observed as a luminous cone (Tyndall's cone) visible against a dark background.
It is typical for solutions of colloidal systems (for example, metal sols, dilute latexes, tobacco smoke), in which particles and their environment differ in refractive index. A number of optical methods for determining the size, shape and concentration of colloidal particles and macromolecules are based on the Tyndall effect. .
19. Zoli - these are poorly soluble substances (salts of calcium, magnesium, cholesterol, etc.) existing in the form of lyophobic colloidal solutions.
A Newtonian fluid is a viscous fluid that obeys Newton's law of viscous friction in its flow, that is, the tangential stress and velocity gradient in such a fluid are linearly dependent. The proportionality factor between these quantities is known as the viscosity.
The Newtonian fluid continues to flow even if the external forces are very small, as long as they are not strictly zero. For a Newtonian fluid, viscosity, by definition, depends only on temperature and pressure (and also on chemical composition if the fluid is not pure), and does not depend on the forces acting on it. A typical Newtonian fluid is water.
A non-Newtonian fluid is a fluid in which its viscosity depends on the velocity gradient. Typically, such liquids are highly inhomogeneous and consist of large molecules that form complex spatial structures.
The simplest illustrative household example is a mixture of starch with a small amount of water. The faster the external impact on the binder macromolecules suspended in the liquid, the higher its viscosity.
***An apple fell on Newton, the Chinese admired the drops on the lotus flowers, and John Tyndall, probably walking through the forest, noticed a cone of light. Story? Maybe. But it is in honor of the last hero that one of the most beautiful effects of our world is named - the Tyndall effect....***
Light scattering is one of the general characteristics of highly dispersed systems.
Under side illumination of a dispersed system, a characteristic iridescent, as a rule, bluish glow is observed, which is especially clearly visible against a dark background.
This property, associated with the scattering of light by particles of the dispersed phase, is called opalescence, from the name of opal - opalus (lat.), A translucent mineral of a bluish or yellowish-white color. In 1868, he discovered that when a colloidal solution is illuminated from the side with a beam of light from a strong source, a bright uniformly luminous cone is observed - Tyndall cone, or Tyndall effect, while in the case of a low molecular weight solution, the liquid appears to be optically empty, i.e. the trace of the beam is invisible.
on the left - 1% starch solution, on the right - water.
The Tyndall effect occurs during scattering by suspended particles, the size of which exceeds the size of atoms by tens of times. When the suspension particles are enlarged to sizes of the order of 1/20 of the wavelength of light (from about 25 nm and above), the scattering becomes polychromatic, that is, the light begins to scatter evenly over the entire visible range of colors from violet to red. As a result, the Tyndall effect disappears. That's why dense fog or cumulus clouds appear white to us - they consist of a dense suspension of water dust with particle diameters from microns to millimeters, which is well above the Tyndall scattering threshold.
You might think that the sky looks blue to us due to the Tyndall effect, but it is not. In the absence of clouds or smoke, the sky turns blue-blue due to the scattering of "daylight" on air molecules. This type of scattering is called Rayleigh scattering (after Sir Rayleigh). Rayleigh scattering scatters blue and cyan light even more than the Tyndall effect: for example, blue light with a wavelength of 400 nm scatters in clean air nine times stronger than red light with a wavelength of 700 nm. This is why the sky appears blue to us - sunlight scatters over the entire spectral range, but in the blue part of the spectrum it is almost an order of magnitude stronger than in the red. The ultraviolet rays that cause sunburn are even more scattered. That is why the tan is distributed fairly evenly over the body, covering even those areas of the skin that are not exposed to direct sunlight.
Gerasimenko Evgenia
This presentation is devoted to the description of the Tyndall Effect and its practical application.
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Completed by: student of grade 11 "B" Evgenia Gerasimenko Checked by: chemistry teacher Yurkina T.I. 2012/2013 academic year tyndall effect
John Tyndall Irish physicist and engineer. Born in Lylin Bridge, County Carlow. After graduating from high school, he worked as a topographer-surveyor in military organizations and in the construction of railways. At the same time he graduated from the Mechanical Institute in Preston. Dismissed from the military geodetic service for protesting against poor working conditions. He taught at Queenwood College (Hampshire), while continuing his self-education. In 1848–51 listened to lectures at Marburg and Berlin universities. Returning to England, he became a teacher, and then a professor at the Royal Institute in London. The main works of the scientist are devoted to magnetism, acoustics, absorption of thermal radiation by gases and vapors, light scattering in turbid media. Studied the structure and movement of glaciers in the Alps. Tyndall was extremely passionate about the idea of popularizing science. He regularly gave public lectures, often in the form of free lectures for everyone: for workers in the factory yards at lunchtime, Christmas lectures for children at the Royal Institute. Tyndall's fame as a popularizer also reached the other side of the Atlantic - the entire print run of the American edition of his book Fragments of Science was sold out in one day. He died an absurd death in 1893: while preparing dinner, the scientist's wife (who outlived him by 47 years) mistakenly used one of the chemical reagents stored in the kitchen instead of table salt.
Description Tyndall effect - the glow of an optically inhomogeneous medium due to the scattering of light passing through it. It is caused by the diffraction of light on individual particles or elements of the structural inhomogeneity of the medium, the size of which is much smaller than the wavelength of the scattered light. It is typical for colloidal systems (for example, hydrosols, tobacco smoke) with a low concentration of particles of the dispersed phase, which have a refractive index different from the refractive index of the dispersion medium. It is usually observed as a light cone on a dark background (Tyndall's cone) when a focused light beam is passed from the side through a glass cell with plane-parallel walls filled with a colloidal solution. The short-wave component of white (non-monochromatic) light is scattered by colloidal particles stronger than the long-wave component, therefore the Tyndall cone formed by it in non-absorbing ash has a blue tint. The Tyndall effect is essentially the same as opalescence. But traditionally, the first term refers to the intense scattering of light in a limited space along the path of the beam, and the second term refers to the weak scattering of light by the entire volume of the observed object.
The Tyndall effect is perceived by the naked eye as a uniform glow of some part of the volume of the light-scattering system. The light comes from individual dots - diffraction spots, well distinguishable under an optical microscope with sufficiently strong illumination of the diluted sol. The intensity of the light scattered in a given direction (at constant parameters of the incident light) depends on the number of scattering particles and their size.
Timing Initiation time (log to -12 to -6); Lifetime (log tc -12 to 15); Degradation time (log td -12 to -6); Optimal development time (log tk -9 to -7). Technical implementation of the effect The effect can be easily observed when a helium-neon laser beam is passed through a colloidal solution (simply uncolored starch jelly). Diagram
Application of the effect Based on the Tyndall effect, methods for detecting, determining the size and concentration of colloidal particles (ultramicroscopy, nephelometry are widely used in scientific research and industrial practice).
Example. Ultramicroscope. An ultramicroscope is an optical instrument for detecting the smallest (colloidal) particles whose dimensions are smaller than the resolution limit of conventional light microscopes. The possibility of detecting such particles using an ultramicroscope is due to the diffraction of light on them by the Tyndall effect. With strong side illumination, each particle in the ultramicroscope is marked by the observer as a bright point (luminous diffraction spot) against a dark background. Due to diffraction on the smallest particles, there is very little light, therefore, as a rule, strong light sources are used in an ultramicroscope. Depending on the intensity of illumination, the wavelength of light, the difference between the refractive indices of the particle and the medium, particles ranging in size from 20-50 nm to 1-5 μm can be detected. It is impossible to determine the true size, shape and structure of particles from diffraction spots. The ultramicroscope does not provide optical images of the objects under study. However, using an ultramicroscope, it is possible to determine the presence and number concentration of particles, study their movement, and also calculate the average size of particles if their weight concentration and density are known. In the scheme of a slit ultramicroscope (Fig. 1a), the system under study is immobile.
In the scheme of a slit ultramicroscope, the system under study is motionless. Schematic diagram of a slit microscope. Cuvette 5 with the object under study is illuminated by a light source 1 (2 - capacitor, 4 - lighting lens) through a narrow rectangular slit 3, the image of which is projected into the observation area. In the eyepiece of the observation microscope 6, luminous dots of particles located in the image plane of the slit are visible. Above and below the illuminated area, the presence of particles is not detected.
In a flow ultramicroscope, the studied particles move along the tube towards the observer's eye. Schematic diagram of a flow microscope Crossing the illumination zone, they are registered as bright flashes visually or using a photometric device. By adjusting the brightness of the illumination of the observed particles by the movable photometric wedge 7, it is possible to single out for registration particles whose size exceeds a predetermined limit. Using a modern in-line ultramicroscope with a laser light source and optoelectronic particle detection system, the particle concentration in aerosols is determined in the range from 1 to 109 particles per 1 cm3, and the particle size distribution functions are also found. Ultramicroscopes are used in the study of dispersed systems, to control the purity of atmospheric air. Water, the degree of contamination of optically transparent media with foreign inclusions.
Used literature 1. Physics. Big Encyclopedic Dictionary.- M.: Big Russian Encyclopedia, 1999.- P.90, 460. 2. New Polytechnical Dictionary.- M.: Big Russian Encyclopedia, 2000.- P.20, 231, 460. Key words optical glow inhomogeneous two-phase medium light scattering disperse medium
In terms of optical properties, colloidal solutions differ significantly from true solutions of low molecular weight substances, as well as from coarsely dispersed systems. The most characteristic optical properties of colloid-dispersed systems are opalescence, the Faraday-Tyndall effect, and color. All these phenomena are due to the scattering and absorption of light by colloidal particles.
Depending on the wavelength of visible light and the relative sizes of the particles of the dispersed phase, the scattering of light takes on a different character. If the particle size exceeds the wavelength of light, then light is reflected from them according to the laws of geometric optics. In this case, part of the light radiation can penetrate inside the particles, experience refraction, internal reflection, and be absorbed.
If the particle size is smaller than the half-wavelength of the incident light, diffractive light scattering is observed; the light, as it were, bypasses (envelops) the particles encountered on the way. In this case, partial scattering takes place in the form of waves diverging in all directions. As a result of the scattering of light, each particle is a source of new, less intense waves, i.e., it is as if self-luminescence of each particle occurs. The phenomenon of light scattering by tiny particles is called opalescence. It is characteristic mainly of sols (liquid and solid), it is observed only in reflected light, i.e., from the side or against a dark background. This phenomenon is expressed in the appearance of some turbidity of the sol and in the change (“overflows”) of its color compared to the color in transmitted light. Coloring in reflected light, as a rule, is shifted towards the higher frequency of the visible part of the spectrum. So, white sols (silver chloride sol, rosin, etc.) opalescent with a bluish color.
Faraday-Tyndall effect. Diffraction scattering of light was first noticed by M. V. Lomonosov. Later, in 1857, this phenomenon was observed by Faraday in gold sols. The phenomenon of diffraction (opalescence) for liquid and gaseous media was studied in most detail by Tyndall (1868).
If you take one glass with a solution of sodium chloride, and the other with an egg white hydrosol, it is difficult to establish where the colloidal solution is and where the true one is, since both liquids look colorless and transparent (Fig. 6.5). However, these solutions can be easily distinguished by doing the following experiment. Let's put on a light source (table lamp) an opaque case with a hole, in front of which, in order to obtain a narrower and brighter beam of light, we put a lens. If both glasses are placed in the path of the light beam, we will see a light path (cone) in the glass with the sol, while the beam is almost invisible in the glass with sodium chloride. By the name of the scientists who first observed this phenomenon, a luminous cone in a liquid was called the Faraday-Tyndall cone (or effect). This effect is characteristic of all colloidal solutions.
The appearance of the Faraday-Tyndall cone is explained by the phenomenon of light scattering by colloidal particles with a size of 0.1-0.001 microns.
The wavelength of the visible part of the spectrum is 0.76-0.38 microns, so each colloidal particle scatters the light falling on it. It is visible in the Faraday-Tyndall cone when the line of sight is directed at an angle to the beam passing through the sol. Thus, the Faraday-Tyndall effect is a phenomenon identical to opalescence, and differs from the latter only in the form of a colloidal state, i.e., the microheterogeneity of the system.
The theory of light scattering by colloidal dispersed systems was developed by Rayleigh in 1871. It establishes the dependence of the intensity (amount of energy) of scattered light (I) during opalescence and in the Faraday-Tyndall cone on external and internal factors. Mathematically, this dependence is expressed in the form of a formula called the Rayleigh formula:
 | 6.1 |
where I is the scattered light intensity in the direction perpendicular to the incident light beam; K is a constant depending on the refractive indices of the dispersion medium and the dispersed phase; n is the number of particles per unit volume of the sol; λ is the wavelength of the incident light; V is the volume of each particle.
From formula (6.1) it follows that the scattering of light (I) is proportional to the concentration of particles, the square of the volume of the particle (or for spherical particles - the sixth power of their radius) and inversely proportional to the fourth power of the wavelength of the incident light. Thus, the scattering of short waves occurs relatively more intensively. Therefore, colorless sols appear reddish in transmitted light, and blue in diffused light.
Coloring of colloidal solutions. As a result of the selective absorption of light (absorption) in combination with diffraction, one or another color of the colloidal solution is formed. Experience shows that most colloidal (especially metallic) solutions are brightly colored in a wide variety of colors, ranging from white to completely black, with all shades of the color spectrum. So, As 2 S 3 sols are bright yellow, Sb 2 S 3 - orange, Fe (OH) 3 - reddish brown, gold - bright red, etc.
The same sol has a different color depending on whether it is viewed in transmitted or reflected light. Sols of the same substance, depending on the method of preparation, can acquire a different color - the phenomenon of polychromy (multicolor). The color of the sols in this case depends on the degree of dispersion of the particles. Thus, coarsely dispersed gold sols have a blue color, a greater degree of dispersion - violet, and highly dispersed - bright red. It is interesting to note that the color of the metal in the non-dispersed state has nothing to do with its color in the colloidal state.
It should be noted that the color intensity of sols is tens (or even hundreds) times greater than that of molecular solutions. Thus, the yellow color of the As 2 S 3 sol in a layer 1 cm thick is clearly visible at a mass concentration of 10 -3 g/l, and the red color of the gold sol is noticeable even at a concentration of 10 -5 g/l.
The beautiful and bright color of many precious and semiprecious stones (rubies, emeralds, topazes, sapphires) is due to their content of negligible (not detectable even on the best analytical balance) amounts of impurities of heavy metals and their oxides, which are in a colloidal state. So, to artificially obtain bright ruby glass used for automobile, bicycle and other lamps, it is enough to add only 0.1 kg of colloidal gold per 1000 kg of glass mass.
