Optical properties of colloidal solutions. Start in science What is the Tyndall effect

… He began to think what was what.
Apparently, the light is afraid of torment.
So the flour is perfect
So that the wave diffracts!
All kinds of dust, and suspension, and turbidity
A beam of light can collapse...
From "Ode to Tyndall" (E.Nickelsparg)

Element "AIR"

An apple fell on Newton, the Chinese admired the drops on lotus flowers, and John Tyndall, probably walking through the forest, noticed a cone of light. Fairy tale? 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. Why is it beautiful - judge for yourself!

This is an optical effect that occurs when a light beam passes through an optically inhomogeneous medium. Typically observed as a luminous cone visible against a dark background. What is an optically inhomogeneous medium? In this case, dust or smoke, which is formed by colloidal particles that form aerosols. The size of the particles does not matter, because even nanoparticles in the atmosphere, be it particles of sea salt or volcanic dust, can cause such a beautiful spectacle. Studying light, Tyndall is rightfully the founder of fiber-optic communications, which have already become vital in our everyday life, which in the modern world has been improved to the nanolevel.

Element "WATER"

Take a look at the solutions shown in the figure. Outwardly, they appear almost identical: colorless and transparent. However, there is one “but”: the laser beam passes unhindered through the right glass, but is strongly scattered in the left glass, leaving a red trace. What's the secret?

In the right glass there is ordinary water, but in the left one there is a colloidal solution of silver. Unlike an ordinary or, as chemists say, a “true” solution, a colloidal solution does not contain molecules or ions of a dissolved substance, but its smallest particles. However, even the smallest nanoparticles can scatter light. This is the Tyndall effect.

What should the particle size be for their solution to be called “colloidal”? In various textbooks, it is suggested that particles whose size ranges from 1 nm to 100 nm, from 1 nm to 200 nm, from 1 nm to 1 micron are considered colloidal. However, the classification of sizes, like any other, is very conditional. The Tyndall effect in liquid media is used, for example, to assess the quality of wine. To assess the clarity of wines, a glass of wine is tilted slightly and placed between the light source and the eye, but not in line. The degree of transparency is determined not by the passage of rays through the wine, but by their reflection from suspended particles even of nanometer size! (Tyndall effect). To characterize the degree of transparency, a verbal scale is used, which includes such definitions as “light opal”, “opalescent”, “dull, with significant opalescence”. A number of optical methods for determining the size, shape and concentration of colloidal particles are based on the Tyndall effect.

“Although nanocolloidal particles are so small that they cannot be observed with an optical microscope, their content in a platinum-silver colloidal solution has been proven by using a laser beam directed into the colloidal solution and observing the Tyndall effect, i.e. scattering of light and bright radiance of the light beam,” from the annotation of Noadada cosmetics (Japan).

Element "EARTH"

The concept of “opalescence” is also directly related to John Tyndall. OPAL is a precious stone, from the play of light of which the term comes opalescence, denoting a special type of radiation scattering characteristic only of this crystal.

This is how Pliny described the opal: “The fire of opal is similar to the fire of a carbuncle, only softer and more gentle, while it glows purple like an amethyst and the green of the sea like emerald; everything merges together into unimaginable, sparkling splendor. The unimaginable charm and beauty of the stone earned it from many the name “paideros” - “love of a youth”. It is second only to emerald.”

Opal contains spherical silica particles with a diameter of 150-450 nanometers, which, in turn, are composed of small globules with a diameter of 50-100 nanometers, arranged in concentric layers or randomly. They form a fairly ordered packing (pseudocrystalline structure of opal). The spheres act as a three-dimensional diffraction grating, causing a characteristic scattering of light - opalescence. Thus, opal is a natural photonic crystal. The opal cluster superlattice served as a prototype for the creation of artificial photonic crystals. For example, in one of the very first works on the synthesis of photonic crystals, carried out at the Physico-Technical Institute (St. Petersburg) and Moscow State University in 1996, a technology was created for producing optically perfect synthetic opals based on microscopic spheres of silicon dioxide. The technology made it possible to vary the parameters of synthetic opals: sphere diameter, porosity, refractive index.

In opal, the lattices formed by closely packed spheres of silicon dioxide contain voids, occupying up to 25% of the total volume of the crystal, which can be filled with substances of a different type. The change in the optical properties of opals when filling voids with water was already known to scientists of the ancient world: a very rare variety of opal - hydrophane (hydrophane), in Old Russian - water light, becomes transparent when immersed in water. In modern developments, this property of a photonic crystal is used to create a light switch - an optical transistor.

Element "FIRE"

Possessing a rare talent as a lecturer and an extraordinary skill as an experimenter, Tyndall brought the “SPARK” of knowledge to the masses. Tyndall created an era with his popular lectures on physics, and may justly be considered the father of the modern popular lecture. His lectures were for the first time accompanied by brilliant and varied experiments, which are now included in the basic course of physics; all subsequent popularizers of physics followed in Tyndall's footsteps. He wrote: “In order to see the picture as a whole, its creator needs to distance himself from it, and in order to evaluate the general scientific achievements of any era, it is advisable to take the point of view of the subsequent one.” I would like to end with a poem I wrote on the topic of light and life:

Walk on the edge of a knife
Standing on the tip of a needle
Where macro force is not important
Compared to the power of the wave.
Where gravity is weak
If you are light as a charge,
Only variable fields
They will launch you like a missile.
Interference lights
They burn with the northern lights.
And like spring streams
The charges are quick and in a hurry.
Perhaps this world of wonders
Not visible to my eye,
But he is the basis of all substances,
Which means I live in it!

Tyndall effect, Tyndall scattering(English) Tyndall effect) - optical effect, light scattering when a light beam passes through an optically inhomogeneous medium. Usually observed as a luminous cone ( Tyndall cone), visible against a dark background.

The Tyndall effect is named after John Tyndall, who discovered it.

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Excerpt describing the Tyndall Effect

“Okay, okay, you’ll tell me later,” said Princess Marya, blushing.
“Let me ask her,” said Pierre. -Have you seen it yourself? - he asked.
- Why, father, you yourself have been honored. There is such a radiance on the face, like heavenly light, and from my mother’s cheek it keeps dripping and dripping...
“But this is a deception,” said Pierre naively, who listened attentively to the wanderer.
- Oh, father, what are you saying! - Pelageyushka said with horror, turning to Princess Marya for protection.
“They are deceiving the people,” he repeated.
- Lord Jesus Christ! – the wanderer said, crossing herself. - Oh, don't tell me, father. So one anaral did not believe it, he said: “the monks are deceiving,” and as he said, he became blind. And he dreamed that Mother of Pechersk came to him and said: “Trust me, I will heal you.” So he began to ask: take me and take me to her. I’m telling you the real truth, I saw it myself. They brought him blind straight to her, he came up, fell, and said: “Heal! “I will give you,” he says, “what the king gave you.” I saw it myself, father, the star was embedded in it. Well, I have received my sight! It's a sin to say that. “God will punish,” she instructively addressed Pierre.
- How did the star end up in the image? asked Pierre.
- Did you make your mother a general? - said Prince Andrei, smiling.

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Introduction

Each of us in our daily lives has more than once encountered and is confronted with phenomena that are ordinary on the one hand, but at the same time amazing on the other hand, without thinking at all about what remarkable 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 study this topic by conducting an experiment on my own.

Why do we observe different results when passing light of different spectral colors through cloudy glass, smoky air or a starch solution? Why does thick fog or cumulus clouds appear white to us, and haze from forest fires appears bluish-purple. Let's try to explain these phenomena.

Objective of the project:

detect colloids using the Tyndall effect;

investigate the influence of factors determining the passage of a light beam through a colloidal solution.

Research objectives:

study of the influence of 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;

searching for additional information on the Tyndall effect;

generalization of acquired knowledge.

Tyndall effect

Light refraction, reflection, dispersion, interference, diffraction and much more: optical effects are all around us. One of them is the Tyndall effect, discovered by the English physicist John Tyndall.

John Tyndall - surveyor, Faraday Fellow, Director of the Royal Institution in London, glaciologist and optician, acoustician and specialist in magnetism. His surname gave the 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 caused by the diffraction of light on individual particles or elements of inhomogeneity of the medium, the size of which is much smaller than the wavelength of the scattered light.

What is a heterogeneous medium? An inhomogeneous medium is a medium characterized by a variable refractive index. Those. n ≠const.

What characteristic feature of this effect can be identified? The Tyndall effect is characteristic of colloidal systems (systems in which one substance in the form of particles of different 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 that of refraction of the medium. Typically observed as a light cone on a dark background (Tyndall cone) when a focused light beam is passed from the side through a glass vessel with plane-parallel walls filled with a colloidal solution. (Colloidal 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 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, a directional light source (for example, a laser pointer), table salt, a surfactant solution (for example, liquid detergent), 1 chicken egg, a diluted solution of hydrochloric acid.

Conducting the experiment:

Pour water into a glass container and completely dissolve a little table salt in it.

We illuminate the glass with the resulting solution from the side with a narrow beam of light (the beam of a laser pointer). 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 stripe is visible in the path of the beam - the appearance of the Tyndall effect.

Then add a dilute solution of hydrochloric acid to the protein solution. The protein will coagulate (denature) to form a whitish precipitate. At the top of the glass, the light beam will again not be visible.

Experiment results: If you direct a beam of light from the side onto a glass beaker containing a salt solution, the beam will be invisible in the solution. If a beam of light is passed through a glass with a colloidal solution (surfactant solution), it will be visible because light is scattered by the colloidal particles.

Influence of wavelength, particle size and concentration on the implementation of the Tyndall effect

Wavelength. Since blue waves have the shortest wavelength in the visible spectrum, it is these waves that are reflected from particles during the Tyndall effect, while longer red waves are scattered less well.

Particle size. If the particle size increases, they can affect the scattering of light of any wavelength, and the "split" rainbow folds back into completely white light.

Particle concentration. The intensity of the scattered light is directly proportional to the concentration of particles in the colloidal solution.

Application of the Tyndall effect

Methods based on the Tyndall effect for detecting, determining the size and concentration of colloidal particles are widely used in scientific research and industrial practice (for example, in ultramicroscopes).

Depending on the intensity of illumination, the length of the light wave, the difference in the refractive indices of the particle and the medium, particles ranging in size from 20-50 nm to 1-5 microns can be detected. It is impossible to determine the true size, shape and structure of particles from diffraction spots. An ultramicroscope does not provide optical images of the objects under study. However, using an ultramicroscope, it is possible to determine the presence and numerical concentration of particles, study their movement, and also calculate the average particle size 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, 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 take a fresh look at both some branches of physics and our wonderful world as a whole.

In addition to the aspects discussed in this work, in my opinion, it would be interesting to study the possibilities of wider practical application of the Tyndall effect.

As for the purpose of the study, it may be useful and interesting for school students who are interested in optics, as well as for anyone interested in physics and various kinds of experiments.

Bibliography

Gavronskaya Yu.Yu. Colloidal chemistry: Textbook. SPb.: Publishing house of the Russian State Pedagogical University named after. A. I. Herzen, 2007. - 267 p.

New Polytechnic Dictionary. - M.: Great Russian Encyclopedia, 2000. - .20 p. , 231 p. , 460 p.

Guide to performing experiments for "NanoSchoolBox". NanoBioNet e.V/ Scince Park Translation 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

The appearance of a luminous cone on a dark background when light is scattered in a turbid medium with particle sizes an order of magnitude smaller than the wavelength of light

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Description

The Tyndall effect is 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 structural heterogeneity of the medium, the size of which is much smaller than the wavelength of the scattered light. Characteristic of colloidal systems (for example, hydrosols, tobacco smoke) with a low concentration of dispersed phase particles having a refractive index different from the refractive index of the dispersion medium. Typically observed as a light cone on a dark background (Tyndall cone) when a focused light beam is passed from the side through a glass cuvette with plane-parallel walls filled with a colloidal solution. The short-wave component of white (non-monochromatic) light is scattered by colloidal particles more strongly than the long-wave component, therefore the Tyndall cone formed by it in a 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 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 a light-scattering system. Light comes from individual points - diffraction spots, clearly visible under an optical microscope with sufficiently strong illumination of the dilute sol. The intensity of light scattered in a given direction (at constant parameters of the incident light) depends on the number of scattering particles and their size.

Timing characteristics

Initiation time (log to -12 to -6);

Lifetime (log tc from -12 to 15);

Degradation time (log td from -12 to -6);

Time of optimal development (log tk from -9 to -7).

Diagram:

Technical implementations of the effect

Technical implementation of the effect

The effect can be easily observed when passing a helium-neon laser beam through a colloidal solution (simply uncolored starch jelly).

Applying an effect

Methods based on the Tyndall effect 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 tiny (colloidal) particles whose sizes are less than the resolution limit of conventional light microscopes. The ability to detect such particles using an ultramicroscope is due to the diffraction of light by the Tyndall effect. Under strong lateral illumination, each particle in the ultramicroscope is marked by the observer as a bright point (luminous diffraction spot) on a dark background. Due to diffraction on the smallest particles there is very little light, so in an ultramicroscope, as a rule, strong light sources are used. Depending on the intensity of illumination, the length of the light wave, the difference in the refractive indices of the particle and the medium, particles ranging in size from 20-50 nm to 1-5 microns can be detected. It is impossible to determine the true size, shape and structure of particles from diffraction spots. An ultramicroscope does not provide optical images of the objects under study. However, using an ultramicroscope, it is possible to determine the presence and numerical concentration of particles, study their movement, and also calculate the average particle size if their weight concentration and density are known.

In the scheme of a slit ultramicroscope (Fig. 1a), the system under study is motionless.

Schematic diagram of a slit microscope

Rice. 1a

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 zone. Through the eyepiece of observation microscope 6, luminous points 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 (Fig. 1b), the particles under study move along the tube towards the observer’s eye.

Schematic diagram of a flow microscope

Rice. 1b

As they cross the illumination zone, they are recorded as bright flashes visually or using a photometric device. By adjusting the brightness of illumination of the observed particles with a movable photometric wedge 7, it is possible to select for registration particles whose size exceeds a given limit. Using a modern flow ultramicroscope with a laser light source and an optical-electronic particle registration system, the concentration of particles in aerosols is determined in the range from 1 to 109 particles per 1 cm3, and particle size distribution functions are also found.

Scattering of light. From a classical point of view, light scattering is that

Electromagnetic waves passing through matter cause vibrations of electrons in atoms. Explanation: if the particle size is small, then the electrons making

forced vibrations in atoms are equivalent to an oscillating dipole. This dipole oscillates with the frequency of the light wave incident on it. Hence, the short-wave part of the spectrum is scattered much more intensely than the long-wave part. Blue light is scattered almost 5 times more intensely than red light. Therefore, scattered light is blue, and transmitted light is reddish. At very high altitudes (hundreds of kilometers) the concentration of atmospheric molecules is very small, scattering practically disappears, the sky should appear black, and the stars are visible in the presence of the Sun. During space flights, all these predictions were completely confirmed.

The Rayleigh-Jeans law is the law of radiation for the equilibrium radiation density of an absolutely black body and for the emissivity of an absolutely black body.

Tyndall effect, Tyndall effect - optical effect, light scattering when a light beam passes through an optically inhomogeneous medium. Typically observed as a luminous cone (Tyndall cone) visible against a dark background.

Characteristic of solutions of colloidal systems (for example, sols, metals, diluted latexes, tobacco smoke), in which the particles and their environment differ in refractive index.

Nephelometry is a method of studying and analyzing a substance based on the intensity of the light flux scattered by suspended particles of this substance.

The essence of the method

The intensity of the scattered light flux depends on many factors, in particular on the concentration of particles in the analyzed sample. The volume of particles scattering light is of great importance in nephelometry. An important requirement for reactions used in nephelometry is that the reaction product must be practically insoluble and be a suspension (suspension). To keep solid particles in suspension, various stabilizers (for example, gelatin) are used to prevent particle coagulation.

50. Thermal radiation of bodies. Laws of black body radiation (Stephan–Boltzmann, Wien).

There is an endless process of energy exchange between all bodies of nature. Bodies continuously emit and absorb energy. If the excitation of atoms occurs as a result of their collision with other atoms of the same body in the process of thermal motion, then the resulting electromagnetic radiation is called thermal radiation.

Thermal radiation occurs at any temperature. In this case, regardless of temperature, the body emits all wavelengths without exception, i.e. the spectrum of thermal radiation is continuous and extends from zero to infinity. However, the higher the temperature, the more short-wave radiation is the main one in the radiation spectrum. The process of emission of electromagnetic waves by the body occurs simultaneously and independently with their absorption.

A body that completely absorbs energy over the entire range of wavelengths, i.e. for which α = 1 is called absolutely black (black)

STEPHAN-BOLZMANN LAW. Wien's displacement law

Stefan and Boltzmann obtained an integral expression for the energetic luminosity of a black body, which does not take into account the distribution of energy over wavelengths:

R = σT 4, σ is the Stefan-Boltzmann constant (σ = 5.6696·10 -8 W/(m 2 ·K 4)).

For gray bodies, Kirchhoff's law allows us to write r λ = α λ ε λ , then for the energetic luminosity of gray bodies we have: .

Analyzing the curves, Wien found that the wavelength at which the maximum spectral density of energy luminosity falls is determined by the relation: .

This is Wien's law, where b = 0.28978·10 -2 m·K is Wien's constant.

Let us determine the value of the wavelength for which ε λ has a maximum value at a given temperature, based on the relationship. According to the rules for finding extrema, this will be provided . Calculations show that this will take place if λ = b/T.

From the relationship it is clear that with increasing temperature, the wavelength at which the maximum emissivity of an absolutely black body occurs shifts to the short-wave region. For this reason, the relationship is also known in the scientific literature as Wien's displacement law. This law is also true for gray bodies.

The Stefan-Boltzmann and Wien laws make it possible to determine their temperatures based on measurements of the energy emitted by a body. This branch of physics is called optical pyrometry.