Radiant energy and spectral composition of optical radiation. Two sides of the same coin

Living nature cannot exist without light, since solar radiation reaching the Earth's surface is practically the only source of energy for maintaining the thermal balance of the planet, creating organic substances by phototrophic organisms of the biosphere, which ultimately ensures the formation of an environment that can satisfy the vital needs of all living beings.
The light regime of any habitat depends on its geographical latitude, height above sea level, the state of the atmosphere, vegetation, season and time of day, solar activity, etc. Therefore, the variety of light conditions on our planet is extremely large: from such highly illuminated areas as highlands , deserts, steppes, to twilight illumination in the water depths and caves.

The biological effect of sunlight depends on its spectral composition, duration, intensity, daily and seasonal periodicity.

Solar radiation is electromagnetic radiation in a wide range of waves, constituting a continuous spectrum from 290 to 3,000 nm. Ultraviolet rays (UFL) shorter than 290 nm, harmful to living organisms, are absorbed by the ozone layer and do not reach the Earth. The Earth is reached mainly by infrared (about 50% of total radiation) and visible (45%) rays of the spectrum. The share of UFL, having a wavelength of 290-380 nm, accounts for 5% of radiant energy. Long-wave UVL, which have high photon energy, are distinguished by high chemical activity. In small doses, they have a powerful bactericidal effect, promote the synthesis of certain vitamins and pigments in plants, and in animals and humans - vitamin D; in addition, they cause sunburn in humans, which is a protective reaction of the skin. Infrared rays with a wavelength of more than 710 nm have a thermal effect.

In ecological terms, the most important is the visible region of the spectrum (390-710 nm), or photosynthetically active radiation (PAR), which is absorbed by chloroplast pigments and thus is of decisive importance in plant life. Visible light is needed by green plants for the formation of chlorophyll, the formation of the structure of chloroplasts; it regulates the functioning of the stomatal apparatus, affects gas exchange and transpiration, stimulates the biosynthesis of proteins and nucleic acids, and increases the activity of a number of photosensitive enzymes. Light also affects the division and elongation of cells, growth processes and the development of plants, determines the timing of flowering and fruiting, and has a shaping effect.
Light with different radiation frequencies (and different colors in the visible range) affects the growth, development of plants and photosynthesis in different ways. In general, plants absorb blue and red, while green reflects or transmits. As a result, green light is used the least efficiently by the leaves. That is why the leaves of plants are mostly green. The dependence of the absorption and assimilation of energy by plants on the wavelength of light radiation is called the energy spectrum of photosynthetic active radiation (radiation). In essence, photosynthetic active radiation is a flow of energy of a certain spectrum, usually the radiation power

The light energy absorbed by plants is spent on photosynthesis, photomorphogenesis, synthesis of chlorophyll, and part of the energy is used for heating and reradiation. The activity of these processes depends on the wavelength in different ways. By changing the radiation components of the blue, green and red parts of the spectrum, it is possible to influence the germination, growth or inhibition of various biological processes and stages of photosynthesis. Studies have shown that PAR - radiation affects not only plants, but also significantly slows down the development of pathogenic fungi and bacteria on irradiated plants.

All plants perceive different wavelengths in the PAR spectrum differently. This is due to the different absorption of different types of pigments in the leaves. The main leaf pigments, chlorophylls a and b, absorb blue and red light, carotenoids absorb blue light. The generalization of light absorption data by leaves of different crops allowed specialists of the Optimum Design Bureau to calculate the effective spectral absorption curve of the “average” green leaf and the spectra for the main agricultural crops (tomatoes, cucumbers, peppers).

Remember: a sunny summer day - and suddenly a cloud appeared in the sky, it began to rain, which seemed to “not notice” that the sun continues to shine. Such rain is popularly called blind. The rain had not yet ended, and a multi-colored rainbow was already shining in the sky (Fig. 13.1). Why did she appear?

Breaking down sunlight into a spectrum.

Even in ancient times, it was noticed that the sunbeam, passing through a glass prism, becomes multi-colored. It was believed that the reason for this phenomenon is the property of a prism to color light. Is this really so, the outstanding English scientist Isaac Newton (1643-1727) found out in 1665 by conducting a series of experiments.

Rice. 13.1. A rainbow can be observed, for example, in the spray of a fountain or waterfall.

To get a narrow beam of sunlight, Newton made a small round hole in the shutter. When he installed a glass prism in front of the hole, a multi-colored strip appeared on the opposite wall, which the scientist called the spectrum. On the strip (as in the rainbow), Newton singled out seven colors: red, orange, yellow, green, blue, indigo, violet (Fig. 13.2, a).

Then, using a screen with a hole, the scientist singled out narrow single-color (monochromatic) beams of light from a wide multi-colored beam of rays and directed them again to the prism. Such beams were deflected by the prism, but were no longer decomposed into a spectrum (Fig. 13.2, b). In this case, the violet light beam was deflected more than others, and the red light beam was deflected less than others.

The results of the experiments allowed Newton to draw the following conclusions:

1) a beam of white (sunlight) light consists of light of different colors;

2) the prism does not “color” white light, but separates it (spreads it into a spectrum) due to the different refraction of light beams of different colors.

rice. 13.2. Scheme of I. Newton's experiments to determine the spectral composition of light

Compare fig. 13.1 and 13.2: the colors of the rainbow are the colors of the spectrum. And this is not surprising, because in fact the rainbow is a huge spectrum of sunlight. One of the reasons for the appearance of a rainbow is that many small water droplets refract the white sunlight.

Learn about the dispersion of light

Newton's experiments demonstrated, in particular, that when refracted in a glass prism, violet light beams always deviate more than red light beams. This means that for light beams of different colors, the refractive index of glass is different. That is why a beam of white light is decomposed into a spectrum.

The phenomenon of decomposition of light into a spectrum, due to the dependence of the refractive index of the medium on the color of the light beam, is called light dispersion.

For most transparent media, violet light has the highest refractive index, and red light has the lowest.

What color beam of light - violet or red - propagates in glass with greater speed? Hint: Remember how the refractive index of a medium depends on the speed of light in that medium.

We characterize colors

In the spectrum of sunlight, seven colors are traditionally distinguished, and more can be distinguished. But you will never be able to highlight, for example, brown or lilac. These colors are composite - they are formed as a result of superposition (mixing) of spectral (pure) colors in different proportions. Some spectral colors, when superimposed on each other, form white. Such pairs of spectral colors are called complementary (Fig. 13.3).

For human vision, the three main spectral colors - red, green and blue - are of particular importance: when superimposed, these colors give a wide variety of colors and shades.

The color image on the screens of a computer, TV, telephone is based on the superposition of the three primary spectral colors in different proportions (Fig. 13.4).

Rice. 13.5. Different bodies reflect, refract and absorb sunlight in different ways, and thanks to this we see the world around us in different colors.

Find out why the world is colorful

Knowing that white light is composite, it is possible to explain why the world around us, illuminated by only one source of white light - the Sun, we see as multi-colored (Fig. 13.5).

So, the surface of a sheet of office paper equally well reflects the rays of all colors, so a sheet illuminated with white light seems white to us. A blue backpack, illuminated by the same white light, predominantly reflects the blue rays, while absorbing the rest.

What color do you think most sunflower petals reflect? plant leaves?

Blue light directed at red rose petals will be almost completely absorbed by them, since the petals reflect predominantly red rays, while the rest absorb. Therefore, a rose illuminated with blue light will appear almost black to us. If white snow is illuminated with blue light, it will appear blue to us, because white snow reflects the rays of all colors (including blue). But the black fur of a cat absorbs all the rays well, so the cat will appear black when illuminated by any light (Fig. 13.6).

Note! Since the color of the body depends on the characteristics of the incident light, in the dark the concept of color is meaningless.

Rice. 13.6. The color of a body depends both on the optical properties of its surface and on the characteristics of the incident light.

Summing up

A beam of white light consists of light of different colors. There are seven spectral colors: red, orange, yellow, green, blue, indigo, violet.

The refractive index of light, and hence the speed of propagation of light in a medium, depends on the color of the light beam. if The dependence of the refractive index of the medium on the color of the light beam is called the dispersion of light. We see the world around us in different colors due to the fact that different bodies reflect, refract and absorb light in different ways.

test questions

1. Describe the experiments of I. Newton to determine the spectral composition of light.

2. Name seven spectral colors. 3. What color light beam is refracted in matter more than others? less than others? if 4. Define the dispersion of light. What natural phenomenon is associated with dispersion? 5. What colors are called complementary? 6. Name the three primary colors of the spectrum. Why are they called that? 7. Why do we see the world around us in different colors?

Exercise number 13

1. What will black letters on white paper look like when viewed through green glass? What will the color of the paper look like?

2. What colors of light pass through blue glass? absorbed by it?

3. Through what color glass can you not see text written in purple ink on white paper?

4. Light beams of red, orange and blue colors propagate in the water. Which beam propagates the fastest?

5. Use additional sources of information and find out why the sky is blue; Why is the sun often red at sunset?

Experimental task

"Rainbow Creators" Fill a shallow vessel with water and place it against a light wall. Place a flat mirror at an angle on the bottom of the vessel (see figure). Direct a beam of light at the mirror - a "sunbeam" will appear on the wall. Examine it and explain the observed phenomenon.

Physics and technology in Ukraine

Kyiv National University. Taras Shevchenko (KNU) was founded in November 1833 as the Imperial University of St. Vladimir. The first rector of the university is an outstanding scientist-encyclopedist Mikhail Aleksandrovich Maksimovich.

The names of well-known scientists - mathematicians, physicists, cybernetics, astronomers - are associated with KNU: D. A. Grave, M. F. Kravchuk, G. V. Pfeiffer, N. N. Bogolyubov, V. M. Glushkov, A. V. Skorokhod , I. I. Gikhman, B. V. Gnedenko, V. S. Mikhalevich, M. P. Avenarius, N. N. Schiller, I. I. Kosonogov, A. G. Sitenko, V. E. Lashkarev, R F. Vogel, M. F. Khandrikov, S. K. Vsekhsvyatsky.

Scientific schools of KNU are known in the world - algebraic, probability theory and mathematical statistics, mechanics, semiconductor physics, physical electronics and surface physics, metallogenic, optics of new materials, etc. Gubersky.

This is textbook material.

Light - electromagnetic radiation emitted by a heated or excited substance, perceived by the human eye. Often, light is understood not only as visible light, but also as wide areas of the spectrum adjacent to it. One of the characteristics of light is its color, which for monochromatic radiation is determined by the wavelength, and for complex radiation - by its spectral composition.

Main the source of light is the sun. The light it emits is considered to be white. Light comes from the sun at different wavelengths.

Light has a temperature that depends on the power of light radiation. In turn, the power depends on the wavelength.

The light from an incandescent lamp appears white, but its spectrum is red-shifted.

The light from a fluorescent lamp is shifted towards the violet part of the spectrum, has a bluish color and a high color temperature.

The light of sunlight in the highlands is shifted towards violet waves. This is due to the rarefied atmosphere at high altitude.

In the sandy desert, the spectrum will be shifted towards the red waves, because. the radiation of hot sand is added to the sunlight.

When shooting, it is necessary to take into account these facts, to know the spectrum of the available light radiation in order to get a high-quality picture with the shades available in the original.

That. Photons of different lengths come from different light sources.

Color is the sensation evoked in the human eye and brain by light of varying wavelengths and intensities.

Radiation of different intensity objectively exists and causes the sensation of a certain color. But by itself it has no color. Color occurs in the organs of human vision. It does not exist independently of them. Therefore, it cannot be considered an objective value.

To describe color, subjective qualitative and quantitative assessments of its characteristics are used.

The causes of color sensations are electromagnetic radiation, light, the objective characteristics of which are associated with the subjective characteristics of color, its saturation, tone, brightness.

Color tone is subjective. due to the properties of human visual perception, light, intensity wave definition.

The temperature at which a black body emits light of the same spectral composition as the light under consideration is called the color temperature. It indicates only the spectral distribution of the radiation energy, and not the temperature of the source. Thus, the light of the blue sky corresponds to a color temperature of about 12,500-25,000 K, i.e., much higher than the temperature of the sun. Color temperature is expressed in Kelvin (K).

The concept of color temperature is applicable only to thermal (hot) light sources. The light of an electric discharge in gases and metal vapors (sodium, mercury, neon lamps) cannot be characterized by the value of the color temperature.

The chemical composition of the substance- the most important characteristic of the materials used by mankind. Without his exact knowledge, it is impossible to plan technological processes in industrial production with any satisfactory accuracy. Recently, the requirements for determining the chemical composition of a substance have become even more stringent: many areas of industrial and scientific activity require materials of a certain "purity" - these are the requirements for an exact, fixed composition, as well as a strict restriction on the presence of impurities of foreign substances. In connection with these trends, more and more progressive methods for determining the chemical composition of substances are being developed. These include the method of spectral analysis, which provides an accurate and fast study of the chemistry of materials.

fantasy of light

The nature of spectral analysis

(spectroscopy) studies the chemical composition of substances based on their ability to emit and absorb light. It is known that each chemical element emits and absorbs a light spectrum characteristic only for it, provided that it can be reduced to a gaseous state.

In accordance with this, it is possible to determine the presence of these substances in a particular material by their inherent spectrum. Modern methods of spectral analysis make it possible to establish the presence of a substance weighing up to billionths of a gram in a sample - the indicator of radiation intensity is responsible for this. The uniqueness of the spectrum emitted by an atom characterizes its deep relationship with the physical structure.

Visible light is radiation from 3,8 *10 -7 before 7,6*10 -7 m responsible for different colors. Substances can emit light only in an excited state (this state is characterized by an increased level of internal ) in the presence of a constant source of energy.

Receiving excess energy, the atoms of matter emit it in the form of light and return to their normal energy state. It is this light emitted by the atoms that is used for spectral analysis. The most common types of radiation include: thermal radiation, electroluminescence, cathodoluminescence, chemiluminescence.

Spectral analysis. Flame coloring with metal ions

Types of spectral analysis

Distinguish between emission and absorption spectroscopy. The method of emission spectroscopy is based on the properties of elements to emit light. To excite the atoms of a substance, high-temperature heating is used, equal to several hundred or even thousands of degrees - for this, a sample of the substance is placed in a flame or in the field of powerful electric discharges. Under the influence of the highest temperature, the molecules of a substance are divided into atoms.

Atoms, receiving excess energy, radiate it in the form of light quanta of various wavelengths, which are recorded by spectral devices - devices that visually depict the resulting light spectrum. Spectral devices also serve as a separating element of the spectroscopy system, because the light flux is summed from all substances present in the sample, and its task is to divide the total light array into spectra of individual elements and determine their intensity, which will allow in the future to draw conclusions about the value of the element present in the total mass of substances.

• Depending on the methods of observing and recording spectra, spectral instruments are distinguished: spectrographs and spectroscopes. The former register the spectrum on photographic film, while the latter make it possible to view the spectrum for direct observation by a person through special telescopes. To determine the dimensions, specialized microscopes are used, which allow to determine the wavelength with high accuracy.
• After registration of the light spectrum, it is subjected to a thorough analysis. Waves of a certain length and their position in the spectrum are identified. Further, the ratio of their position with belonging to the desired substances is performed. This is done by comparing the data of the position of the waves with the information located in the methodical tables, indicating the typical wavelengths and spectra of chemical elements.
• Absorption spectroscopy is carried out similarly to emission spectroscopy. In this case, the substance is placed between the light source and the spectral apparatus. Passing through the analyzed material, the emitted light reaches the spectral apparatus with "dips" (absorption lines) at certain wavelengths - they constitute the absorbed spectrum of the material under study. The further sequence of the study is similar to the above process of emission spectroscopy.

Discovery of spectral analysis

Significance of spectroscopy for science

Spectral analysis allowed humanity to discover several elements that could not be determined by traditional methods of registering chemicals. These are elements such as rubidium, cesium, helium (it was discovered using the spectroscopy of the Sun - long before its discovery on Earth), indium, gallium and others. The lines of these elements were found in the emission spectra of gases, and at the time of their study were unidentifiable.

It became clear that these are new, hitherto unknown elements. Spectroscopy has had a serious impact on the formation of the current type of metallurgical and machine-building industries, the nuclear industry, and agriculture, where it has become one of the main tools for systematic analysis.

Spectroscopy has become of great importance in astrophysics.

Provoking a colossal leap in understanding the structure of the universe and asserting the fact that everything that exists consists of the same elements, which, among other things, abound on the Earth. Today, the method of spectral analysis allows scientists to determine the chemical composition of stars, nebulae, planets and galaxies located billions of kilometers from the Earth - these objects, of course, are not accessible to direct analysis methods due to their great distance.

Using the method of absorption spectroscopy, it is possible to study distant space objects that do not have their own radiation. This knowledge makes it possible to establish the most important characteristics of space objects: pressure, temperature, features of the structure of the structure, and much more.

Newton's experiments found that sunlight has a complex character. In a similar way, that is, by analyzing the composition of light with a prism, one can be convinced that the light of most other sources (incandescent lamp, arc lamp, etc.) has the same character. Comparing the spectra of these luminous bodies, we find that the corresponding sections of the spectra have different brightness, i.e., the energy is distributed differently in different spectra. You can verify this even more reliably if you study the spectra with the help of a thermoelement (see § 149).

For ordinary sources, these differences in the spectrum are not very significant, but they can be easily detected. Our eye, even without the help of a spectral apparatus, detects differences in the quality of the white light given by these sources. Thus, the light of a candle seems yellowish or even reddish compared to an incandescent lamp, and this latter is noticeably yellower than sunlight.

Even more significant is the difference if the source of light instead of a hot body is a tube filled with a gas that glows under the action of an electric discharge. Such tubes are currently used for luminous inscriptions or street lighting. Some of them discharge lamps give bright yellow (sodium lamps) or red (neon lamps) light, others glow with a whitish light (mercury), clearly different in shade from the sun. Spectral studies of light from such sources show that their spectrum contains only individual, more or less narrow, colored regions.

At present, they have learned how to manufacture gas-discharge lamps, the light of which has a spectral composition very close to that of the sun. Such lamps are called fluorescent lamps(see § 186).

If you examine the light of the sun or an arc lamp, filtered through colored glass, then it will be noticeably different from the original. The eye will evaluate this light as colored, and spectral decomposition will reveal that more or less significant parts of the source spectrum are absent or very weak in its spectrum.

§ 165. Light and colors of bodies. The experiments described in § 164 show that the light that causes the sensation of one color or another in our eye has a more or less complex spectral composition. It turns out that our eye is a rather imperfect apparatus for analyzing light, so that rays of diverse spectral composition can sometimes produce almost the same color impression. Nevertheless, it is with the help of the eye that we gain knowledge about the whole variety of colors in the world around us.

Cases where light from a source is directed directly into the observer's eye are relatively rare. Much more often, light first passes through bodies, being refracted and partially absorbed in them, or more or less completely reflected from their surface. Thus, the spectral composition of the light that has reached our eye can be significantly changed due to the processes of reflection, absorption, etc. described above. In the vast majority of cases, all such processes lead only to the weakening of certain spectral regions and can even completely eliminate some from such areas, but do not add to the light that came from the source, the radiation of those wavelengths that were not in it. However, such processes can also take place (for example, in fluorescence phenomena).

§ 166. Coefficients of absorption, reflection and transmission. The color of various objects illuminated by the same light source (for example, the sun) is very diverse, despite the fact that all these objects are illuminated by light of the same composition. The main role in such effects is played by the phenomena of reflection and transmission of light. As has already been clarified, the luminous flux incident on the body is partially reflected (scattered), partially transmitted and partially absorbed by the body. The proportion of the light flux involved in each of these processes is determined using the appropriate coefficients: reflection r, transmission t and absorption a (see § 76).

Each of the indicated coefficients (a, r, t) can depend on the wavelength (color), due to which various effects arise when illuminating bodies. It is easy to see that any body, for example, for which the transmittance is large for red light, and the reflection coefficient is small, and for green, on the contrary, will appear red in transmitted light and green in reflected light. Such properties are possessed, for example, by chlorophyll - a green substance contained in the leaves of plants and causing their green color. A solution (extract) of chlorophyll in alcohol turns out to be red in the light, and green in the reflection.

Bodies in which absorption is large for all rays, and reflection and transmission are very small, will be black opaque bodies (for example, soot). For a very white opaque body (magnesium oxide), the coefficient r is close to unity for all wavelengths, and the coefficients a and t very small. Completely transparent glass has small reflection coefficients r and absorption coefficients a and a transmittance t close to unity for all wavelengths; on the contrary, for colored glass, for some wavelengths, the coefficients t and r are practically equal to zero and, accordingly, the value of the coefficient a is close to unity. The difference in the values ​​of the coefficients a, t and r and their dependence on color (wavelength) cause an extraordinary variety in the colors and shades of various bodies.

§ 167. Colored bodies illuminated by white light. Painted bodies appear colored when illuminated with white light. If the layer of paint is thick enough, then the color of the body is determined by it and does not depend on the properties of the layers underlying the paint. Typically, paint is small grains that selectively scatter light and are immersed in a transparent mass that binds them, such as oil. The coefficients a, r and t of these grains determine the properties of the paint.

The action of the paint is schematically shown in Fig. 316. The topmost layer reflects almost equally everything

Rice. 316. Scheme of action of a layer of paint

rays, i.e. white light comes from it. Its share is not very significant, about 5%. The remaining 95% of the light penetrates deep into the paint and, being scattered by its grains, goes out. In this case, part of the light is absorbed in the paint grains, and certain spectral regions are absorbed to a greater or lesser extent, depending on the color of the paint. Part of the light penetrating even deeper is scattered on the next layers of grains, etc. As a result, a body illuminated with white light will have a color determined by the values ​​of the coefficients a, t and r for the grains of the paint covering it.

Paints that absorb light falling on them in a very thin layer are called covering. Paints, the action of which is due to the participation of many layers of grains, are called glazing. The latter allow you to achieve very good effects by mixing several types of colored grains (erasing on the palette). As a result, you can get a variety of color effects. It is interesting to note that mixing glazing colors corresponding to complementary colors should result in very dark shades. Indeed, let red and green grains be mixed in the paint. The light scattered by the red grains will be absorbed by the green grains and vice versa, so that almost no light will escape from the paint layer. Thus, mixing colors gives completely different results than mixing light of the corresponding colors. This circumstance should be kept in mind by the artist when mixing paints.

§ 168. Colored bodies illuminated by colored light. All of the above applies to white light illumination. If the spectral composition of the incident light is significantly different from daylight, then the lighting effects can be completely different. Bright colorful areas of a color picture look dark if the incident light lacks precisely those wavelengths for which these areas have a high reflectivity. Even the transition from daylight to artificial evening lighting can significantly change the ratio of shades. In daylight, the relative proportion of yellow, green and blue rays is much greater than in artificial light. Therefore, yellow and green fabrics appear dimmer in the evening light than during the day, and fabrics that are blue in daylight often appear completely black under lamps. This circumstance must be taken into account by artists and decorators who choose colors for a theatrical performance or for a parade taking place during the day in the open air.

In many industries where the correct assessment of shades is important, for example when sorting yarn, working in the evening light is very difficult or even completely impossible. Therefore, under such conditions, it is rational to use fluorescent lamps, that is, lamps whose spectral composition of light would be as close as possible to the spectral composition of daylight (see § 187).

§ 169. Camouflage and unmasking. Even under bright illumination, we are not able to distinguish bodies whose color does not differ from the color of the surrounding background, i.e., bodies for which the coefficient r has practically the same values ​​for all wavelengths as for the background. That is why, for example, it is so difficult to distinguish animals with white fur or people in white clothes on a snowy plain. This is used in military affairs for color camouflage of troops and military facilities. In nature, in the process of natural selection, many animals have acquired protective coloration (mimicry).

From the foregoing, it is clear that the most perfect masking is the selection of such a color, in which the reflection coefficient r for all wavelengths has the same values ​​as that of the surrounding background. In practice, this is very difficult to achieve, and therefore it is often limited to the selection of close reflection coefficients for radiation, which plays a particularly important role in daylight and eye observation. This is predominantly the yellow-green part of the spectrum, to which the eye is especially sensitive and which is more strongly represented in sunlight (daylight). However, if the objects camouflaged in such a way are observed not with the eye, but photographed, then the camouflage may lose its significance. Indeed, violet and ultraviolet radiation is especially strong on a photographic plate. Therefore, if for this region of the spectrum the reflection coefficients of the object and the background are noticeably different from each other, then when observed by the eye, such a masking defect will go unnoticed, but it will sharply make itself felt in the photograph. The imperfection of camouflage will also be clearly visible if one observes through a light filter that practically eliminates those wavelengths for which camouflage is primarily designed, for example, through a blue filter. Despite a significant decrease in the brightness of the entire picture when viewed through such a filter, details that were hidden when observed in white light may appear on it. Pairing a filter with a photograph can have a particularly powerful effect. Therefore, when choosing masking colors, one must be attentive to the definition of r for a fairly wide range of the spectrum, including infrared and ultraviolet.

Light filters are sometimes used to improve the correct transmission of illumination when photographing. Due to the fact that the sensitivity maxima of the eye and the photographic plate lie in different areas (yellow-green for the eye, blue-violet for the photographic plate), visual and photographic impressions can be quite different. The figure of a girl dressed in a yellow blouse and a purple skirt seems to the eye to be light in its upper part and dark in its lower part. On a photographic card, she may appear to be wearing a dark blouse and a light-colored skirt. If, on the other hand, a yellow light filter is placed in front of a photographic lens, it will change the ratio of illumination of the skirt and blouse in a direction approaching the visual impression. By using, moreover, a photographic film with an increased sensitivity to long wavelengths (orthochromatic) compared to ordinary ones, we can achieve a fairly correct reproduction of the illumination of the figure.

§ 170. Saturation of colors. In addition to the designation of color - red, yellow, blue, etc. - we often distinguish color by saturation, that is, by the purity of the hue, the absence of whitishness. An example of deep, or saturated, colors are spectral colors. They represent a narrow range of wavelengths without the admixture of other colors. The colors of fabrics and paints covering objects are usually less saturated and more or less whitish. The reason lies in the fact that the reflection coefficient of most colorants is not equal to zero for any wavelength. Thus, when a dyed fabric is illuminated with white light, we observe in scattered light mainly one color region (for example, red), but a noticeable amount of other wavelengths is mixed with it, giving white light in aggregate. But if such a tissue-scattered light with a predominance of one color (for example, red) is not directed directly into the eye, but is forced to be reflected a second time from the same tissue, then the proportion of the predominant color will increase significantly compared to the rest and the whitishness will decrease .. Multiple repetition of such a process ( fig. 317) can result in a sufficiently saturated color.

Rice. 317. Obtaining a saturated color when reflected from a red drapery

If the intensity of incident light of any wavelength is denoted by I, and the reflection coefficient for the same wavelength - through r, then after a single reflection we get the intensity I r, after double I r 2 , after three I r 3, etc. From this it can be seen that if r for some narrow spectral region is, for example, 0.7, and for the rest it is 0.1, then after a single reflection the white color impurity is 1/7, i.e. That is, about 15%, after a double reflection 1/49, i.e. about 2%, and after a triple reflection 1/343, i.e. less than 0.3%. Such light can be considered quite saturated.

The described phenomenon explains the saturation of the colors of velvet fabrics, draperies falling in folds or flying banners. In all these cases, there are numerous depressions (velvet) or folds of colored matter. Falling on them, white light undergoes multiple reflections before reaching the observer's eye. In this case, of course, the fabric appears darker than, for example, a smooth stretched strip of colored satin; but the saturation of the color increases tremendously, and the fabric wins in beauty.

In § 167 we mentioned that the surface layer of any paint always scatters white light. This circumstance spoils the saturation of the colors of the picture. Therefore, oil paintings are usually covered with a layer of varnish. Filling all the unevenness of the paint, the varnish creates a smooth mirror surface of the picture. White light from this surface is not scattered in all directions, but reflected in a certain direction. Of course, if you look at the picture from an unsuccessfully chosen position, then such light will be very disturbing ("glow"). But if you look at the picture from other places, then thanks to the lacquer coating, the white light from the surface does not spread in these directions, and the colors of the picture win in saturation.

§ 171. Color of the sky and dawns. The change in the spectral composition of light reflected or scattered by the surface of bodies is associated with the presence of selective absorption and reflection, which is expressed as the dependence of the coefficients a and r on the wavelength.

In nature, another phenomenon plays an important role, leading to a change in the spectral composition of sunlight. The light reaching the observer from areas of the cloudless firmament, far from the Sun, is characterized by a rather saturated blue or even blue tint. There is no doubt that the light of the sky is sunlight scattered in the thickness of the air atmosphere and therefore reaching the observer from all sides, even in directions far from the direction to the Sun. Rice. 318 explains the origin of the scattered light of the sky. Theoretical research and experiments have shown that such scattering occurs due to the molecular structure of air; even completely dust-free air dissipates

Rice. 318. Origin of the color of the sky (the light of the Sun scattered by the atmosphere). Both the direct light of the Sun and the light scattered in the thickness of the atmosphere reach the surface of the Earth (for example, point A). The color of this scattered light is called the color of the sky.

sunlight. The spectrum of light scattered by air differs markedly from the spectrum of direct sunlight: in sunlight, the maximum energy falls on the yellow-green part of the spectrum, and in skylight, the maximum is shifted to the blue part. The reason lies in the fact that short light waves scatter much more than long ones. According to the calculations of the English physicist John Strett Lord Rayleigh (1842-1919), confirmed by measurements, the intensity of scattered light is inversely proportional to the fourth power of the wavelength, if the scattering particles are small compared to the wavelength of light, therefore, violet rays are scattered almost 9 times stronger than red ones. Therefore, the yellowish light of the Sun, when scattered, turns into a blue color of the sky. This is the case for scattering in clean air (in the mountains, over the ocean). The presence of relatively large dust particles in the air (in cities) adds to the scattered blue light the light reflected by dust particles, i.e., almost unchanged light from the Sun. Thanks to this impurity, the color of the sky becomes more whitish under these conditions.

The predominant scattering of short waves leads to the fact that the direct light of the Sun reaching the Earth turns out to be more yellow than when viewed from a great height. On its way through the air, the light of the Sun is partially scattered to the sides, and short waves are more strongly scattered, so that the light that reaches the Earth becomes relatively richer in long-wavelength radiation. This phenomenon is especially pronounced at sunrise and sunset (or moon), when direct light passes through a much greater thickness of air (Fig. 319). Due to this, the Sun and Moon at sunrise (or sunset) have a copper-yellow, sometimes even reddish hue. In those cases

Rice. 319. Explanation of the red color of the Moon and the Sun at sunrise and sunset: S 1 - the luminary at the zenith - a short path in the atmosphere (AB); S 2 - star on the horizon - a long way in the atmosphere (CB)

when there are very small (significantly smaller wavelengths) dust particles or moisture droplets (fog) in the air, the scattering caused by them also follows the law,

Rice. 320. Scattering of light by a turbid liquid: incident light - white, scattered light - bluish, transmitted light - reddish

close to the Rayleigh law, i.e., short waves are predominantly scattered. In these cases, the rising and setting Sun can be completely red. Clouds floating in the atmosphere also turn red. This is the origin of the beautiful pinks and reds of the morning and evening dawns.

You can observe the described color change during scattering if you pass a beam of light from a lantern through a vessel (Fig. 320) filled with a cloudy liquid, that is, a liquid containing small suspended particles (for example, water with a few drops of milk). The light going to the sides (diffused) is noticeably bluer than the direct light of the lantern. If the thickness of the turbid liquid is quite significant, then the light that has passed through the vessel loses such a significant part of the short-wave rays (blue and violet) during scattering that it turns out to be orange and even red. In 1883, there was a strong volcanic eruption on the island of Krakatoa, which half destroyed the island and threw a huge amount of the smallest dust into the atmosphere. For several years, this dust, dispersed by air currents over vast distances, littered the atmosphere, causing intense red dawns.