Dispersion of light conditions of occurrence. Light dispersion

(or wavelength) of light (frequency dispersion), or, the same thing, the dependence of the phase speed of light in matter on the wavelength (or frequency). Experimentally discovered by Newton around 1672, although theoretically quite well explained much later.

Spatial dispersion is the dependence of the dielectric constant tensor of a medium on the wave vector. This dependence causes a number of phenomena called spatial polarization effects.

One of the most illustrative examples dispersion - the decomposition of white light when passing through a prism (Newton's experiment). The essence of the dispersion phenomenon is the unequal speed of propagation of light rays with different wavelengths in a transparent substance - an optical medium (while in a vacuum the speed of light is always the same, regardless of the wavelength and therefore color). Typically, the higher the frequency of the wave, the higher the refractive index of the medium and the lower its speed of light in it:

in red maximum speed in the medium and the minimum degree of refraction,
at purple the minimum speed of light in a medium and the maximum degree of refraction.

However, in some substances (for example, iodine vapor), an anomalous dispersion effect is observed, in which blue rays are refracted less than red ones, while other rays are absorbed by the substance and elude observation. More strictly speaking, anomalous dispersion is widespread, for example, it is observed in almost all gases at frequencies near absorption lines, but in iodine vapor it is quite convenient for observation in the optical range, where they absorb light very strongly.

Light dispersion made it possible for the first time to demonstrate quite convincingly the composite nature of white light.

White light is decomposed into a spectrum as a result of passing through a diffraction grating or reflection from it (this is not related to the phenomenon of dispersion, but is explained by the nature of diffraction). The diffraction and prismatic spectra are somewhat different: the prismatic spectrum is compressed in the red part and stretched in the violet and is arranged in descending order of wavelength: from red to violet; normal (diffraction) spectrum is uniform in all areas and is arranged in order of increasing wavelengths: from violet to red.

By analogy with the dispersion of light, similar phenomena of the dependence of the propagation of waves of any other nature on the wavelength (or frequency) are also called dispersion. For this reason, for example, the term dispersion law, used as the name of a quantitative relationship relating frequency and wave number, applies not only to an electromagnetic wave, but to any wave process.

Dispersion explains the fact that a rainbow appears after rain (more precisely, the fact that the rainbow is multi-colored and not white).

Dispersion is the cause of chromatic aberrations - one of the aberrations of optical systems, including photographic and video lenses.

Cauchy came up with a formula expressing the dependence of the refractive index of a medium on the wavelength:

…,

Dispersion of light in nature and art

Due to dispersion it can be observed different colors.

Rainbow, whose colors are due to dispersion, is one of the key images of culture and art.
Thanks to light dispersion, it is possible to observe the colored “play of light” on the facets of a diamond and other transparent faceted objects or materials.
To one degree or another, rainbow effects are found quite often when light passes through almost any transparent objects. In art they can be specifically intensified and emphasized.
The decomposition of light into a spectrum (due to dispersion) when refracted in a prism is a fairly common topic in fine arts. For example, the cover of the album Dark Side Of The Moon by Pink Floyd depicts the refraction of light in a prism with decomposition into a spectrum.

Literature

Yashtold-Govorko V. A. Photography and processing. Photography, formulas, terms, recipes. - Ed. 4th, abbr. - M.: Art, 1977.

Links

Wikimedia Foundation. 2010.

See what “Light dispersion” is in other dictionaries:

The dependence of the refractive index n in VA on the frequency n (wavelength l) of light or the dependence of the phase speed of light waves on their frequency. Consequence D. s. decomposition into a spectrum of a beam of white light when passing through a prism (see SPECTRA... ... Physical encyclopedia

light dispersion- Phenomena caused by the dependence of the speed of light propagation on the frequency of light vibrations. [Collection of recommended terms. Issue 79. Physical optics. Academy of Sciences of the USSR. Committee of Scientific and Technical Terminology. 1970] Topics… … Technical Translator's Guide

light dispersion- šviesos skaida statusas T sritis radioelektronika atitikmenys: engl. dispersion of light vok. Lichtdispersion, f; Zerteilung des Lichtes, f rus. light dispersion, f pranc. dispersion de la lumière, f… Radioelektronikos terminų žodynas

light dispersion- šviesos dispersija statusas T sritis fizika atitikmenys: engl. dispersion of light vok. Lichtdispersion, f; Zerlegung des Lichtes, f rus. light dispersion, f pranc. dispersion de la lumière, f … Fizikos terminų žodynas

The dependence of the refractive index n of a substance on the frequency ν (wavelength λ) of light or the dependence of the phase speed (See Phase speed) of light waves on frequency. Consequence D. s. decomposition into spectrum of a beam of white light when passing... ... Great Soviet Encyclopedia

Dependence of the refractive index n in va on the frequency of light v. In the region frequencies of light, for which v is transparent, n increases with increasing v normal d.s. In the region frequencies corresponding to the bands of intense absorption of light in the field, n decreases with... ... Big Encyclopedic Polytechnic Dictionary

Dependence of the absolute refractive index of a substance on the wavelength of light... Astronomical Dictionary

What would you like to improve this article?: Add illustrations. Find and arrange in the form of footnotes links to authoritative sources that confirm what is written. Put down a template card that is... Wikipedia

Dependence of the phase velocity of harmonic waves in a medium on the frequency of their oscillations. wave dispersion is observed for waves of any nature. The presence of wave dispersion leads to distortion of the signal shape (for example, an audio pulse) when propagating in a medium... Big Encyclopedic Dictionary

A beam of light passing through a triangular prism is deflected towards the face opposite the refracting angle of the prism. However, if it is a beam of white light, then after it passes through the prism, it will not only be deflected, but will also decompose into colored beams. This phenomenon is called light dispersion. It was first studied in a series of remarkable experiments.

The light source in Newton's experiments was a small round hole located in the shutter of a window illuminated by the rays of the Sun. When a prism was installed in front of the hole, instead of a round spot, a colored strip appeared on the wall, which Newton called the spectrum. Such a spectrum consists of seven main colors: red, orange, yellow, green, blue, indigo and violet, which gradually turned into one another. Each of them occupies a different size space in the spectrum. The purple stripe has the greatest length, the red stripe the shortest.

The next experiment consisted of using a screen with a small hole to separate out narrow beams of a certain color from a wide beam of colored rays obtained using a prism and direct them to a second prism.

The prism, deflecting them, does not change the color of these rays. Such rays are called simple or monochromatic (one-color).

Experience shows that red rays experience less deflection than violet rays, i.e. Rays of different colors are refracted differently by a prism.

By collecting beams of rays emerging from the prism, Newton received a white image of a hole on a white screen instead of a colored stripe.

From all his experiments, Newton drew the following conclusions:

white light by its nature is a complex light that consists of colored rays;
For rays of light of different colors, the refractive indices of the substance are also different; as a result of this, when a beam of white light is deflected by a prism, it is decomposed into a spectrum;
If you combine the colored rays of the spectrum, you will again get white light.

Thus, light dispersion is a phenomenon that is due to the dependence of the substance on the wavelength (or frequency).

Light dispersion is noted not only when light passes through a prism, but also in various other cases of refraction of light. Thus, in particular, the refraction of sunlight in water droplets is accompanied by its decomposition into multi-colored rays, which explains the formation of a rainbow.

To obtain a spectrum, Newton directed a fairly wide cylindrical beam of sunlight onto a prism through a round hole made in a shutter.

The spectrum obtained in this way is a series of multi-colored images of a round hole, partially overlapping each other. To obtain a cleaner spectrum, when studying such a phenomenon as the dispersion of light, Newton proposed using not a round hole, but a narrow slit parallel to the refractive edge of the prism. Using a lens, a clear image of the slit is obtained on the screen, after which a prism is installed behind the lens, which produces a spectrum.

The purest and brightest spectra are obtained using special instruments - spectroscopes and spectrographs.

Light absorption is a phenomenon in which the energy of a light wave decreases as it passes through matter. This occurs due to the conversion of the energy of a light wave into the energy of secondary radiation or, in other words, a substance that has a different spectral composition and other directions of propagation.

The absorption of light can cause heating of a substance, ionization or excitation of molecules or atoms, photochemical reactions, as well as other processes in the substance.

DEFINITION

Light dispersion call the dependence of the refractive index of a substance (n) on the frequency () or wavelength () of light in a vacuum (often the index 0 is omitted):

Sometimes dispersion is defined as the dependence of the phase velocity (v) of light waves on frequency.

The well-known consequence of dispersion is the decomposition of white light into a spectrum when passing through a prism. I. Newton was the first to record his observations of light dispersion. Dispersion is a consequence of the dependence of the polarization of atoms on frequency.

Graphic dependence of the refractive index on frequency (or wavelength) - dispersion curve.

Dispersion occurs as a result of vibrations of electrons and ions.

Dispersion of light in a prism

If a monochromatic beam of light hits a prism, the refractive index of which is equal to n, at an angle (Fig. 1), then after double refraction the beam deviates from the original direction by an angle:

If angles A, are small, therefore all other angles in formula (2) are small. In this case, the law of refraction can be written not through the sines of these angles, but directly through the values of the angles themselves in radians:

Knowing that , we have:

Consequently, the angle of deflection of rays using a prism is directly proportional to the value of the refractive angle of the prism:

and depends on the size. And we know that the refractive index is a function of wavelength. It turns out that the rays having different lengths After passing through the prism, the waves are deflected at different angles. It becomes clear why a beam of white light will decompose into a spectrum.

Dispersion of a substance

Value (D) equal to:

called dispersion of matter. It shows the rate of change in the refractive index depending on the wavelength.

The refractive index for transparent substances increases monotonically with decreasing wavelength, which means that the magnitude of D increases with decreasing wavelength. This dispersion is called normal. The phenomenon of normal dispersion is the basis for the operation of prism spectrographs, which can be used to study spectral composition Sveta.

Examples of problem solving

EXAMPLE 1

Exercise

What are the main differences in the diffraction and prismatic spectra?

Solution

A diffraction grating sorts light into wavelengths. From the obtained and measured angles to the directions of the corresponding maxima, the wavelength can be calculated. Unlike a diffraction grating, a prism sorts light according to refractive index values, therefore, to find the wavelength of light it is necessary to have a dependence.

In addition to the above, the colors in the spectrum obtained as a result of diffraction and the prismatic spectrum are located differently. For a diffraction grating, it was found that the sine of the deflection angle is proportional to the wavelength. This means that the diffraction grating rejects red rays more than violet rays. The prism separates the rays according to the refractive index, and for all transparent substances it monotonically decreases with increasing wavelength. It turns out that red rays, which have a lower refractive index, will be deflected by the prism less than violet rays (Fig. 2).

EXAMPLE 2

Exercise

What will be the angle of deflection () of the beam by a glass prism if it falls normally on its face? The refractive index of the prism substance is n=1.5. The refractive angle of the prism is thirty degrees ().

Solution

When solving the problem, you can use Fig. 1 in the theoretical part of the article. It should be noted that. From Fig. 1 it follows that

According to the law of refraction we write:

Since , we get that . From formula (2.1) we obtain that:

Every hunter wants to know where the pheasant is sitting. As we remember, this phrase means the sequence of colors of the spectrum: red, orange, yellow, green, blue, indigo and violet. Who showed that white this is the totality of all colors, what does a rainbow, beautiful sunsets and sunrises, shine have to do with this precious stones? All these questions are answered by our lesson, the topic of which is: “Dispersion of Light.”

Until the second half of the 17th century, it was not completely clear what color was. Some scientists said that this is a property of the body itself, some stated that these are different combinations of light and dark, thereby confusing the concepts of color and illumination. Such color chaos reigned until Isaac Newton conducted an experiment on transmitting light through a prism (Fig. 1).

Rice. 1. Path of rays in a prism ()

Let us remember that a ray passing through a prism undergoes refraction when passing from air to glass and then another refraction - from glass to air. The trajectory of the ray is described by the law of refraction, and the degree of deviation is characterized by the refractive index. Formulas describing these phenomena:

Rice. 2. Newton's experiment ()

In a dark room, a narrow beam of sunlight penetrates through the shutters; Newton placed a glass triangular prism in its path. A beam of light passing through a prism was refracted in it, and a multi-colored strip appeared on the screen behind the prism, which Newton called a spectrum (from the Latin “spectrum” - “vision”). White color turned into all colors at once (Fig. 2). What conclusions did Newton make?

1. Light has a complex structure (speaking modern language- white light contains electromagnetic waves different frequencies).

2. Light of different colors differs in the degree of refraction (characterized by different indicators refraction in a given medium).

3. The speed of light depends on the medium.

Newton outlined these conclusions in his famous treatise “Optics”. What is the reason for this decomposition of light into a spectrum?

As Newton's experiment showed, red was the weakest refracted color, and violet was the most refracted. Recall that the degree of refraction of light rays is characterized by the refractive index n. Red color differs from violet in frequency; red has a lower frequency than violet. Since the refractive index increases as we move from the red end of the spectrum to the violet end, we can conclude that the refractive index of glass increases as the frequency of light increases. This is the essence of the phenomenon of dispersion.

Let's remember how the refractive index is related to the speed of light:

n ~ ν; V ~ => ν =

n - refractive index

C - speed of light in vacuum

V - speed of light in the medium

ν - frequency of light

This means that the higher the frequency of light, the lower the speed of light propagating in glass, thus highest speed inside the glass prism is red, and lowest speed- violet.

The difference in the speed of light for different colors carried out only in the presence of a medium; naturally, in a vacuum, any ray of light of any color propagates at the same speed m/s. Thus, we found out that the reason for the decomposition of white color into a spectrum is the phenomenon of dispersion.

Dispersion- dependence of the speed of light propagation in a medium on its frequency.

The phenomenon of dispersion, discovered and studied by Newton, awaited its explanation for more than 200 years; only in the 19th century, the Dutch scientist Lawrence proposed the classical theory of dispersion.

The reason for this phenomenon is the interaction of external electromagnetic radiation, that is, light with the medium: the higher the frequency of this radiation, the stronger the interaction, which means the more the beam will deviate.

The dispersion that we talked about is called normal, that is, the frequency indicator increases if the frequency of electromagnetic radiation increases.

In some rare media, anomalous dispersion is possible, that is, the refractive index of the medium increases as the frequency decreases.

We saw that each color corresponds to a specific wavelength and frequency. Wave corresponding to the same color in different environments has the same frequency but different wavelengths. Most often, when talking about the wavelength corresponding to a certain color, they mean the wavelength in vacuum or air. The light corresponding to each color is monochromatic. “Mono” means one, “chromos” means color.

Rice. 3. Arrangement of colors in the spectrum according to wavelengths in the air ()

The longest wavelength is red (wavelength - from 620 to 760 nm), the shortest wavelength is violet (from 380 to 450 nm) and the corresponding frequencies (Fig. 3). As you can see, there is no white color in the table, white color is the sum of all colors, this color does not correspond to any strictly defined wavelength.

What explains the colors of the bodies that surround us? They are explained by the body’s ability to reflect, that is, scatter radiation incident on it. For example, a white color, which is the sum of all colors, falls on some body, but this body best reflects red color, and absorbs other colors, then it will seem exactly red to us. The body that best reflects blue will appear blue and so on. If the body reflects all colors, it will end up appearing white.

It is the dispersion of light, that is, the dependence of the refractive index on the wave frequency, that explains the beautiful phenomenon of nature - the rainbow (Fig. 4).

Rice. 4. The phenomenon of the rainbow ()

Rainbows occur when sunlight is refracted and reflected by droplets of water, rain, or fog floating in the atmosphere. These droplets deflect light of different colors in different ways, as a result, white color is decomposed into a spectrum, that is, dispersion occurs; an observer who stands with his back to the light source sees a multi-colored glow that emanates from space along concentric arcs.

Dispersion also explains the remarkable play of color on the facets of precious stones.

1. The phenomenon of dispersion is the decomposition of light into a spectrum, due to the dependence of the refractive index on the frequency of electromagnetic radiation, that is, the frequency of light. 2. Body color is determined by the body’s ability to reflect or scatter a particular frequency of electromagnetic radiation.

References

Tikhomirova S.A., Yavorsky B.M. Physics ( basic level) - M.: Mnemosyne, 2012.
Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M.: Mnemosyne, 2014.
Kikoin I.K., Kikoin A.K. Physics - 9, Moscow, Education, 1990.

Homework

What conclusions did Newton draw after his experiment with a prism?
Define dispersion.
What determines body color?

Internet portal B -i-o-n.ru ().
Internet portal Sfiz.ru ().
Internet portal Femto.com.ua ().

Light dispersion

Each of us has ever seen how rays shimmer on cut glass products or, for example, on diamonds. This can be observed due to a phenomenon called light dispersion. This is an effect that reflects the dependence of the refractive index of an object (substance, medium) on the length (frequency) of the light wave that passes through this object. The consequence of this dependence is the decomposition of the beam into a color spectrum, for example, when passing through a prism.

Light dispersion is expressed by the following equality:

where n is the refractive index, ƛ is the frequency, and ƒ is the wavelength. The refractive index increases with increasing frequency and decreasing wavelength. We often observe dispersion in nature.

Its most beautiful manifestation is the rainbow, which is formed due to dispersion sun rays when passing through numerous raindrops.

History of discovery and research.

In 1665-1667, a plague epidemic raged in England, and young Isaac Newton decided to take refuge from it in his native Woolsthorpe. Before leaving for the village, he purchased glass prisms in order to “perform experiments with the famous phenomena of flowers.”

Already in the 1st century new era It was known that when passing through a transparent single crystal with the shape of a hexagonal prism, sunlight is decomposed into a colored stripe - a spectrum. Even earlier, in the 4th century BC, the ancient Greek scientist Aristotle put forward his theory of colors. He believed that the main thing is sunlight (white) light, and all other colors are obtained from it by adding to it various quantities dark light. This idea of light dominated science until the 17th century, despite the fact that numerous experiments were carried out on the decomposition of sunlight using glass prisms.

While exploring the nature of flowers, Newton came up with and performed a whole range of different optical experiments. Some of them, without significant changes in the methodology, are still used in physics laboratories.

The first experiment on dispersion was traditional. Having made a small hole in the shutter of the window of a darkened room, Newton placed a glass prism in the path of the beam of rays passing through this hole. On the opposite wall he received an image in the form of a strip of alternating colors. Newton divided the spectrum of sunlight obtained in this way into seven colors of the rainbow - red, orange, yellow, green, blue, indigo, violet.

The establishment of exactly seven primary colors of the spectrum is to a certain extent arbitrary: Newton sought to draw an analogy between the spectrum of sunlight and musical sound. If we consider the spectrum without such a prejudice, then the spectrum band arising due to dispersion breaks up into three main parts - red, yellow-green and blue-violet. The remaining colors occupy relatively narrow areas between these basic ones. In general, the human eye is capable of distinguishing up to 160 different color shades in the spectrum of sunlight.

In subsequent dispersion experiments, Newton succeeded in combining colored rays into white light.

As a result of his research, Newton, in contrast to Aristotle, came to the conclusion that when “white and black are mixed, no color arises...”. All the colors of the spectrum are contained in the sunlight, and a glass prism only separates them, since different colors are refracted differently by glass. Violet rays are refracted most strongly, red rays refract weakest.

Subsequently, scientists established the fact that, considering light as a wave, each color should be associated with its own wavelength. It is very important that these wavelengths change in a continuous manner, corresponding to the different shades of each color.

The change in the refractive index of a medium depending on the length of the wave propagating in it is called dispersion (from the Latin verb “to scatter”). The refractive index of ordinary glass is close to 1.5 for all wavelengths of visible light.

The experiments of Newton and other scientists showed that as the wavelength of light increases, the refractive index of the substances under study monotonically decreases. However, in 1860, while measuring the refractive index of iodine vapor, the French physicist Leroux discovered that red rays are refracted by this substance more strongly than blue ones. He called this phenomenon anomalous dispersion of light. Subsequently, anomalous dispersion was discovered in many other substances.

In modern physics, both normal and anomalous dispersion of light are explained in the same way. The difference between normal and anomalous dispersion is as follows. Normal dispersion occurs with light rays whose wavelength is far from the region where the waves are absorbed by the substance. Anomalous dispersion is observed only in the absorption region.

If you look closely at the dispersion of light, you can discover its connection with the penetrating ability of electromagnetic radiation. Indeed, the shorter the wavelength of electromagnetic radiation, the greater the chance of radiation penetrating through matter, in the space between atoms. That is why X-ray and gamma radiation have a very high penetrating power.

Dispersion of light in nature and art

Due to dispersion, different colors of light can be observed.

The rainbow, whose colors are due to dispersion, is one of the key images of culture and art.

Thanks to light dispersion, it is possible to observe the colored “play of light” on the facets of a diamond and other transparent faceted objects or materials.

To one degree or another, rainbow effects are found quite often when light passes through almost any transparent objects. In art they can be specifically intensified and emphasized.

The decomposition of light into a spectrum (due to dispersion) when refracted in a prism is a fairly common topic in the visual arts. For example, the cover of the album Dark Side Of The Moon by Pink Floyd depicts the refraction of light in a prism with decomposition into a spectrum.

The discovery of dispersion was very significant in the history of science. On the scientist’s tombstone there is an inscription with the following words: “ Here lies Sir Isaac Newton, the nobleman who... was the first to explain, with the torch of mathematics, the movements of the planets, the paths of comets, and the tides of the oceans.

He investigated the difference in light rays and the various properties of colors that manifest themselves, which no one had previously suspected. …Let mortals rejoice that such an adornment of the human race existed.”