Who discovered the phenomenon of dispersion. School Encyclopedia

Lesson objectives:

• Educational:
  • introduce the concepts of spectrum, dispersion of light;
  • to acquaint students with the history of the discovery of this phenomenon.
  • visually demonstrate the process of decomposition of a narrow light beam into components of various color shades.
  • identify the differences between these elements of the beam of light.
  • continue the formation of the scientific worldview of students.
• Educational:
  • development of attention, figurative and logical thinking, memory in the study of this topic.
  • stimulation of cognitive motivation of students.
  • development of critical thinking.
• Educational:
  • fostering interest in the subject;
  • fostering a sense of beauty, the beauty of the world.

Lesson type: lesson of studying and primary consolidation of new knowledge.

Teaching methods: conversation, story, explanation, experiment. (Information-developing)

Requirements to basic level preparation: be able to describe and explain the phenomenon of dispersion.

Equipment and materials: computer, color cards, plane-parallel plates

Lesson plan:

Before the start of the lesson, at the break, conduct a diagnosis of “Class Coloring”. Each student, entering the class, chooses a card with a certain color that matches his mood, a “Class color scheme” diagram is drawn up at the beginning of the lesson.

• yellow is good
• Orange - very good
• Red - joyful
• Green - calm
• Blue - sad
• Brown - alarming
• Black is bad
• White - indifferent

Epigraph to the lesson:

Nature cannot be caught sloppy and half-dressed, she is always beautiful.

R. Emerson (American philosopher of the 19th century)

DURING THE CLASSES

1. Motivation

Sunlight has always been and remains for a person a symbol of joy, eternal youth, all the good, the best that can be in life:

“May there always be the Sun.
May there always be heaven...

Such words are in the famous song, the author of the words is Lev Oshanin.
Even a physicist. Accustomed to dealing with facts, with accurate registration of phenomena, sometimes feels embarrassed, saying that light is electromagnetic waves of a certain wavelength and nothing more.
The wavelength of light is very short. Imagine an average sea ​​wave, which would have increased so much that it alone occupied the entire Atlantic Ocean - from America to Lisbon in Europe. The wavelength of light at the same magnification would only slightly exceed the width of a book page.
Question:
Where do these electromagnetic waves come from?
Answer:
- Their source is the Sun.
Together with visible radiation, the Sun sends us thermal radiation, infrared and ultraviolet. The high temperature of the sun is the main reason for the birth of these electromagnetic waves.

2. Organizing moment

Formulation of the topic and objectives of the lesson.

The theme of our lesson is "Dispersion of Light". Today we need:

• Introduce the concept of "spectrum", "dispersion of light";
• To identify the features of this phenomenon - the dispersion of light;
• Get acquainted with the history of the discovery of this phenomenon.

Activation of mental activity:

student reads a poem

Flavor of the Sun

The scent of the sun? What nonsense!
No, not nonsense.
Sounds and dreams in the sun
Fragrances and flowers
All merged into a consonant choir,
All intertwined in one pattern.
The sun smells like herbs
fresh baths,
Awakened spring
And resinous pine
Gently light-colored
Drunk lilies of the valley
that blossomed victoriously
In the sharp smell of the earth.
The sun shines with bells
green leaves,
Breathes the external song of birds,
Breathes the laughter of young faces.
So say to all the blind:
Will you!
Do not see the gates of heaven,
The sun has a fragrance
Sweetly intelligible only to us,
Visible to birds and flowers!
A. Balmont

3. Learning new material

A bit of history

Speaking about these ideas, one should start with Aristotle's theory of colors (4th century BC). Aristotle argued that the difference in color is determined by the difference in the amount of darkness "mixed" into the sunlight (white) light. Violet color, according to Aristotle, occurs with the greatest addition of darkness to light, and red - with the least. Thus, the colors of the rainbow are complex colors, and the main one is white light. Interestingly, the appearance of glass prisms and the first experiments on observing the decomposition of light by prisms did not give rise to doubts about the correctness of Aristotle's theory of the origin of colors. Both Khariot and Martzi remained followers of this theory. This should not be surprising, since at first glance, the decomposition of light by a prism into different colors would seem to confirm the idea that color arises as a result of mixing light and darkness. The rainbow strip appears just at the transition from the shadow strip to the illuminated one, i.e., at the border of darkness and white light. From the fact that the violet ray travels the longest distance inside the prism compared to other colored rays, it is not surprising to conclude that the violet color occurs when white light loses its “whiteness” the most when passing through the prism. In other words, the greatest mixing of darkness into white light takes place on the longest path. It was not difficult to prove the falsity of such conclusions by setting up the corresponding experiments with the same prisms. However, no one had done this before Newton.

Sunlight has many secrets. One of them - dispersion phenomenon. It was first discovered by the great English physicist Isaac Newton in 1666 while improving the telescope.

Light dispersion(light decomposition) is a phenomenon due to the dependence of the absolute refractive index of a substance on the frequency (or wavelength) of light (frequency dispersion), or, the same thing, the dependence of the phase velocity of light in a substance on the wavelength (or frequency).

Experimentally, the dispersion of light was discovered by I. Newton around 1672, although it was theoretically well explained much later.
One of the most good examples dispersion - the decomposition of white light as it passes through a prism (Newton's experiment). The essence of the phenomenon of dispersion is the unequal speed of propagation of light rays with different wavelengths in a transparent substance - an optical medium (whereas in vacuum the speed of light is always the same, regardless of the wavelength and hence the color). Usually, the higher the frequency of the wave, the higher the refractive index of the medium and the lower its speed of light in it:

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

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

White light is also decomposed into a spectrum as a result of passing through a diffraction grating or reflecting 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; the normal (diffraction) spectrum is uniform in all areas and is arranged in ascending order of wavelengths: from violet to red.

Knowing that white light has a complex structure, one can explain the amazing variety of colors in nature. If an object, such as a sheet of paper, reflects all the rays of various colors falling on it, then it will appear white. Covering the paper with a layer of red paint, we do not create light of a new color, but retain some of the existing one on the sheet. Only red rays will now be reflected, while the rest will be absorbed by a layer of paint. Grass and tree leaves appear green to us because of all the sun's rays falling on them, they reflect only green ones, absorbing the rest. If you look at the grass through red glass, which transmits only red rays, it will appear almost black.

The phenomenon of dispersion, discovered by Newton, is the first step towards understanding the nature of color. The depth of understanding of dispersion came after the dependence of color on the frequency (or length) of a light wave was clarified.

Thomas Young (1773-1829) was the first to measure the wavelengths of different colors in 1802.

After the discovery of the dispersion of light, the wavelength became the main quantity that determines the color of light. The main color receiver is the retina.

Color- there is a sensation that occurs in the retina of the eye when it is excited by a light wave of a certain length. Knowing the wavelength of the emitted light and the conditions for its propagation, it is possible to predict in advance with a high degree of accuracy what color the eye will see.

It may be that the retina of the eye does not perceive one of the primary colors well or does not react to it at all, then this person's color perception is disturbed. This lack of vision is called colorblind.

Good color perception is very important for a number of professions: sailors, pilots, railway workers, surgeons, artists. Special devices have been created anomaloscopes for the study of color vision disorders.

Dispersion explains the fact that the rainbow appears after the rain (more precisely, the fact that the rainbow is multi-colored, not white).
First attempt to explain rainbow as a natural phenomenon was made in 1611 by Archbishop Antonio Dominis.

1637 The scientific explanation of the rainbow was first given by Rene Descartes. He explained the rainbow based on the laws of refraction and reflection sunlight in drops of rain. The phenomenon of dispersion had not yet been discovered, so Descartes' rainbow turned out to be white.

After 30 years Isaac Newton supplemented the theory of Descartes, explained how colored rays are refracted in raindrops.

“Descartes hung the rainbow in the right place in the sky, and Newton colored it with all the colors of the spectrum”

American scientist A. Fraser

Rainbow- This is an optical phenomenon associated with the refraction of light rays on numerous raindrops. However, not everyone knows exactly how the refraction of light on raindrops leads to the appearance of a giant multi-colored arc in the sky. Therefore, it is useful to dwell in more detail on the physical explanation of this spectacular optical phenomenon.

Rainbow through the eyes of a careful observer. First of all, a rainbow can only be observed in the direction opposite to the Sun. If you stand facing the rainbow, then the Sun will be behind. A rainbow occurs when the Sun illuminates a curtain of rain. As the rain subsides and then stops, the rainbow fades and gradually disappears. The colors observed in the rainbow alternate in the same sequence as in the spectrum obtained by passing a beam of sunlight through a prism. In this case, the inner (facing the surface of the Earth) extreme region of the rainbow is colored purple, and the outer extreme region is red. Often, another (secondary) rainbow appears above the main rainbow - wider and blurry. The colors in the secondary rainbow alternate in reverse order, from red (the innermost region of the arc) to violet (the outermost region).

For an observer located on a relatively flat earth's surface, a rainbow appears provided that the angular height of the Sun above the horizon does not exceed about 42 °. The lower the Sun, the greater the angular height of the rainbow apex and, consequently, the larger the observed region of the rainbow. A secondary rainbow can be observed if the height of the Sun above the horizon does not exceed about 52.

The rainbow can be thought of as a gigantic wheel, which, like an axle, is put on an imaginary straight line passing through the Sun and the observer.

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

Dispersion of light in nature and art

• Due to dispersion, one can observe different colors Sveta.
• The rainbow, whose colors are caused by dispersion, is one of the key images of culture and art.
• Due to the dispersion of light, one can observe the color "play of light" on the facets of a diamond and other transparent faceted objects or materials.
• To some extent, iridescent effects are found quite often when light passes through almost any transparent object. In art, they can be specially amplified, emphasized.
• The decomposition of light into a spectrum (due to dispersion) during refraction in a prism is a fairly common topic in fine arts. For example, the cover of Pink Floyd's album Dark Side Of The Moon depicts the refraction of light in a prism with decomposition into a spectrum.

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

He explored the difference in light rays and the different properties of colors manifested in this, which no one had previously suspected. ... Let mortals rejoice that such an adornment of the human race existed.

4. Fixing

• Answer questions about the topic.
• Heading "Think..."
• Q: Why is the rainbow round?
• Compilation of "Sinkwine" on the topic "Dispersion"

5. Summing up the lesson

At the end of the lesson, again carry out the diagnostics “Color painting class”. Find out what the mood was at the end of the lesson, on the basis of which the “Class Coloring” diagram is compiled and the result is compared, what mood the students had at the beginning of the lesson and at the end.

6. Homework:§66

Literature:

1. Myakishev G.Ya., Bukhovtsev B.B. Physics: Textbook for grade 11 high school. – M.: Enlightenment, 2006.
2. Rymkevich A.P. Collection of problems in physics for grades 9-11 of high school. – M.: Enlightenment, 2006.
3. Reader in physics: Tutorial for students in grades 8-10 of secondary school / Ed. B.I. Spassky. - M .: Education, 1987.
4. Journal "Physics at School" No. 1/1998

Sometimes when after heavy rain the sun is shining again, you can see the rainbow. This is because the air is saturated with fine water dust. Each drop of water in the air plays the role of a tiny prism, crushing the light into different colors.

About 300 years ago, I. Newton missed Sun rays through the prism. He discovered that white light is a "wonderful mixture of colors."

It is interesting… Why are there only 7 colors in the white light spectrum?

So, for example, Aristotle indicated only three colors of the rainbow: red, green, purple. Newton first identified five colors in the rainbow, and later ten. However, later, he settled on seven colors. The choice is explained, most likely, by the fact that the number seven was considered "magical" (seven wonders of the world, seven weeks, etc.).

The dispersion of light was first observed experimentally by Newton in 1666, when a narrow beam of sunlight was passed through a glass prism. In the spectrum of white light he obtained, he singled out seven colors: From this experience, Newton concluded that "light beams that differ in color differ in the degree of refraction." Violet rays are most strongly refracted, red ones are least refracted.

White light is a complex light consisting of waves of different wavelengths (frequency). Each color has its own wavelength and frequency: red, orange, green, blue, blue, violet - this decomposition of light is called the spectrum.

Waves of different colors are refracted differently in a prism: less red, more violet. A prism deflects waves of different colors to different angles.. Their behavior is explained by the fact that during the transition of light waves from air to a glass prism, the speed of the “red” waves changes less than that of the “violet”. Thus, the shorter the wavelength (the greater the frequency), the greater the refractive index of the medium for such waves.

Dispersion is the dependence of the refractive index of light on the oscillation frequency (or wavelength).

For waves of different chromaticity, the refractive indices given substance different; as a result, when deflected by a prism, white light decomposes into spectrum.

When a monochromatic light wave passes from air to matter, the wavelength of the light decreases, oscillation frequency remains unchanged. The color remains unchanged.

When all the colors of the spectrum are superimposed, white light is formed.

Why do we see objects colored? Paint doesn't create color, it selectively absorbs or reflects light.

Basic summary:

Questions for self-control on the topic "Dispersion of light"

1. What is the dispersion of light?
2. Draw diagrams for obtaining the spectrum of white light using a glass prism.
3. Why does white light pass through a prism giving off a spectrum?
4. Compare the refractive indices for red and violet light.
5. Which light travels faster in a prism, red or violet?
6. How to explain the diversity of colors in nature in terms of wave optics?
7. What color will be visible through the red light filter surrounding objects? Why?

DEFINITION

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

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

A 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 the dispersion of light. The dispersion is a consequence of the frequency dependence of the polarization of atoms.

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

Dispersion arises as a result of oscillations 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 the angles A, are small, then all other angles in formula (2) are small. In this case, the law of refraction can be written not in terms of the sines of these angles, but directly in terms of the angles themselves in radians:

Knowing that , we have:

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

and depends on the value. And we know that the refractive index is a function of the wavelength. It turns out that rays having different wavelengths, after passing through a prism, 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 substance dispersion. It shows how quickly the refractive index changes with wavelength.

The refractive index for transparent substances increases monotonically with decreasing wavelength, which means that the value of D in absolute value increases with decreasing wavelength. This dispersion is called normal. Phenomenon normal dispersion formed the basis for the operation of prism spectrographs, which can be used to study the spectral composition of light.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

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

The source of light 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 placed in front of the hole, instead of a round spot, a colored strip appeared on the wall, called Newton's spectrum. Such a spectrum consists of seven main colors: red, orange, yellow, green, blue, indigo and violet, which gradually passed one into another. Each of them occupies space in the spectrum different size. The purple stripe is the longest and the red stripe the shortest.

The next experiment consisted in the fact that from a wide beam of colored rays obtained with the help of a prism, narrow beams of a certain color stood out by a screen with a small hole and were directed 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 feel less deflection than violet ones, i.e. Rays of different colors are refracted differently by a prism.

Collecting beams of rays that emerged from the prism, Newton received on a white screen instead of a colored stripe a white image of a hole.

From all the experiments carried out, Newton drew the following conclusions:

• white light is inherently complex light, which consists of colored rays;
• rays of light of different colors also have different refractive indices of the substance; as a result, when a beam of white light is deflected by a prism, it decomposes into a spectrum;
• if you combine the colored rays of the spectrum, then again you get white light.

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

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

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

The spectrum obtained in this way is a series of multi-colored images of a round hole, partially superimposed on each other. To obtain a purer spectrum, when studying such a phenomenon as light dispersion, Newton suggested using not a round hole, but a narrow slot parallel to the refracting 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 gives the 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 a substance. This is due to the transformation of the energy of a wave of light into the energy of secondary radiation or, in other words, a substance that has another spectral composition and other areas of distribution.

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

One of the results of the interaction of light with matter is its dispersion.

Dispersion of light is called the dependence of the refractive indexn substances from frequencyν (wavelengthsλ) light or the dependence of the phase velocity of light waves on their frequency.

The dispersion of light is represented as a dependence:

The consequence of dispersion is the decomposition into a spectrum of a beam of white light when it passes through a prism (Fig. 10.1). The first experimental observations of the dispersion of light were made in 1672 by I. Newton. He explained this phenomenon by the difference in masses of corpuscles.

Consider the dispersion of light in a prism. Let a monochromatic beam of light fall on a prism with refractive angle BUT and refractive index n(Fig. 10.2) at an angle.

Rice. 10.1	Rice. 10.2

After double refraction (on the left and right faces of the prism), the beam is refracted from the original direction by an angle φ. From fig. follows that

Let's assume the angles BUT and are small, then the angles , , will also be small, and instead of the sines of these angles, you can use their values. Therefore, , and since , then or .

Hence it follows that

those. the angle of deflection of the rays by the prism is the greater, the greater the refractive angle of the prism.

From expression (10.1.1) it follows that the angle of deflection of the rays by the prism depends on the refractive index n, a n is a function of the wavelength, so rays different lengths waves after passing through the prism are deflected at different angles. A beam of white light behind a prism decomposes into a spectrum called dispersive or prismatic which Newton observed. Thus, with the help of a prism, as well as with the help of a diffraction grating, by decomposing light into a spectrum, one can determine its spectral composition.

Consider differences in the diffraction and prismatic spectra.

· Diffraction grating decomposes light directly by wavelength, therefore, from the measured angles (in the directions of the corresponding maxima), one can calculate the wavelength (frequency). The decomposition of light into a spectrum in a prism occurs according to the values ​​of the refractive index, therefore, to determine the frequency or wavelength of light, one must know the dependence or .

· Composite colors in diffraction and prismatic spectra are located differently. We know that the sine of the angle in a diffraction grating is proportional to the wavelength . Consequently, red rays, which have a longer wavelength than violet, are deflected by the diffraction grating more strongly.. The prism, on the other hand, decomposes the rays of light in the spectrum according to the values ​​​​of the refractive index, which for all transparent substances decreases with increasing wavelength (that is, with decreasing frequency) (Fig. 10.3).

Therefore, red rays are deflected by the prism to a lesser extent than by a diffraction grating.

Value(or )called substance dispersion, shows how quickly the refractive index changes with wavelength.

From fig. 10.3 it follows that the refractive index for transparent substances increases with increasing wavelength, therefore, the modulus value also increases with decreasing λ. This dispersion is called normal . Near the lines and absorption bands, the course of the dispersion curve will be different, namely n decreases with decreasing λ. This course of addiction n from λ is called anomalous dispersion . Let's take a closer look at these types of dispersion.