Light dispersion experience

Experiment scenario

"Decomposition of white light into a spectrum"

The purpose of the experiment: to form in students a single, whole idea of ​​the physical nature of the phenomenon of light dispersion, to consider the conditions for the occurrence of a rainbow.

Tasks:

• using the methods of scientific knowledge, explain the nature of the dispersion spectrum, apply the knowledge gained to explain atmospheric optical phenomena;
• to form research skills: to obtain the phenomenon of dispersion, to establish causal relationships between facts, to put forward hypotheses, to justify them and to check their reliability;
• to form empathic qualities of students through heuristic methods of work, to realize the needs of a teenager in communication, to promote the development of the qualities of cooperation, motivation in the study of physics;

Equipping experience:

• Equipment: demonstration equipment for wave optics, instrument for demonstrating rainbows in laboratory conditions.
• Demonstration experiments and practical observations: experiment on light dispersion with prisms, practical work "Observation of light dispersion", indecomposability into the spectrum of monochromatic light, addition of spectral colors.

Practical purpose of the experiment: contributes to the development of skills in working with equipment - to obtain and study the dispersion spectrum, contributes to the formation of a holistic picture of the world, to improve the skills of expressing one's own opinion, public speaking, working with an audience, applying the obtained theoretical knowledge when explaining natural phenomena.
Experience is an integral part of the work on self-improvement of the student's competencies, because students in their subject "Portfolio" will mark their successes and achievements, will be able to analyze their activities at an open event.

Conceptual apparatus: refraction, speed of light, dispersion, spectrum, order of colors in the spectrum, monochromatic wave.

Experiment

Position the prism so that a ray of light falls on one of its faces. To achieve a directed beam of light from an incandescent lamp, a screen with a narrow slit is installed between the prism and the lamp. As a result of the passage of the ray through the prism, it experiences a number of refractions, because passes through media with different optical density. And at the exit from the prism, the beam is decomposed into a spectrum, which we track on a screen installed behind the prism. For the convenience of the experiment, the laboratory should be dark.

If on the path of the beam between the prism and the narrow slit we place a light filter, for example, a red filter, then we will not see the decomposition of the red light, because light monochrome

Cognitive motivation

- How can you explain the amazing variety of colors in nature? I want to invite you to listen to a poem by F.I. Tyutchev:

How unexpected and bright
On the damp blue sky
Aerial arch erected
In your momentary celebration!
I stuck one end into the woods,
She hugged half the sky
And I was exhausted in height.

- What phenomenon is described in these poetic lines? (Rainbow)

- Until 1666 it was believed that color is a property of the body itself. Since ancient times, rainbow color separation has been observed, and it has been known that the formation of a rainbow is associated with the illumination of raindrops. There is a belief: whoever passes under the rainbow will remain happy for life. Is it a fairy tale or a reality? Can you walk under a rainbow and be HAPPY? One amazing physical phenomenon will help to understand this, thanks to which you can see our world around us in color. Why can we see beautiful flowers, amazing colors of paintings by artists: Why does the world give us a whole gamut of landscapes of different beauty and originality? This phenomenon is dispersion. Let's try to formulate the name of the experience. (Students suggest different variations of names)

Target: study the variance and find out the reasons for the appearance of the rainbow.

Tasks:

• find out what variance is;
• dispersion discovery history;
• explain the reasons for the appearance of variance;
• conduct an experiment to obtain a dispersion;
• consider a natural phenomenon - a rainbow.

Hypothesis: if you know the phenomenon of dispersion, then you can explain natural phenomena and get a rainbow in laboratory conditions. Any research involves the choice of an object and subject of research

Object of study: light waves, dispersion

Subject of study: Rainbow

Dispersion sounds great,
The phenomenon itself is beautiful,
It is close and familiar to us from childhood,
We have watched it hundreds of times!

I. Newton's experiments on dispersion

The phenomenon of dispersion was discovered by I. Newton and is considered one of his most important achievements. "He investigated the difference between light rays and the resulting different properties of colors that no one had previously suspected." About 300 years ago, Isaac Newton sent the sun's rays through a prism. It is not for nothing that on his gravestone monument, erected in 1731 and decorated with figures of young men who hold the emblems of his most important discoveries, one figure holds a prism, and the inscription on the monument contains the words: “He investigated the difference between light rays and the various properties manifested at the same time, which no one had previously suspected. " He discovered that white light is "a wonderful mixture of colors."
So what did Newton do? Let's repeat Newton's experiment.
If you look closely at the passage of light through a triangular prism, you can see that the decomposition of white light begins as soon as the light passes from air to glass. In the experiments described, a glass prism was used. Instead of glass, you can take other materials transparent to light. It is remarkable that this experience survived for centuries, and its methodology is still used without significant changes.

Shows a continuous spectrum of white light

Before we understand the essence of this phenomenon, let's remember the refraction of light waves.

- What is the peculiarity of passing a light beam through a prism?
1 Newton's conclusion: light has a complex structure, i.e. white light contains electromagnetic waves of different frequencies.
2 Newton's conclusion: light of different colors differs in the degree of refraction, i.e. characterized by different refractive indices in a given environment.

The violet rays are most strongly refracted, the red ones least of all.
The set of color images of the slit on the screen is a continuous spectrum... Isaac Newton conditionally identified seven primary colors in the spectrum:
The order of the colors is easy to remember by the abbreviation of the words: every hunter wants to know where the pheasant is sitting... There is no sharp border between colors.
Different colors correspond to different wavelengths. There is no specific wavelength for white light. Nevertheless, the boundaries of the ranges of white light and its constituent colors are usually characterized by their wavelengths in a vacuum. Thus, white light is a complex light, a collection of wavelengths from 380 to 760 nm.

Conclusions from the experiments:

• The speed of light depends on the environment.
• The prism decomposes the light.
• White light is a complex light made up of light waves of various colors.

Output: when light passes through a substance having a refractive angle, light is decomposed into colors.

Output: In matter, the speed of propagation of short-wavelength radiation is less than that of long-wavelength. This means that the refractive index for violet light is greater than for red.
The dispersion mechanism is explained as follows. An electromagnetic wave excites forced vibrations of electrons in atoms and molecules in a substance. Since dispersion occurs due to the interaction of particles of a substance with a light wave, this phenomenon is associated with the absorption of light - the conversion of the energy of an electromagnetic wave into the internal energy of a substance.
Separation of colors in a beam of white light occurs due to the fact that waves of different wavelengths are refracted or scattered by matter in different ways. Rainbow - separation of light when refracted by water droplets.
The maximum energy absorption occurs at resonance, when the frequency v incident light is v vibrations of atoms. Once again, we draw the students' attention to the fact that when a wave passes from one medium to another, both the speed and the wavelength change, but the frequency of the oscillations remains unchanged.

Game "Finish the sentence"

• The prism does not change the light, but only ... (decomposes)
• White light as an electromagnetic wave consists of ... (seven colors)
• Refracts most strongly ... (violet light)
• Less refraction ... (red light)

Issues for discussion:

• How can the phenomenon of light dispersion be observed?
• What explains the decomposition of white into colored beams?
• A ray of red light is directed onto a glass prism. Will this light decompose into any colored rays?
• Is light dispersion observed when passing through a vacuum?
• Will dispersion be observed if light passes from one medium to another, both media have the same refractive indices?

Let's continue the study of light phenomena using the rainbow as an example.

The rainbow is "created" by water drops: in the sky - rains, on the poured asphalt - droplets, splashes from a water jet. However, not everyone knows exactly how the refraction of light on raindrops leads to the appearance of a giant multicolored arc in the sky. The bright rainbow that occurs after rains or in the spray of a waterfall is the primary rainbow. The colored stripes differ greatly in brightness, but the order is always the same: there is always a purple stripe inside the arc, which turns into blue, green, yellow, orange and red - on the outside of the rainbow. Above the first, in the sky, a second, less bright arc appears, in which the color stripes are arranged in reverse order.

In 1704, the famous work of Isaac Newton (1642-1727) "Optics" was published, in which an experimental method for studying color vision was first described. It is called the additive color mixing method, and the results obtained by this method laid the foundation for the experimental science of color.

Thus, Newton showed that colors are not formed by a prism, but ...! And here it is necessary to stop for a minute, because until now there have been physical experiments with light and only here the experiments on mixing colors begin. So, seven colored rays mixed together give a white ray, which means that it was the composition of the light that caused the color to appear, but where do they go after mixing? Why, no matter how you look at the white light, there is no hint of the colored rays that make up it?

It was this phenomenon, which would make it possible to formulate one of the laws of color mixing, that led Newton to develop a method for mixing colors. Referring again to Fig. 1.1. Instead of solid screen 1, we put another screen 1, in which holes are cut out so that only part of the rays (two, three or four of the seven) pass through, and the rest are obscured by opaque partitions. And this is where miracles begin. On screen 2, colors appear from nowhere and in an unknown manner. For example, we blocked the path of the violet, cyan, blue, yellow and orange rays and let the green and red rays pass. However, after passing through the lens and reaching screen 2, these rays disappeared, but yellow appeared instead. If we look at screen 1, we are convinced that the yellow ray is delayed by this screen and cannot reach screen 2, but nevertheless the same yellow color is displayed on screen 2. Where did he come from?

The same miracles happen if you stop all the rays except the blue and orange. Again, the original rays will disappear, and white light will appear, the same as if it consisted not of two rays, but of seven. But the most surprising phenomenon arises when only the extreme rays of the spectrum - violet and red - are skipped. A completely new color appears on screen 2, which was not among the original seven colors, nor among their other combinations - magenta.

These startling phenomena made Newton take a close look at the rays of the spectrum and their various mixtures. If we look at the spectral series, we will see that the individual components of the spectrum are not separated from each other by a sharp border, but gradually merge into each other so that the neighboring rays in the spectrum seem more similar to each other than the distant ones. And here Newton discovered another phenomenon.

It turns out that for the extreme violet ray of the spectrum, the closest in color are not only blue, but also non-spectral magenta. And this same magenta, together with orange, makes up a pair of neighboring colors for the extreme red ray of the spectrum. That is, if you arrange the colors of the spectrum and the mixture in accordance with their perceived similarity, then they form not a line, like a spectrum, but a vicious circle (Fig. 1.2), so that the most different in position in the radiation spectrum, that is, the most physically different rays will be very similar in color.

“When I speak of light and rays as colored or evocative colors, it should be understood that I am not speaking in a philosophical sense, but as ordinary people say about these concepts. In essence, the rays are not colored; they have nothing but a certain ability and disposition to evoke the sensation of one color or another. Just like sound ... in any sounding body there is nothing more than movement, which is perceived by the senses in the form of sound, so the color of an object is nothing more than a predisposition to reflect this or that kind of rays to a greater extent than others. , the color of the rays is their predisposition in one way or another to influence the senses, and their sensation takes the form of flowers ”(Newton, 1704).

Considering the relationship between light rays of different physical composition and the color sensations they cause, Newton was the first to understand that color is an attribute of perception, for which an observer is needed who can perceive the rays of light and interpret them as colors. Light itself is no more colored than radio waves or X-rays.

Thus, Newton was the first to experimentally prove that color is a property of our perception, and its nature is in the device of the senses, capable of interpreting the effect of electromagnetic radiation in a certain way.

Now we know that Newton was mistaken in assuming a resonant mechanism for generating color (unlike hearing, where the first stage of the transformation of mechanical vibrations into sound is carried out precisely by the resonant mechanism, color vision is arranged fundamentally differently), but for us something else is more important, that Newton first identified a specific triad: physical radiation - physiological mechanism - mental phenomenon, in which color is determined by the interaction of physiological and psychological levels. Therefore, we can call Newton's point of view the idea of ​​the psychophysiological nature of color.

In 1704, the famous work of Isaac Newton (1642-1727) "Optics" was published, in which an experimental method for studying color vision was first described. It is called the additive color mixing method, and the results obtained by this method laid the foundation for the experimental science of color.

Newton's experiments are described in many manuals, so we will consider them only in connection with the question of the nature of color. Rice. 1.1 is a diagram of Newton's setup and illustrates the essence of the experiments.

If you take a thick sheet of white cardboard as screen 1, then after the passage of the sun's ray through the prism, the screen will reflect the usual linear color spectrum. To test the hypothesis where colored rays arise - in light or a prism - Newton removed screen 1 and passed the spectral rays onto the lens, which again collected them into a beam on screen 2, and this beam was as colorless as the original light.

Thus, Newton showed that colors are not formed by a prism, but ...! And here it is necessary to stop for a minute, because until now there have been physical experiments with light and only here the experiments on mixing colors begin. So, seven colored rays mixed together give a white ray, which means that it was the composition of the light that caused the color to appear, but where do they go after mixing? Why, no matter how you look at the white light, there is no hint of the colored rays that make up it? It was this phenomenon, which would make it possible to formulate one of the laws of color mixing, that led Newton to develop a method for mixing colors. Referring again to Fig. 1.1. Instead of a solid screen 1, we put another screen 1, in which holes are cut so that only part of the rays (two, three or four of the seven) pass through, and the rest are obstructed

opaque partitions. And this is where miracles begin. On screen 2, colors appear from nowhere and in an unknown manner. For example, we blocked the path of the violet, cyan, blue, yellow and orange rays and let the green and red rays pass. However, after passing through the lens and reaching screen 2, these rays disappeared, but yellow appeared instead. If we look at screen 1, we are convinced that the yellow ray is delayed by this screen and cannot reach screen 2, but nevertheless the same yellow color is displayed on screen 2.

Rice. 1.1. Scheme of Newton's setup for additive color mixing. The various types of screens used in the experiments are shown at the top. Spectral color range projected on screen A1 is shown on the first side of the binding of the book

Where did he come from? The same miracles happen if you stop all the rays except the blue and orange. Again, the original rays will disappear, and white light will appear, the same as if it consisted not of two rays, but of seven. But the most surprising phenomenon arises when only the extreme rays of the spectrum - violet and red - are skipped. A completely new color appears on screen 2, which was not among the original seven colors, nor among their other combinations - magenta.

These amazing phenomena made Newton carefully examine the rays of the spectrum and their various mixtures. If we look closely into the spectral series, we will see that the individual components of the spectrum are not separated from each other by a sharp border, but gradually pass into each other so that the neighboring components in the spectrum

the rays seem to be more similar to each other than the distant ones. And here Newton discovered another phenomenon. It turns out that for the extreme violet ray of the spectrum, the closest in color are not only blue, but also non-spectral magenta. And this same magenta, together with orange, makes up a pair of neighboring colors for the extreme red ray of the spectrum. That is, if you arrange the colors of the spectrum and the mixture in accordance with their perceived similarity, then they form not a line, like a spectrum, but a vicious circle (Fig. 1.2), so that the most different in position in the radiation spectrum, that is, the most physically different rays will be very similar in color.

Rice. 1.2. Newton's color wheel. In contrast to the linear physical scale, the closed shape of the circle reflects the subjective similarity of the colors of the spectrum. This meant that the physical structure of the spectrum and the color structure of sensations are completely different phenomena. And this was the main conclusion that Newton drew from his experiments in Optics:

“When I speak of light and rays as colored or evocative colors, it should be understood that I am not speaking in a philosophical sense, but as ordinary people say about these concepts. In essence, the rays are not colored; they have nothing but a certain ability and disposition to evoke the sensation of one color or another. Just like sound ... in any sounding body there is nothing more than movement, which is perceived by the senses in the form of sound, so the color of an object is nothing more than a predisposition to reflect this or that kind of rays to a greater extent than others. , the color of the rays is their predisposition in one way or another to influence the senses, and their sensation takes the form of flowers ”(Newton, 1704).

Considering the relationship between light rays of different physical composition and the color sensations they cause, Newton was the first to understand that color is an attribute of perception, for which an observer is needed who can perceive the rays of light and interpret them as colors. Light itself is no more colored than radio waves or X-rays.

Thus, Newton was the first to experimentally prove that color is a property of our perception, and its nature is in the device of the senses, capable of interpreting the effect of electromagnetic radiation in a certain way. Since Newton was a supporter of the corpuscular theory of light, he assumed that the transformation of electromagnetic radiation into

color is carried out by vibration of nerve fibers, so that a certain combination of vibrations of different fibers causes a certain sensation of color in the brain. Now we know that Newton was mistaken in assuming a resonant mechanism for generating color (unlike hearing, where the first stage of the transformation of mechanical vibrations into sound is carried out precisely by the resonant mechanism, color vision is arranged in a fundamentally different way), but for us something else is more important, that Newton first identified a specific triad: physical radiation - physiological mechanism - mental phenomenon, in which color is determined by the interaction of physiological and psychological levels. Therefore, we can call Newton's point of view the idea of ​​the psychophysiological nature of color.

For the first time, the experiment on the decomposition of light into a spectrum was made by Isaac Newton in 1666. He made a small hole in the window shutter and on a sunny day received a narrow beam of light, in the path of which he placed a triangular glass prism. The beam refracted in it, and a colored stripe appeared on the opposite wall, where all the colors of the rainbow were arranged in a certain order: red, orange, yellow, green, light blue, blue and violet. This color band Newton called spectrum(from the Latin "spectrum" - visible).

The smallest deviation from the initial direction of incidence is experienced by the red rays, and the largest - by the violet ones.

After such an experiment, Newton made first conclusion: the decomposition of white light into a color spectrum means that white light has a complex structure, that is, it is a composite, that is, a mixture of all the colors of the rainbow.

Second conclusion Newton was that light of different colors is characterized by different refractive indices in a certain environment... This means that the absolute refractive index for violet colors is higher than for reds.

The dependence of the refractive index of light on its colors Newton called variance(from the Latin word dispersio - "dispersion").

However, Newton was a supporter of the corpuscular theory and could not explain the phenomenon of dispersion.

Light dispersion

According to wave theory the colors of light are determined by the frequency of the electromagnetic wave which is light. Red light has the lowest frequency and violet light has the highest. Based on Newton's experiments and based on the wave theory of light, the conclusion follows: the refractive index of light depends on the frequency of the light wave.

Light dispersion- This is the phenomenon of decomposition of light into a spectrum due to the dependence of the absolute refractive index of the medium on the frequency of the light wave.

What depends on what.?

Different speeds of wave propagation correspond to different absolute refractive indices of the medium
.

This means that the red ray is refracted less due to the fact that it has the highest speed in the substance, and the violet ray - the least.

Frequency and wavelength are related

The formula shows that the wavelength is directly proportional to the speed of light and inversely proportional to the frequency. Hence it follows that the wavelength is longer in the environment where the wave velocity is greater(at a given frequency).

It can be seen from the formulas that

Therefore, it can be argued that the absolute the refractive index decreases correspondingly to an increase in the light wavelength and increases correspondingly to a decrease in the wavelength of the light wave.

Hence, during the transition from one environment to another speed propagation of a light wave, which means and the wavelength changes , a frequency, which means and the color of the light remains unchanged .

How does the eye distinguish colors?

On the retina of the eye there are light-sensitive elements - nerve endings, which are called "rods" and "cones". Sticks distinguish only light from dark. There are three types of cones - they are conventionally called "red", "green" and "blue". Because "red" cones are most sensitive to red, "green" to green, and "blue" to blue. And all the variety of colors we see is due to the "signals" sent to the brain by just three types of cones.

Color addition

Subtraction of colors

Around 1666, Newton made the following simple but extremely important experiment (Fig. 157): “I took an oblong piece of thick black paper with parallel sides and divided it into two equal halves with a line. I painted one part red and the other blue. The paper was very black, the colors were intense, and thickly applied so that the phenomenon could be seen more clearly. I viewed this paper through a solid glass prism, the sides of which were flat and well polished.

Examining the paper, I held it and the prism in front of the window. The wall of the room behind the prism, under the window, was covered with a black cloth in the dark; thus, light could not be reflected from it, which, passing the edges of the paper into the eye, would mix with the light from the paper and obscure the phenomenon. Having set the objects in this way, I found that in the case when the refractive angle of the prism is turned upwards, so that the paper appears to be raised due to refraction (image), then the blue side rises by refraction higher than the red side.If the refractive angle of the prism is turned down and the paper appears lowered due to refraction (the image then the blue part will be slightly lower than the red

Thus, in both cases, the light coming from the blue half of the paper through the prism to the eye, under the same circumstances, undergoes more refraction than the light coming from the red half. "

From a modern point of view, this phenomenon is explained by the fact that the refractive index of the glass from which the prism is made depends on the wavelength of the transmitted light. The prism refracts rays with different wavelengths in different ways. Glass has a higher refractive index for blue rays than for red ones, that is, the refractive index decreases with increasing wavelength.

Rice. 157. Scheme of Newton's experiment proving the existence of dispersion.

Newton describes a second, no less important experiment in the same area. In a completely dark room, he made a small hole in the shutter of the window through which a white sunbeam passed (Fig. 158). Having passed through the prism, this ray gave a whole colored spectrum on the wall. Thus, it was proved that white light is a mixture of colors and that this mixture can be decomposed into composite colors, taking advantage of the difference in refraction for rays of different colors.

However, one should not think that the very discovery of prismatic colors belongs to Newton. SI Vavilov, one of the most subtle connoisseurs of Newton, wrote: “Newton did not at all discover prismatic colors, as they often write and especially say: they were known long before him, Leonardo da Vinci, Galileo and many others knew about them; glass prisms were sold in the 17th century. precisely because of the prismatic colors. " Newton's merit lies in conducting clear and subtle experiments that clarified the dependence of the refractive index on the color of the rays (see, for example, the first experiment).

The dependence of the refractive index on the wavelength of the transmitted light is called light dispersion. In fig. 159 depicts dispersion curves for a number of crystals.

In practice, dispersion is characterized by setting a series of refractive index values ​​for several wavelengths corresponding to dark Fraunhofer lines in the solar spectrum.

In Soviet optical factories, four values ​​of the refractive index of glass are usually used: the refractive index for red light with a wavelength of 656.3 nanometers for yellow light with a wavelength for blue light with a wavelength and - for blue light with a wavelength

Rice. 158. Dispersion spectrum of white light.

Rice. 159. Dispersion curves of various substances.

Glasses with a low specific gravity - crowns - have less dispersion, heavy glasses - flints - more dispersion.

The table contains numerical data on the dispersion of Soviet optical glasses and some liquid and crystalline bodies.

(see scan)

A number of interesting consequences follow from the figures given in the table. Let's dwell on some of them. Dispersion affects in the most extreme case only in the change of the second decimal place in the value of the refractive index. At the same time, as we will see below, dispersion plays a colossal role in the operation of optical instruments. Further, although the variance is large as