Newton's experiments. Light dispersion
The first 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 was refracted in it, and a colored strip appeared on the opposite wall, where all the colors of the rainbow were located in a certain order: red, orange, yellow, green, blue, indigo and violet. Newton called this color band spectrum(from the Latin "spectrum" - visible).
Red rays experience the smallest deviation from the original direction of incidence, and violet rays experience the largest.
After such an experiment, Newton made first conclusion: the decomposition of white light into the 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 particular medium. This means that the absolute refractive index for violets is greater than for reds.
Newton called the dependence of the refractive index of light on its colors dispersion(from the Latin word dispersio - "scattering").
However, Newton was a supporter of the corpuscular theory and could not explain the phenomenon of dispersion.
Light dispersion
According to the 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 frequency. Based on Newton's experiments and relying 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 wave propagation velocities 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 has the lowest.
Frequency and wavelength are related
From the formula it can be seen that the wavelength is directly proportional to the speed of light and inversely proportional to the frequency. From this it follows that the wavelength is longer in the medium where the wave speed 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 the increase in the wavelength of the light wave and increases as the wavelength decreases.
Consequently, during the transition from one environment to another speed propagation of a light wave, which means and the wavelength changes , but frequency, which means and the color of the light stays the same .
How does the eye distinguish colors?
On the retina of the eye 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 conditionally 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.
Addition of colors
Color subtraction
Topics of the USE codifier: light dispersion.
Let the sunbeam pass from the air into a transparent medium (for example, water or glass). If the angle of incidence is not equal to zero, then, as you remember, the angle of refraction is determined from the law of refraction:
The value , called the refractive index, characterizes the medium and does not depend on the angle of incidence.
It turns out, however, that the medium reacts differently to the passage of electromagnetic waves of different frequencies. Occurs dispersion - dependence of the refractive index of the medium on the frequency of light.
Newton's experience.
The classic experiment on the observation of dispersion was set by Newton. A narrow beam of sunlight was directed to a triangular glass prism (Fig. 1).
Appeared on the screen behind the prism range- rainbow stripe One end of the spectrum turned out to be red, the other - violet, and the colors within the spectrum continuously changed into each other.
Selecting a beam of any color (for example, red or blue) and launching it into another prism, we will no longer see a change in the color of the refracted beam. Therefore, the components of the rainbow are the simplest colors that cannot be further decomposed. They can be collected back with a second prism, and then white light is again obtained. Therefore, white light is a mixture of light beams of different colors continuously filling the range of visible light from red to violet.
We see, therefore, that the glass prism is the simplest spectral instrument- it allows you to explore the spectral composition of white light. With the action of a more complex spectral device - a diffraction grating - we met in the previous topic.
As Newton's experiment shows, red light is refracted the weakest, and violet light is the strongest. In the visible range, red light has the lowest frequency and violet light has the highest frequency. Since the refractive index becomes larger as we move from the red end of the spectrum to the violet, we conclude that The refractive index of glass increases with increasing light frequency.
But the refractive index is the ratio of the speed of light in air to the speed of light in a medium: . This means that the higher the frequency of light, the slower the speed of light propagates in glass.. Red light has the highest speed inside a glass prism, and violet light has the lowest speed.
The difference in the speeds of light for different frequencies appears only in the presence of a medium. In vacuum, the propagation speed of electromagnetic waves does not depend on frequency and is equal to .
Discovered and studied by Newton, the dispersion of light has been waiting for its explanation for more than two hundred years - appropriate information about the structure of matter was needed. The classical theory of dispersion was proposed by Lorentz only at the end of the 19th century. A more accurate quantum theory of dispersion appeared in the first half of the last century.
Chromatic aberration.
]Assume that a beam of white light is incident on a converging lens parallel to the main optical axis. Being refracted in the lens, it would seem that it should gather at its focus. However, due to dispersion, chromatic aberration- some defocusing of the beam caused by different refraction of different components of white light.
The phenomenon of chromatic aberration is shown in fig. 2.
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| Rice. 2. Chromatic aberration |
The refractive index of the lens material takes on the lowest value for red light, and therefore red light is refracted the least. Red rays are collected on the main optical axis at the point farthest from the lens. The yellow rays converge closer to the lens, the green rays converge even closer, and finally, the violet rays converge at the point closest to the lens.
Chromatic aberration degrades image quality - reduces sharpness, gives extra color bands. But chromatic aberration can be dealt with. For this purpose, so-called achromatic lenses are used in optical technology, obtained by superimposing an additional diverging lens on a converging lens. Guess - why do you need a diverging lens?
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 we take a thick sheet of white cardboard as screen 1, then after the passage of the sun's ray through the prism, the usual linear color spectrum will be reflected on the screen. To test the hypothesis where colored rays originate - in light or in a prism - Newton removed screen 1 and passed the spectral rays to a lens, which again collected them in a beam on screen 2, and this beam was as colorless as the original light.
Thus, Newton showed that colors are formed not by a prism, but ...! And here it is necessary to stop for a moment, because so far there have been physical experiments with light, and only here 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 white light, there is no hint of the colored rays that make it up?
It is this phenomenon, which will make it possible to formulate one of the laws of color mixing, and led Newton to develop a method of color mixing. Let us turn again to fig. 1.1. Instead of a solid screen 1, we put another screen 1 in which holes are cut out so that only a part of the rays (two, three or four out of seven) pass through, and the rest are blocked by opaque partitions. And here miracles begin. Colors appear on screen 2 from no one knows where and no one knows how. For example, we blocked the violet, blue, blue, yellow and orange rays and let the green and red rays through. However, after passing through the lens and reaching screen 2, these rays disappeared, but yellow appeared instead. If we look at screen 1, we see that the yellow beam is blocked by this screen and cannot reach screen 2, but screen 2 still has exactly the same yellow color. Where did he come from?
The same miracles occur if all the rays are delayed, except for blue and orange. Again, the original rays will disappear, and a white light will appear, the same as if it consisted not of two rays, but of seven. But the most amazing phenomenon occurs if you miss only the extreme rays of the spectrum - violet and red. On screen 2, a completely new color appears, which was neither among the original seven colors, nor among their other combinations - magenta.
These amazing phenomena forced Newton to carefully consider the rays of the spectrum and their various mixtures. If we also look into the spectral series, we will see that the individual components of the spectrum are not separated from each other by a sharp boundary, but gradually pass 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 purple. And the same purple, together with orange, makes up a pair of neighboring colors for the extreme red ray of the spectrum. That is, if the colors of the spectrum and the mixture are arranged in accordance with their perceived similarity, then they do not form a line, like a spectrum, but a vicious circle (Fig. 1.2), so that the most different in position in the radiation spectrum, i.e., the most physically different rays will be very close in color.
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 made from his experiments in "Optics"
“When I speak of light and rays as colored or causing colors, it should be understood that I am not speaking in a philosophical sense, but in the way ordinary people speak about these concepts. Essentially, the rays are not colored; they have nothing but a certain ability and disposition to evoke the sensation of this or that color. Just as sound ... in any sounding body there is nothing but movement, which is perceived by the senses as sound, so the color of an object is nothing but a predisposition to reflect one or another type 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 colors ”(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, which requires an observer who is able to 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 sense organs, capable of interpreting the impact of electromagnetic radiation in a certain way.
Since Newton was a supporter of the corpuscular theory of light, he believed that the conversion of electromagnetic radiation into colors is carried out by vibration of nerve fibers, so that "a certain combination of vibrations of various fibers causes a certain sensation of color in the brain.
Now we know that Newton was mistaken in assuming a resonant mechanism of color generation (in contrast to 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 something else is more important for us, that Newton was the first to single out 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.
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 with red paint, the other with blue. The paper was very black, the colors were intense and applied in thick layers so that the phenomenon could be more distinct. 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 black matter, which was in darkness; thus, no light could be reflected from it, which, passing past the edges of the paper into the eye, would mix with the light from the paper and obscure the phenomenon. By placing objects in this way, I found that in the case when the refractive angle of the prism is turned upwards, so that the paper seems 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 seems 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 experiences, under the same circumstances, 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 of different wavelengths in different ways. In glass, the refractive index for blue rays is greater than for red ones, i.e., the refractive index decreases with increasing wavelength.
Rice. 157. Diagram of Newton's experiment proving the existence of dispersion.
Newton also describes a second, no less important experience in the same area. In a completely dark room, he made a small hole in the window shutter, through which a white sunbeam passed (Fig. 158). Passing through the prism, this beam produced 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 its component colors, using the difference in refraction for rays of different colors.
One should not, however, think that Newton owns the very discovery of prismatic colors. S. I. Vavilov, one of the finest connoisseurs of Newton, wrote: “Newton did not discover prismatic colors at all, as is often written and especially said: 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 carrying out clear and subtle experiments that elucidated 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 the dispersion of light. On fig. 159 shows 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 millimicrons for yellow light with a wavelength for blue light with a wavelength and for blue light with a wavelength
Rice. 158. Dispersive spectrum of white light.
Rice. 159. Dispersion curves of various substances.
Glasses with a low specific gravity - crowns - have a lower dispersion, heavy glasses - flints - a larger dispersion.
The table lists 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. In the most extreme case, dispersion affects only the change in the second decimal place in the refractive index. At the same time, as we shall see below, dispersion plays a colossal role in the operation of optical instruments. Further, although a large variance, as
In 1672, Isaac Newton did a simple experiment that is described in all school textbooks. Having closed the shutters, he made a small hole in them, through which a ray of sunlight passed. A prism was placed in the path of the beam, and a screen was placed behind the prism. On the screen, Newton observed a "rainbow": a white sunbeam, passing through a prism, turned into several colored rays - from purple to red. This phenomenon is called light dispersion.
Sir Isaac was not the first to observe this phenomenon. Already at the beginning of our era, it was known that large single crystals of natural origin have the property of decomposing light into colors. Even before Newton, the first studies of light dispersion in experiments with a glass triangular prism were carried out by the Englishman Khariot and the Czech naturalist Marci.
However, prior to Newton, such observations were not subjected to serious analysis, and the conclusions drawn from them were not rechecked by additional experiments. Both Chariot and Martzi remained followers of Aristotle, who argued that the difference in color is determined by the difference in the amount of darkness "mixed" with white light. Violet color, according to Aristotle, occurs with the greatest addition of darkness to light, and red - with the least. Newton did additional experiments with crossed prisms, when light passed through one prism then passes through another. Based on the totality of his experiments, he concluded that “no color arises from whiteness and blackness mixed together, except for intermediate dark ones; the amount of light does not change the appearance of the color." He showed that white light must be considered as a composite light. The main colors are from purple to red.
This experiment of Newton is a wonderful example of how different people, observing the same phenomenon, interpret it differently, and only those who question their interpretation and make additional experiments come to the right conclusions.
Henry Cavendish experiment
Establishment Newton law of gravity was the most important event in the history physics. Its value is determined, first of all, by the universality of the gravitational interaction. One of the central sections of astronomy, celestial mechanics, is based on the law of universal gravitation. We feel the force of attraction to the Earth, but the attraction of small bodies to each other is imperceptible. It was required to experimentally prove the validity of the law of universal gravitation for ordinary bodies as well. This is exactly what G. Cavendish did, simultaneously determining the average density of the Earth.
where m 1 and m 2 are the masses of material points, R is the distance between them, a F is the strength of interaction between them. Before the beginning of the 19th century G was not introduced into the law of universal gravitation, since for all calculations in celestial mechanics it is sufficient to use the constants GM having kinematic dimension. Constant G appeared for the first time, apparently, only after the unification of units and the transition to a single metric system of measures at the end of the 18th century. Numerical value G can be calculated through the average density of the Earth, which had to be determined experimentally. Obviously, with known values of the density c and radius R of the Earth, as well as the free fall acceleration g on its surface can be found G:
The experiment was originally proposed John Michell. It was he who designed the main part in the experimental setup - a torsion balance, but died in 1793 without putting experience. After his death, the experimental facility was taken over by Henry Cavendish. Cavendish modified the installation, conducted experiments and described them in Philosophical Transactions in 1798.
Installation
Torsion scales
The installation is a wooden rocker with small lead balls attached to its ends. It is suspended on a silver-plated copper thread 1 m long. Larger balls weighing 159 kg, also made of lead, are brought to the balls. As a result of the action of gravitational forces, the rocker twists at a certain angle. The stiffness of the thread was such that the rocker made one oscillation in 15 minutes. The angle of rotation of the rocker was determined using a beam of light cast on a mirror on the rocker and reflected into a microscope. Knowing the elastic properties of the thread, as well as the angle of rotation of the rocker, it is possible to calculate gravitational constant.
To prevent convection currents, the setup was enclosed in a windproof chamber. The deflection angle was measured with a telescope.
Having attributed the twisting of the thread to the magnetic interaction of an iron rod and lead balls, Cavendish replaced it with a copper one, obtaining the same results.
Computed value
IN Britannica it is stated that G. Cavendish obtained the value G=6.754 10 -11 m³/(kg s³) . This is what E. P. Cohen, K. Crove and J. Dumond say. and A. Cook. .
L. Cooper in his two-volume physics textbook gives a different value: G = 6.71 10 -11 m³ / (kg s³) .
O.P. Spiridonov -- third: G=(6.6 ± 0.04) 10 -11 m/(kg s) .
However, in the classic work of Cavendish, no value of G was given. He calculated only the value of the average density Earth: 5.48 density water(modern value 5.52 g/cm³). The conclusion of Cavendish that the average density of the planet is 5.48 g/cm3 is greater than the surface density ~2 g/cm3, confirmed that heavy substances are concentrated in the depths.
The gravitational constant was apparently introduced for the first time only S. D. Poisson in "Treatise on Mechanics" (1811) . The value of G was calculated later by other scientists from the data of the Cavendish experiment. Who first calculated the numerical value of G is unknown to historians.
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