The phenomenon that polychromatic light breaks down into monochromatic light is called light dispersion. Newton first observed the dispersion of light with a prism in 1666, and decomposed white light into colored bands (spectra). Dispersion phenomenon indicates the speed (or dispersion) of light in the medium.
The refractive index n=c/v) varies with the frequency of light. The dispersion of light can be realized by prism, diffraction grating, interferometer and so on. The dispersion of light proves that light fluctuates.
polychromatic light
White light consists of red, orange, yellow, green, blue, indigo, purple and other colors, and the light mixed with monochromatic light is called polychromatic light. Color light that cannot be decomposed is called monochromatic light.
chromatic dispersion
The phenomenon that polychromatic light is decomposed into monochromatic light to form spectrum is called optical dispersion. Using prism or grating as "dispersion system" can realize dispersion. After entering the prism, polychromatic light has different refractive indexes for light with various frequencies, and the propagation direction of polychromatic light is deflected to varying degrees, so when leaving the prism, they are dispersed separately to form a spectrum.
yellow,cyan,magenta
Red, green and blue. In addition, the phosphor of the TV we watch is also in this combination. Just look at the CRT in front of the color TV, not the computer monitor in front. Its pixels are too small to be distinguished by the naked eye. The combination of these three colors, RGB, forms almost all colors. Red, green and blue are called the "three primary colors" of light, because the three colors of red, green and blue in nature cannot be mixed with other colors, and other colors can be obtained by properly mixing them, so the three colors of red, green and blue are called the "three primary colors" of light. After sunlight passes through the prism, it is decomposed into various colors of light. If it is accepted by the white screen, a line will be formed on the white screen. This shows that white light is a mixture of various colors, and rainbow is the dispersion of water droplets in the air during the propagation of sunlight. Colored light is a typical "brightening and brightening", that is, when different colors are superimposed, the brightness also increases.
The phenomenon that the refractive index of a medium changes with the frequency or wavelength of light waves in vacuum. When polychromatic light refracts at the interface of the medium, the medium has different refractive indexes for light with different wavelengths, and the light of each color is separated from each other due to different refraction angles. In 1672, Newton used a prism to disperse dispersion.
Sunlight is broken down into colored bands, which is the first dispersion experiment that people have done. The relationship between refractive index n or dispersion ratio DN/D λ and wavelength λ is usually used to describe the dispersion law. The dispersion of any medium can be divided into normal dispersion and abnormal dispersion.
Edit this spectrum
The phenomenon that polychromatic light is decomposed into monochromatic light to form a spectrum. Let a beam of white light shine on the glass prism, and after the light is refracted by the prism, a colored light band will be formed on the white screen on the other side. The color arrangement is red near the top corner of the prism, purple at the end near the bottom, and orange, yellow, green and indigo in the middle. This band of light is called spectrum. Each color of light in the spectrum cannot be decomposed into other colors of light. It is called monochromatic light. Light mixed with monochromatic light is called polychromatic light. The light emitted by sunlight, incandescent lamps and fluorescent lamps in nature is polychromatic light. When light shines on an object, part of the light is reflected by the object and part of the light is absorbed by the object. If the object is transparent, there is still a part passing through it. Different objects reflect, absorb and transmit different colors, so they show different colors.
The phenomenon that polychromatic light breaks down into monochromatic light is called light dispersion. Newton first observed the dispersion of light with a prism in 1666, and decomposed white light into colored bands (spectra). Dispersion phenomenon shows that the speed (or refractive index n=c/v) of light in the medium varies with the frequency of light. The dispersion of light can be achieved by using prisms, diffraction gratings, interferometers, etc.
White light consists of red, orange, yellow, green, blue, indigo and purple, which is called polychromatic light. Colors such as red, orange, yellow and green are called monochromatic light.
Dispersion: The phenomenon that polychromatic light is decomposed into monochromatic light to form spectrum is called optical dispersion. Using prism or grating as "dispersion system" can realize dispersion. After entering the prism, polychromatic light has different refractive indexes for light with various frequencies, and the propagation direction of polychromatic light is deflected to varying degrees, so when leaving the prism, they are dispersed separately to form a spectrum.
The three primary colors of light: red, green and blue.
In addition, the phosphor of the TV we watch is also in this combination. You can watch CRT in front of color TV, but don't look at the computer monitor in front of you. Its pixels are too small to be distinguished by the naked eye. The combination of these three colors, RGB, forms almost all colors.
scattering of light
The phenomenon that the refractive index of a medium changes with the frequency or wavelength of light waves in vacuum. When polychromatic light refracts at the interface of the medium, the medium has different refractive indexes for light with different wavelengths, and the light of each color is separated from each other due to different refraction angles. 1672, Newton used a prism to decompose sunlight into colored bands, which was the first dispersion experiment made by people. The relationship between refractive index n or dispersion ratio DN/D λ and wavelength λ is usually used to describe the dispersion law. The dispersion of any medium can be divided into normal dispersion and abnormal dispersion.
The phenomenon that polychromatic light is decomposed into monochromatic light to form a spectrum. Let a beam of white light shine on the glass prism, and after the light is refracted by the prism, a colored light band will be formed on the white screen on the other side. The color arrangement is red near the top corner of the prism, purple at the end near the bottom, and orange, yellow, green and indigo in the middle. This band of light is called spectrum. Each color of light in the spectrum cannot be decomposed into other colors of light. It is called monochromatic light. Light mixed with monochromatic light is called polychromatic light. The light emitted by sunlight, incandescent lamps and fluorescent lamps in nature is polychromatic light. When light shines on an object, part of the light is reflected by the object and part of the light is absorbed by the object. If the object is transparent, there is still a part passing through it. Different objects reflect, absorb and transmit different colors, so they show different colors.
Light waves have a certain frequency, and the color of light is determined by the frequency of light waves. In the visible region, the frequency of red light is the smallest and that of violet light is the largest. The speed of light with various frequencies in vacuum is the same, equal to 3.0× 10 ˇ 8m/s, while monochromatic light with different frequencies is slower than that in vacuum due to the action of medium. The propagation speed of violet light is small, so the medium has a small refractive index for red light and a large refractive index for violet light. When different colors of light are incident on the prism at the same incident angle, the refractive index of red light is the smallest, which is close to the top corner in the spectrum. Violet light has a high frequency and a high refractive index in the medium, and it is also arranged at the end closest to the bottom of the prism in the spectrum.
In summer, colorful arcs facing the sun often appear in the sky after rain. This is the rainbow. The reason why rainbows are formed is that many tiny water droplets are suspended in the sky after rain. When the sun shines at a certain angle, these small water droplets will disperse. Looking at water droplets, colorful rainbows will appear. The colors of the rainbow are red outside and purple inside, arranged in turn.
First, the dispersion phenomenon of ancient light in China.
China's understanding of light dispersion in ancient times originated from the understanding of natural dispersion phenomenon-rainbow, which is a complex dispersion phenomenon caused by refraction and reflection caused by sunlight entering water droplets in the air at a certain angle. China recorded rainbows in Oracle Bone Inscriptions as early as the Yin and Shang Dynasties. At that time, the word "rainbow" was written as "". According to the Chu Ci in the Warring States Period, the color of rainbow is divided into five colors. Cai Yong in the Eastern Han Dynasty (132~ 192) and Kong in the early Tang Dynasty (574~648) roughly revealed the optical origin of rainbows in Notes to the Book of Rites: "If the clouds are thin and the sun leaks, there will be raindrops when it rains", which shows that raindrops are a natural phenomenon caused by sunlight. In the middle of the eighth century, Zhang (744~773) used experimental methods for the first time in the spirit of Xuanzhen Zidao. Moreover, it is the first white light dispersion experiment consciously carried out: "The water sprayed from the back of the sun looks like a rainbow, but it can't be straight and looks like a shadow." After the Tang Dynasty, similar experiments have been repeated. For example, an experiment conducted by Cai Bian in the Southern Song Dynasty to simulate "raindrops in the sun" linked the rainbow with the halo phenomenon of the sun and the moon, deliberately indicating that the rainbow is a process of dispersion. It also points out the relationship between the rainbow and the position of sunlight. In the Southern Song Dynasty, Cheng Dachang (1123 ~1195) described the phenomenon of dew splitting in Fan Yanlu, pointing out that sunlight can be converted into many colors through a drop of water, which is actually dispersive, but this color is not possessed by the drop itself.
In China, since the Jin Dynasty, many ancient books have recorded the phenomenon of crystal dispersion. For example, it is recorded that peacock hair and the epidermis of an insect constantly change color in the sun, and mica sheets can observe various colors of light. Li Shizhen also pointed out that larger hexagonal crystals and smaller crystal beads can form dispersion. At the end of the Ming Dynasty, Fang Yizhi (161~1671year) made a wonderful summary of the dispersion phenomenon in his book Introduction to Physics. He divided white light into five colors with natural crystals with edges and triangular crystals artificially fired, and then mixed them with artificial rainbows with five colors. And natural phenomena such as rainbow color, sun and moon halo and five-color clouds. It is considered that "all are dispersion of white light". All these indicate that China had a comprehensive understanding of the nature of dispersion phenomenon before the Ming Dynasty, but it also reflects that most of China's knowledge of ancient physics is scattered and empirical.
Second, Western Newton's previous understanding of the dispersion of light
In the early days of optical development, it was particularly difficult to explain colors. Before Newton, Aristotle's view was popular in European understanding of color. Aristotle believes that color is not an objective attribute of an object, but a subjective feeling of human beings. All colors are the result of a proportional mixture of light and dark, black and white. Boyle also studied the color of an object in 6438+0663. He believes that the color of an object does not belong to the essential nature of the object, but is caused by the change of light on the surface of the irradiated object. Objects that can completely reflect light are white, and objects that completely absorb light are black. In addition, Descartes, Hooke and many other scientists have also discussed the problem that white light is dispersed or gathered into color, but they all advocate that red is greatly concentrated light and purple is greatly diluted light, which is such a complex and disorderly theory. So before Newton,
3. Newton's experimental exploration and theoretical research on the dispersion of light.
(1) Design and conduct prism experiments.
When white light passes through colorless glass and fragments of various gems, it will form bright lights of various colors. This fact was known centuries before Newton, but it was not until the middle of the seventeenth century that Newton studied the problem through experiments. This experiment was rated as one of the "most beautiful experiments in physics".
Newton first made a famous prism experiment. He wrote in the book: "1666 At the beginning, I made a triangular glass prism to study the color of light. To this end, I painted the room with ink and water powder, and opened a small hole in the window to let in the right amount of sunlight. I put the prism at the entrance of the light and let the refracted light shine on the opposite wall. When I first saw the bright and strong light color, I felt very happy. " Newton's experimental design is like this: through this experiment, a color point is obtained on the wall, and the color arrangement is red, orange, yellow, green, cyan, blue and purple. Newton called this color speckle spectrum.
(2) Further design experiments to obtain pure spectra.
The Newton spectrum obtained from the above experiment is impure. He thinks that the spectrum is impure because it consists of a series of overlapping images of circular spots. In order to obtain a very pure spectrum, Newton designed a set of optical instruments for experiments, and the experimental design is shown in the following figure:
White light illuminates the slit s through a lens, and a converging lens is placed behind the slit to form an image i of the slit s.. Then the prism is placed in the optical path of the lens. As a result, light passes through the prism due to different deflection angles, thus forming a spectral band from red to purple on the white screen. This spectral band consists of a series of color images of adjacent slits. If the slit is made very narrow, the overlapping phenomenon can be minimized, so the spectrum becomes very pure.
(3) Newton put forward a theory to explain the spectrum.
In order to explain the decomposition of white light in prism experiment, Newton thought that white light was composed of different colors and the refractive index of glass was different. When white light passes through a prism, all colors are refracted at different angles, resulting in separation into chromatograms. When the white light passes through the prism, it is biased to the bottom of the prism, and the purple light is the largest and the red light is the smallest. The phenomenon that a prism divides white light into various colors is called dispersion. Strictly speaking, there are many colors in the spectrum.
(4) Design experiments to verify the correctness of the above theory.
In order to further study the color of light and verify the correctness of the above theory, Newton did another experiment. The experimental design is as follows:
Newton made a small hole in the screen de to observe the spectrum, and then put a screen de with a small hole behind it, so that the light passing through this hole is monochromatic light of a certain color. Newton put a second prism abc in the path of this beam, and put a new observation screen V behind it. Experiments show that the second prism abc only deflects the monochromatic light beam by an angle without changing the color of the light. In the experiment, Newton turned the first prism ABC. Light of different colors in the spectrum passes through the small holes on the de and DE screens. In all these cases, these monochromatic lights of different colors can no longer be decomposed by the second prism, and all of them are deflected only by a certain angle after passing through the second inspection mirror. In addition, it is also found that the light deflection angles of different colors are different.
Through these experiments, Newton came to the conclusion that white light can be decomposed into different colors, which are monochromatic, and the prism can no longer be decomposed.
(5) Experiment of synthesizing white light with monochromatic light.
Since white light can be decomposed into monochromatic light, can monochromatic light also be synthesized into white light? "Newton conducted an experiment for this. As shown in fig. 55, the colored light of the spectrum can be recombined into white light by placing it on a row of small rectangular flat mirrors. Adjust the angle between each plane mirror and incident light, so that all reflected light falls on the same position of the light screen, thus obtaining white light.
Newton pointed out that there is another way to recombine colored light into white light. Draw the spectrum on a fan-shaped disk, and then rotate the disk at high speed, and the disk will appear white. This experimental effect is generally called "visual persistence effect". After the image formed on the retina of the eye disappears, the brain can keep the impression for a few tenths of a second. Therefore, the brain can combine rapidly changing color images to form a static white image. On a TV screen or a movie screen.
(6) Newton's research results on optical dispersion.
Newton summarized the results into several terms through a series of dispersion experiments and theoretical studies, and the main points are as follows:
The refractive index of light is different, so is the color. Color is not the change of light, but the inherent property of light.
(2) the same color belongs to the same refractive index, and vice versa.
The type of color and the degree of refraction are inherent in light and will not change due to refraction, reflection or any other reasons.
It is necessary to distinguish between the original pure colors and the colors composed of them.
⑤ There is no white light. White is produced by properly mixing all colors of light. In fact, experiments can be carried out to synthesize the colors of the spectrum to obtain white light.
According to the above, we can explain the reason why prism makes light produce color and the principle of rainbow.
The color of a natural object is because it reflects a lot of light and less other light.
8 So color is the quality of light (various rays), so light itself can't be quality. Because the quality of color originates from light, there is now sufficient basis to think that light is an entity.
(7) The characteristics of Newton's research method on the dispersion phenomenon of light.
As can be seen from the above, Newton adopted the typical physical law research method of experimental induction-hypothesis theory-experimental test in the study of light dispersion, and infiltrated the physical research methods such as analytical method (decomposing white light into monochromatic light) and comprehensive method (synthesizing monochromatic light into white light).
The dispersion of light indicates that the light fluctuates. Because dispersion is caused by the different refractive indexes of light components (light of different colors), and the refractive index is determined by the frequency of waves.
The most typical example of the particle nature of light is photoelectric effect.
dispersion forces
Due to the constant motion of electrons and nuclei in molecules, the distribution of electron clouds in nonpolar molecules is fluctuating, which leads to instantaneous relative displacement between them and nuclei, resulting in instantaneous dipole, thus making molecules deform. The more electrons, atoms and atomic radii in a molecule, the easier it is to deform. The instantaneous dipole can make another non-polar molecule adjacent to it produce an instantaneous induced dipole, and the two instantaneous dipoles always adopt the state of being adjacent to each other. The force between instantaneous dipoles generated by molecules at any time is dispersion force (named because the expression of its action energy is similar to the dispersion formula of light). Although the instantaneous dipole is short-lived, the neighboring states of different poles are repeated one after another, so there is always a dispersion force between molecules. Undoubtedly, the dispersion force exists not only between nonpolar molecules, but also between polar molecules and between polar and nonpolar molecules.
There is a dispersion force between all molecules. Dispersion is related to the deformability of molecules. The stronger the deformability, the easier it is to polarize and the stronger the dispersion. Rare gas molecules do not form chemical bonds, but when they are close to each other, they can liquefy and release energy, which proves the existence of dispersion force.
These three intermolecular forces are collectively called van der Waals forces. It was put forward when people studied the deviation between actual gas and ideal gas. Intermolecular force has the following characteristics: ① Intermolecular force is inversely proportional to the 6th power of intermolecular distance. So when the molecules are slightly away, the intermolecular force suddenly weakens. Their working distance is about 300 ~ 500 pm. When there is a certain contact distance between molecules and the electron clouds do not overlap, half of the nuclear distance between two atoms in adjacent molecules is called the van der Waals radius of atoms. The van der Waals radius of chlorine atom is 180pm, which is much larger than its valence radius of 99pm. ② Intermolecular force has no directionality and saturation. ③ The intermolecular force energy is generally 2 ~ 20kJ mol-1,which is about 2 orders of magnitude smaller than the chemical bond energy (100 ~ 600kJ mol-1).
The physical properties of halogen molecules can be easily explained qualitatively by intermolecular forces: F2, Cl2, Br2 and I2 are all nonpolar molecules. With the increase of molecular weight, atomic radius and electron number, the dispersion force, molecular deformability and intermolecular force increase. Therefore, halogen molecules melt in turn and the boiling point rises rapidly. At room temperature, F2 and Cl2 are gases, Br2 is liquid and I2 is solid. However, the molecular weights of HF, H2O and NH3 are obviously smaller than those of the corresponding homologous hydrides, but their melting points and boiling points are extremely high, because there are helium bonds between these molecules.
refractive index
Chromogenic refractive index
Purple 1.532
Blue 1.528
Green 1.5 19
Huang 1.5 17
Orange 1.5 14
Red 1.5 13
The refractive index of a medium is equal to the ratio of the speed of light in a vacuum to the speed in this medium. The speed of all colors of light in vacuum is the same, which is equal to C. Their refractive indices in the same medium (such as glass) are different, indicating that their speeds in the same medium are different. The refractive index of red light is smaller than other colors, indicating that the speed of red light in the medium is greater than other colors. Waves with shorter wavelengths are easy to scatter, while waves with longer wavelengths are not easy to scatter.