In 1911, E. Rutherford, on the basis of his experiments, substantiated ...
Light is a form of energy visible to the human eye that is emitted by moving charged particles.
Sunlight plays an important role in the life of wildlife. It is essential for plant growth. Plants convert sunlight energy into chemical form through the process of photosynthesis. Oil, coal and natural gas are the remains of plants that lived millions of years ago. We can say that this is the energy of converted sunlight.
Scientists have shown through experiments that at times light behaves like a particle and at other times like a wave. In 1900 quantum theory Max Planck combined two points of view of scientists on the world. And in modern physics light is considered as transverse electromagnetic waves, visible person, which are emitted by light quanta (photons) - particles that have no mass and move at a speed
Light characteristics
Like any wave, light can be characterized by length (λ), frequency (υ) and propagation speed in any medium (v). The relationship between these quantities is shown by the formula:
Visible light lies in the wavelength range of electromagnetic radiation from m (in ascending order of wavelength: violet, blue, green, yellow, orange, red). The frequency of a light wave is related to its color.
When a light wave passes from vacuum to a medium, its length and propagation speed decrease, the frequency of the light wave remains unchanged:
n is the refractive index of the medium, c is the speed of light in vacuum.
It must be remembered that the speed of light:
- in vacuum is a universal constant in all reporting systems;
- in a medium is always less than the speed of light in a vacuum;
- depends on the environment through which it passes;
- in a vacuum is always greater than the speed of any particle with mass.
Wave nature of light
The wave nature of light was first illustrated through diffraction and interference experiments. Like all electromagnetic waves, light can travel through a vacuum and be reflected and refracted. The transverse nature of light is proved by the phenomenon of polarization.
Interference
light waves, having a constant phase difference and the same frequency, produce a visible interference effect when the resulting wave is strengthened or attenuated.
Isaac Newton was one of the first scientists to study the phenomenon of interference. In his famous "Newton's Rings" experiment, he connected a convex lens with a large radius of curvature to a flat glass plate. If we consider this optical system through the reflected sunlight, a series of concentric light and dark strongly colored circles of light are observed. Rings appear due to a thin layer of air between the lens and the plate. The light reflected from the upper and lower surfaces of the glass interferes and gives a maximum of interference in the form of light, and a minimum in the form of dark rings.
Diffraction
Diffraction is the bending of a light wave around obstacles. The phenomenon can be observed when the obstacle is comparable in size to the wavelength. If the object is much larger than the wavelength from the light source, the phenomenon is almost imperceptible.
The result of diffraction is alternating colored and dark bands of light or concentric circles. This optical effect occurs as a result of the fact that the waves that go around the obstacle interfere. This picture is given by the light reflected from the surface of the CD.
Let's answer the questions: 1. What two types are all waves divided into? 2. What waves are called longitudinal? 3. What waves are called transverse? 4. What oscillates in a transverse mechanical wave? 5. What type of waves is a sound wave? 6. What type of waves does electromagnetic wave? Why?
In 1865, Maxwell concluded that light is an electromagnetic wave. One of the arguments in favor of this statement is the coincidence of the speed of electromagnetic waves, theoretically calculated by Maxwell, with the speed of light, determined experimentally (in the experiments of Roemer and Foucault).
Natural light Light is a transverse wave. In a beam of waves incident from a conventional source, there are oscillations of various directions perpendicular to the direction of wave propagation. A light wave that oscillates in all directions perpendicular to the direction of propagation is called a natural wave.
Polarized light A tourmaline crystal has the ability to transmit light waves with vibrations lying in one specific plane. Such light is called polarized or, more precisely, plane polarized, in contrast to natural light, which can also be called unpolarized.
Polaroid It is a thin (0.1 mm) film of herapatite crystals deposited on a celluloid or glass plate. Transparencies(polymer, single-crystal, etc.), which convert unpolarized light into linearly polarized, because transmit light in only one direction of polarization. Polaroids were invented by the American scientist E. Land in 1932.
If natural light falls on the interface between two dielectrics (for example, air and glass), then part of it is reflected, and part is refracted and propagated in the second medium. By placing an analyzer (for example, tourmaline) in the path of the reflected and refracted beams, one can make sure that the reflected and refracted beams are partially polarized: when the analyzer is rotated around the beams, the light intensity periodically increases and decreases (complete extinction is not observed!). Further studies have shown that in the reflected beam, oscillations perpendicular to the plane of incidence prevail (in the figure they are indicated by dots), in the refracted beam - oscillations parallel to the plane of incidence (shown by arrows).
Experimental verification of the polarization of light emitted by various sources A liquid crystal monitor produces polarized light. When you turn the polarizer, it is weakened, when you turn it by 90, it is completely extinguished. The radiation of the calculator display is also polarized. Polarized display light mobile phone. Light reflected from glass is polarized. Look at the glass through a polaroid. By rotating the polaroid, we achieve the disappearance of glare.
Polarized light in nature Reflected light is polarized, glare, for example, lying on the surface of water, Scattered light of the sky is nothing but sunlight that has undergone multiple reflections from air molecules, refracted in water droplets or ice crystals. Therefore, in a certain direction from the Sun, it is polarized. Many insects, unlike humans, see the polarization of light. Bees and ants use this ability to orient themselves when the Sun is obscured by clouds. The light of some astronomical objects is polarized. Most famous example The Crab Nebula in the constellation Taurus. Some species of beetles, which have a metallic sheen, turn the light reflected from their back into a circularly polarized one. This is the name of polarized light, the plane of polarization of which is twisted in space in a helical direction, to the left or to the right.
Polarized and Anti-Glare Sunglasses Driving safely at night, day, dusk, fog and winter. Polarized lenses cut out glare from windshield, from wet roads, from snow, protect from the headlights of oncoming cars, relieve fatigue, improve visibility in any weather. They are indispensable for polar explorers who constantly have to look at a dazzling reflection sun rays from the frozen snow field.
Obtaining a stereo image, stereo monitor To obtain the effect of volume (stereo effect), it is necessary to show each eye its own picture, as if different eyes look at the object from different angles; everything else our brain will complete and calculate on its own. In a stereo monitor, even and odd rows of pixels on the screen must have a different direction of light polarization. The lenses of the glasses are polarizers rotated relative to each other by 90 degrees - only even lines are visible through one lens of the glasses, and odd lines through the other. Each eye will see only the picture that is intended for it, so the image becomes three-dimensional.
The principle of operation of LCD displays The operation of LCD displays is based on the phenomenon of polarization of the light flux. Liquid crystals are organic matter, capable of turning into electric field. Liquid crystals have anisotropic properties. In particular, depending on the orientation, they reflect and transmit light in different ways, and rotate its plane of polarization. The TFT panel is like a sandwich sandwich. The liquid crystal layer is located between two polarizing panels. The voltage causes the crystals to act like a shutter, blocking or letting light through. The intensity of light passing through a polarizer depends on the voltage.
Conclusions: Tourmaline crystal (polaroid) converts natural light into plane polarized. Polarization is one of the wave properties of light. Various sources light can emit both polarized and unpolarized light. With the help of polaroids, you can control the intensity of light; The phenomenon of light polarization is found in nature, widely used in modern technology. Light is a transverse wave. cbbb15dd9463b3/gDM
hardware/monitors/24761http://hardware/monitors/
The phenomena of interference and diffraction of light confirm its wave nature. V early XIX century, T. Jung and O. Fresnel, having created the wave theory of light, considered light waves to be longitudinal, i.e. similar sound waves. To do this, they had to introduce some kind of hypothetical environment called ether, in which the propagation of longitudinal light waves took place. At that time it seemed incredible that light is transverse waves, since, by analogy with mechanical waves, one would have to assume that ether is solid(transverse mechanical waves cannot propagate in a gaseous or liquid medium). However, already at that time there were facts contradicting the longitudinality of light waves.
Back in the Middle Ages, sailors brought unusual transparent stones from Iceland, which were later called Icelandic spar. Their unusualness lay in the fact that if a piece of Icelandic spar is put on any inscription, then through it the inscription will be seen bifurcated.
In 1669, the Danish scientist Bartholin reported interesting results from his experiments with Icelandic spar crystals. When passing through such a crystal, the beam splits into two (Fig. 2.6.1). These rays are named ordinary beam and extraordinary beam, and the phenomenon itself birefringence.
An ordinary ray obeys the ordinary law of refraction, and an extraordinary ray does not obey this law. The rays split in two even when they were normally incident on a crystal of Icelandic spar. If the crystal is rotated relative to the direction of the original beam, then both beams that have passed through the crystal are rotated. Bartholin also discovered that there is a certain direction in the crystal along which the incident beam does not split. However, he could not explain these phenomena.
A few years later, this Bartholin discovery attracted the attention of Huygens, who introduced the concept optical axis of the crystal(Bartolin actually discovered it).
The optical axis of the crystal called the selected direction in the crystal, along which the ordinary and extraordinary rays propagate without separating.
In 1809, the French engineer E. Malus conducted an experiment with tourmaline crystals (transparent greenish crystals). In this experiment, light was successively passed through two identical tourmaline plates. If the second plate is rotated relative to the first, then the intensity of the light passing through the second plate changes from the maximum value to zero (Fig. 2.6.2). Light intensity dependence I from the corner j between the optical axes of both plates has the form:
(Malus' law ), (2.6.1)
where I 0 is the intensity of the incident light.
Rice. 2.6.3 a. Rice. 2.6.3 b.
Neither double refraction nor Malus' law can be explained within the framework of the theory of longitudinal light waves. For longitudinal waves, the direction of propagation of the beam is the axis of symmetry. In a longitudinal wave, all directions in a plane perpendicular to the beam are equal.
To understand how a transverse wave behaves, consider a wave traveling along a cord in a vertical plane. If a box with a vertical slot is placed in the path of this wave (Fig. 2.6.3 a), then the wave passes freely through the slot. If the slot in the box is located horizontally, then the wave no longer passes through it (Fig. 2.6.3 b). This wave is also called plane polarized, because vibrations in it occur in one (vertical) plane.
Experiments with crystals of Icelandic spar and tourmaline made it possible to prove that the light wave is transverse. T. Jung (1816) was the first to suggest that light waves are transverse. Fresnel, independently of Jung, also put forward the concept of transverse light waves, substantiated it with numerous experiments and created the theory of birefringence of light in crystals.
In the mid-60s of the XIX century, Maxwell came to the conclusion that light is an electromagnetic wave. This conclusion was made on the basis of the coincidence of the propagation velocity of electromagnetic waves, which is obtained from Maxwell's theory, with known value the speed of light. By the time Maxwell concluded that electromagnetic waves existed, the transverse nature of light waves had already been proven experimentally. Therefore, Maxwell believed that the transverseness of electromagnetic waves is another important proof of the electromagnetic nature of light.
In the electromagnetic theory of light, the difficulties associated with the need to introduce a special medium for the propagation of waves - the ether, which had to be considered as a solid body, also disappeared.
In an electromagnetic wave, the vectors and are perpendicular to each other and lie in a plane perpendicular to the direction of wave propagation. It is accepted that the plane in which the vector oscillates is called vibration plane, and the plane in which the oscillations of the vector occur, plane of polarization. Since in all processes of interaction of light with matter the main role is played by the electric field strength vector, it is called light vector. If, during the propagation of an electromagnetic wave, the light vector retains its orientation, such a wave is called linearly polarized or plane polarized.
Linearly polarized light is emitted by lasers. However, light emitted from ordinary sources (such as sunlight, incandescent lamps, etc.) is not polarized. This is due to the fact that atoms emit light in separate trains independently of each other. As a result, the vector in the resulting light wave randomly changes its orientation in time, so that, on average, all directions of oscillations are equal.
A light wave in which the direction of oscillation of the light vector changes chaotically in time is called natural or unpolarized light.
Natural light, passing through a crystal of Icelandic spar or tourmaline, is polarized. The phenomenon of double refraction of light is explained by the fact that in many crystalline substances the refractive indices for two mutually perpendicularly polarized waves are different. Therefore, the crystal bifurcates the rays passing through it (Fig. 2.6.1). Two beams at the output of the crystal are linearly polarized in mutually perpendicular directions. Crystals in which birefringence occurs are called anisotropic.
Light can become polarized when reflected or scattered. In particular, the blue light of the sky is partially or completely polarized. The polarization of reflected light was first observed by Malus when he looked through a crystal of Icelandic spar at the reflection of the setting sun in the windows of the Luxembourg Palace in Paris. Malus found that the reflected light is polarized to some extent. The degree of polarization of the reflected beam depends on the angle of incidence: at normal incidence, the reflected light is not completely polarized, and when incident at an angle called the angle of full polarization or the Brewster angle, the reflected beam is 100% polarized. When reflected at the Brewster angle, the reflected and refracted rays are perpendicular to each other (Fig. 2.5.4). The reflected beam is plane-polarized parallel to the surface.
Because , and , then the Brewster angle is found by the formula .
Polarized light is widely used in many areas of technology (for example, for smooth control of light, in the study of elastic stresses, etc.). The human eye does not distinguish the polarization of light, but the eyes of some insects, such as bees, perceive it.
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The purpose of the lesson
To form the concept of "natural and polarized light" among schoolchildren; to acquaint with the experimental proof of the transverseness of light waves; to study the properties of polarized light, to show the analogy between the polarization of mechanical, electromagnetic and light waves; report examples of the use of polaroids.
The lesson on the polarization of light is the final one in the topic "Wave Optics". In this regard, a lesson using computer simulation can be built as a lesson of generalizing repetition or part of the lesson can be assigned to solving problems on the topics "Interference of light", "Diffraction of light". We offer a model of the lesson in which new material on the topic "Polarization of light", and then the material studied is consolidated on computer model. In this lesson, it is easy to combine a real demonstration with a computer simulation, as the polaroids can be handed to the children and show the extinction of the light when one of the polaroids is rotated.
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Homework: § 74, task No. 1104, 1105.
Explanation of new material
The phenomena of interference and diffraction leave no doubt that propagating light has the properties of waves. But which waves - longitudinal or transverse?
For a long time, the founders of wave optics, Jung and Fresnel, considered light waves to be longitudinal, that is, similar to sound waves. At that time, light waves were considered as elastic waves in the ether that fills space and penetrates into all bodies. Such waves, it seemed, could not be transverse, since transverse waves can only exist in a solid body. But how can bodies move in solid ether without encountering resistance? After all, the ether should not impede the movement of bodies. Otherwise, the law of inertia would not hold.
However, gradually more and more experimental facts were accumulated, which could not be interpreted in any way, considering light waves to be longitudinal.
Experiments with tourmaline
Let us consider in detail only one of the experiments, very simple and effective. This is an experiment with tourmaline crystals (transparent green crystals).
Demonstrate to students how to turn off the light when two polaroids are rotated. Tourmaline crystal has an axis of symmetry and belongs to the so-called uniaxial crystals. Take a rectangular plate of tourmaline, cut in such a way that one of its faces is parallel to the axis of the crystal. If a beam of light from an electric lamp or the sun is directed normally to such a plate, then the rotation of the plate around the beam will not cause any change in the intensity of the light that has passed through it (see Fig.). You might think that the light was only partially absorbed in the tourmaline and acquired a greenish color. Nothing else happened. But it's not. The light wave has acquired new properties.
These new properties are revealed if the beam is forced to pass through a second, exactly the same tourmaline crystal (see Fig. a), parallel to the first. With identically directed axes of the crystals, again, nothing interesting happens: the light beam is simply further weakened due to absorption in the second crystal. But if the second crystal is rotated, leaving the first motionless (Fig. b), then an amazing phenomenon will be revealed - the extinction of light. As the angle between the axes increases, the light intensity decreases. And when the axes are perpendicular to each other, the light does not pass at all (Fig. c). It is completely absorbed by the second crystal. How can this be explained?
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Transverse light waves
From the experiments described above, two facts follow: firstly, that the light wave coming from the light source is completely symmetrical with respect to the direction of propagation (during the rotation of the crystal around the beam in the first experiment, the intensity did not change) and, secondly, that the wave that emerged from of the first crystal does not have axial symmetry (depending on the rotation of the second crystal relative to the beam, this or that intensity of the transmitted light is obtained).
Longitudinal waves have complete symmetry with respect to the direction of propagation (oscillations occur along this direction, and it is the axis of symmetry of the wave). Therefore, it is impossible to explain the experiment with the rotation of the second plate, assuming that the light wave is longitudinal.
A complete explanation of the experience can be obtained by making two assumptions.
The first assumption concerns the light itself. Light is a transverse wave. But in a beam of waves incident from a conventional source, there are oscillations of various directions perpendicular to the direction of wave propagation (see Fig.).
Demonstrate that natural light contains vibrations in all planes.
According to this assumption, a light wave has axial symmetry, while being transverse at the same time. Waves, for example, on the surface of water do not have such symmetry, since the vibrations of water particles occur only in the vertical plane.
A light wave with vibrations in all directions perpendicular to the direction of propagation is called natural. This name is justified because normal conditions light sources create just such a wave. This assumption explains the result of the first experiment. The rotation of the tourmaline crystal does not change the intensity of the transmitted light, since the incident wave has axial symmetry (despite the fact that it is transverse).
The second assumption to be made concerns the crystal. Tourmaline crystal has the ability to transmit light waves with vibrations lying in one specific plane (plane P in the figure).
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On the computer model "Law of Malus"
Demonstrate that a tourmaline crystal highlights only one plane of light vibrations. By rotating the polarizer and then the analyzer, it can be shown that the intensity of the transmitted light changes from a maximum value to zero. To extinguish the light, the angle between the axes of the polaroids must be 90 °. If the axes of the polaroids are parallel, then the second polaroid transmits all the light that has passed through the first one.
Such light is called polarized, or more precisely, plane polarized, as opposed to natural light, which may also be called unpolarized. This assumption fully explains the results of the second experiment. A plane-polarized wave emerges from the first crystal. With crossed crystals (the angle between the axes is 90°), it does not pass through the second crystal. If the axes of the crystals make an angle different from 90° to each other, then oscillations pass, the amplitude of which is equal to the projection of the amplitude of the wave that passed through the first crystal onto the direction of the axis of the second crystal.
So, a tourmaline crystal converts natural light into plane polarized light.
Mechanical model of experiments with tourmaline
It is not difficult to construct a simple illustrative mechanical model of the phenomenon under consideration. It is possible to create a transverse wave in a rubber cord so that the vibrations quickly change their direction in space. This is an analogue of a natural light wave. Let us now pass the cord through the narrow wooden box(see fig.). From the vibrations of all possible directions, the box "separates" the vibrations in one definite plane. Therefore, a polarized wave comes out of the box.
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If on its way there is still exactly the same box, but rotated by 90 ° relative to the first one, then the oscillations do not pass through it. The wave is completely extinguished.
If there is a mechanical model of polarization in the office, you can demonstrate it. If there is no such model, then this model can be illustrated with fragments of the video film “Polarization”.
Polaroids
Not only tourmaline crystals are able to polarize light. The same property, for example, have the so-called polaroids. A polaroid is a thin (0.1 mm) film of herapatite crystals deposited on a celluloid or glass plate. With a polaroid, you can do the same experiments as with a tourmaline crystal. The advantage of polaroids is that you can create large surfaces that polarize light. The disadvantages of polaroids are purple hue, which they give to white light.
Direct experiments have shown that the light wave is transverse. In a polarized light wave, oscillations occur in a strictly defined direction.
In conclusion, we can consider the use of polarization in technology and illustrate this material with fragments of the video film "Polarization".
Diffraction and interference of light confirms the wave nature of light. But waves can be longitudinal and transverse. Consider the following experience.
Light polarization
Let us pass a beam of light through a rectangular tourmaline plate, one of the faces of which is parallel to the crystal axis. There were no visible changes. The light was only partially extinguished in the plate and acquired a greenish color.
picture
Now after we place another plate after the first one. If the axes of both plates are aligned, nothing will happen. But if the second crystal starts to rotate, then the light will be extinguished. When the axes are perpendicular, there will be no light at all. It will be completely absorbed by the second plate.
picture
Let's make two conclusions:
1. The wave of light is symmetrical with respect to the direction of propagation.
2. After passing through the first crystal, the wave ceases to have axial symmetry.
This cannot be explained from the point of view of longitudinal waves. Therefore, light is a transverse wave. The tourmaline crystal is a polaroid. It transmits light waves, the oscillations of which occur in one plane. This property is well illustrated in the following figure.
picture
Transverse light waves and electromagnetic theory of light
The light that is produced after passing through a polaroid is called plane polarized light. In polarized light, vibrations occur only in one direction - the transverse direction.
The electromagnetic theory of light originates in the work of Maxwell. In the second half of the 19th century, Maxwell theoretically proved the existence of electromagnetic waves that can propagate even in a vacuum.
And he suggested that light is also an electromagnetic wave. The electromagnetic theory of light is based on the fact that the speed of light and the speed of propagation of electromagnetic waves are the same.
By the end of the 19th century, it was finally established that light waves arise from the movement of charged particles in atoms. With the recognition of this theory, the need for a luminiferous ether, in which light waves propagate, has disappeared. light waves These are not mechanical, but electromagnetic waves.
Oscillations of a light wave consist of oscillations of two vectors: the intensity vector and the magnetic induction vector. The direction of oscillations of the electric field strength vector is considered to be the direction of oscillations in light waves.
