Lesson topic:
LIGHT AS AN ELECTROMAGNETIC WAVE
The purpose of the lesson : Generalize knowledge on the topic "Geometric and wave optics"; promote awareness of the wave nature of light; continue the formation of the ability to apply theoretical knowledge to explain natural phenomena; to promote the formation of interest in physics; contribute to the development of independent cognitive activity, enrichment vocabulary scientific terminology, to show that science is closely intertwined with art.
During the classes
Theories of the origin and propagation of light began to exist in the 17th century. The first theory is corpuscular. According to its provisions, light is a stream of particles (corpuscles) that move from a source to different sides. The second theory is the wave theory. Light is a wave.
The following examples were given as proof of the wave theory of light:
1. Intersecting light beams do not affect each other.
2. If light is a stream of particles, why does the mass of a luminous object (the Sun) not decrease?
As proof of the corpuscular theory of light, the formation of a shadow was described: particles reach an obstacle and do not pass through it. A shadow is formed.
At the beginning of the 20th century it was proved that when emitting and absorbing light behaves like a stream of particles, when propagating like electromagnetic wave.
A light wave has the following properties:
1.Velocity of propagation in a vacuum
2. In an optically homogeneous medium, light propagates in a straight line. The rectilinearity of the propagation of light explains the shadows and penumbra.
3. The angle of incidence of a light beam is equal to the angle of its reflection. The incident and reflected rays, as well as the perpendicular reconstructed at the point of incidence, lie in the same plane. (Law of reflection of light).
4. The incident and refracted beams, as well as the perpendicular to the interface between two media, restored at the point of incidence of the beam, lie in the same plane. The ratio of the sine of the angle of incidence α to the sine of the angle of refraction β is a constant for the two given media. It is called the relative refractive index. (Law of refraction of light).
5. When a beam passes at a certain angle through the interface between two media, white light can be decomposed into color components (into a spectrum). This phenomenon is called dispersion.
6. Two light waves may form. In this case, an increase or decrease in the resulting oscillation is observed. The phenomenon is called interference. The screen shows alternating light and dark bands. The phenomenon of interference was discovered in 1802. The waves must be coherent, i.e. have the same frequency and phase
Diffraction
Diffraction of light is the phenomenon of deviation of light from the rectilinear direction of propagation when passing near obstacles. During diffraction, light waves bend around the boundaries of opaque bodies and can penetrate into the region of a geometric shadow.
Home building: paragraphs 58, 59.
Preparing for control work on the topic "Electromagnetic field". Repeat paragraphs 42-59
Gymnasium 144
abstract
The speed of light.
Light interference.
standing waves.
11th grade student
Korchagin Sergey
St. Petersburg 1997.
Light is an electromagnetic wave.
In the 17th century, two theories of light arose: wave and corpuscular. The corpuscular 1 theory was proposed by Newton, and the wave theory by Huygens. According to Huygens, light is waves propagating in a special medium - ether, which fills all space. The two theories have existed side by side for a long time. When one of the theories did not explain a phenomenon, it was explained by another theory. For example, the rectilinear propagation of light, leading to the formation of sharp shadows, could not be explained on the basis of wave theory. However, in early XIX centuries, such phenomena as diffraction 2 and interference 3 were discovered, which gave rise to thoughts that the wave theory finally defeated the corpuscular one. In the second half of the 19th century, Maxwell showed that light is a special case of electromagnetic waves. These works served as the foundation for the electromagnetic theory of light. However, at the beginning of the 20th century, it was discovered that when emitted and absorbed, light behaves like a stream of particles.
The speed of light.
There are several ways to determine the speed of light: astronomical and laboratory methods.
The speed of light was first measured by the Danish scientist Roemer in 1676 using the astronomical method. He recorded the time that the largest of Jupiter's moons, Io, was in the shadow of this huge planet. Roemer took measurements at the moment when our planet was closest to Jupiter, and at the moment when we were a little (according to astronomical terms) farther from Jupiter. In the first case, the interval between outbreaks was 48 hours 28 minutes. In the second case, the satellite was late by 22 minutes. From this it was concluded that the light needs 22 minutes to travel the distance from the place of the previous observation to the place of the present observation. Knowing the distance and time delay of Io, he calculated the speed of light, which turned out to be huge, about 300,000 km/s 4 .
For the first time, the speed of light was measured by the laboratory method by the French physicist Fizeau in 1849. He obtained the value of the speed of light equal to 313,000 km/s.
According to modern data, the speed of light is 299,792,458 m/s ±1.2 m/s.
Light interference.
It is rather difficult to obtain a picture of the interference of light waves. The reason for this is that the light waves emitted by different sources are not consistent with each other. They must have the same wavelengths and a constant phase difference at any point in space 5 . Equality of wavelengths is not difficult to achieve using light filters. But it is impossible to achieve a constant phase difference, due to the fact that atoms of different sources emit light independently of each other 6 .
Nevertheless, the interference of light can be observed. For example, iridescent overflow of colors on a soap bubble or on a thin film of kerosene or oil on water. The English scientist T. Jung was the first to come to the brilliant idea that color is explained by the addition of waves, one of which is reflected from the outer surface, and the other from the inner one. In this case, interference of 7 light waves occurs. The result of interference depends on the angle of incidence of light on the film, its thickness and wavelength.
standing waves.
It was noticed that if one end of the rope is swung with a correctly selected frequency (its other end is fixed), then a continuous wave will run to the fixed end, which will then be reflected with the loss of a half-wave. The interference of the incident and reflected wave will result in a standing wave that appears to be stationary. The stability of this wave satisfies the condition:
L=nl/2, l=u/n, L=nu/n,
Where L * is the length of the rope; n * 1,2,3, etc.; u * is the speed of wave propagation, which depends on the tension of the rope.
Standing waves are excited in all bodies capable of oscillating.
The formation of standing waves is a resonant phenomenon that occurs at the resonant or natural frequencies of the body. Points where interference is canceled are called nodes, and points where interference is enhanced are antinodes.
Light ѕ electromagnetic wave………………………………………..2
The speed of light…………………………………………………………2
Light interference………………………………………………….3
Standing waves……………………………………………………………3
Physics 11 (G.Ya. Myakishev B.B. Lukhovtsev)
Physics 10 (N.M. Shakhmaev S.N. Shakhmaev)
Base notes and test items(G.D. Luppov)
1 The Latin word “corpuscle” translated into Russian means “particle”.
2 Rounding obstacles with light.
3 The phenomenon of amplification or attenuation of light when superimposing light beams.
4 Roemer himself received a value of 215,000 km/s.
5 Waves having the same length and constant phase difference are called coherent.
6 The only exceptions are quantum light sources - lasers.
7 The addition of two waves, as a result of which there is a time-stable amplification or weakening of the resulting light vibrations at various points in space.
1. Laws of refraction and reflection of light. 2. Interference and its application. 3. Diffraction. 4. Dispersion. 5. Polarization. 6. Corpuscular-wave dualism.
Light- these are electromagnetic waves in the frequency range 63 10 14 - 8 10 14 Hz, perceived by the human eye, i.e. wavelengths in the range 380 - 770 nm.
Light has all the properties of electromagnetic waves: reflection, refraction, interference, diffraction, polarization. Light can exert pressure on a substance, be absorbed by the medium, and cause the phenomenon of the photoelectric effect. It has a finite propagation velocity in a vacuum of 300,000 km/s, while in a medium the velocity decreases.
The wave properties of light are most clearly revealed in the phenomena of interference and diffraction. . interference light is called the spatial redistribution of the luminous flux when two (or several) coherent light waves are superimposed, as a result of which maxima appear in some places, and intensity minima in others (interference pattern). Light interference explains the color of soap bubbles and thin oil films on water, although the soap solution and oil are colorless. Light waves are partially reflected from the surface of a thin film, and partially pass into it. At the second boundary of the film, a partial reflection of the wave occurs again (Fig. 34). Light waves reflected by two surfaces of a thin film travel in the same direction but travel different paths. With a travel difference I integer multiple of wavelengths l = 2 kλ/2.
With a path difference that is a multiple of an odd number of half-waves l = (2 k+ 1) λ/2, an interference minimum is observed. When the maximum condition is satisfied for one wavelength of light, it is not satisfied for other wavelengths. Therefore, illuminated with white light, thin colored transparencies appears to be colored. The phenomenon of interference in thin films is used to control the quality of the surface treatment of optical coatings. When light passes through a small round hole alternating dark and light rings are observed on the screen around the central bright spot; if light passes through a narrow slit, then a pattern is obtained of alternating light and dark bands.
The phenomenon of light deflection from the rectilinear direction of propagation when passing at the edge of an obstacle is called light diffraction. Diffraction is explained by the fact that the light waves coming as a result of deviation from different points of the hole to one point on the screen interfere with each other. Light diffraction is used in spectral instruments, the main element of which is a diffraction grating. Diffraction grating It is a transparent plate with a system of parallel opaque stripes applied on it, located at equal distances from each other. Let monochromatic (of a certain wavelength) light fall on the grating (Fig. 35). As a result of diffraction at each slit, light propagates not only in the original direction,
but also in all other areas. If a converging lens is placed behind the grating, then on the screen in the focal plane all the rays will be collected in one strip.
Parallel rays coming from the edges of neighboring slots have a path difference l= d sin φ, where d - grating constant - the distance between the corresponding edges of adjacent slots, called grating period,(φ - angle of deviation of light rays from the perpendicular to the grating plane. With a path difference equal to an integer number of wavelengths d sin φ = kλ, an interference maximum is observed for a given wavelength. The interference maximum condition is satisfied for each wavelength at its own value of the diffraction angle φ. As a result, when passing through the diffraction grating, the white light beam decomposes into a spectrum. The diffraction angle matters most for red light, as red light has the longest wavelength in the visible region. The smallest diffraction angle for violet light.
Experience shows that the intensity of a light beam passing through some crystals, such as Icelandic spar, depends on the mutual orientation of the two crystals. With the same orientation of the crystals, light passes through the second crystal without attenuation.
If the second crystal is rotated by 90°, then light does not pass through it. There is a phenomenon polarization, i.e., the crystal transmits only such waves in which the oscillations of the intensity vector electric field occur in one plane, the plane of polarization. The phenomenon of polarization proves the wave nature of light and the transverse nature of light waves.
A narrow parallel beam of white light, when passing through a glass prism, decomposes into beams of light of different colors, with the violet rays having the greatest deviation to the base of the prism. The decomposition of white light is explained by the fact that white light consists of electromagnetic waves with different wavelengths, and the refractive index of light depends on its wavelength. The refractive index is related to the speed of light in the medium, therefore, the speed of light in the medium depends on the wavelength. This phenomenon is called dispersion of light.
Based on the coincidence of the experimentally measured value of the speed of electromagnetic waves, Maxwell suggested that light - it is an electromagnetic wave. This hypothesis is confirmed by the properties that light has.
A person perceives most of the information about the world around him through the organs of vision. But the eyes themselves are able to see only one type of energy - electromagnetic, and even then in a very narrow light range. So what is light? What are the famous sources visible radiation does a person use? What is the dual nature of light? And what are its main properties? Now let's find out the answers to these questions.
Light as an electromagnetic wave
Light is an electromagnetic wave that the human eye can see. To do this, the length of this wave should not go beyond the boundaries from 380-400 nm to 760-780 nm. After 780 nm, the infrared range begins, which a person can feel like heat, and before the visible spectrum goes ultraviolet radiation. Some insects and birds can see it, and human skin can react to it with a tan. The visible range electromagnetic radiation divided into segments, each of which a person perceives as light certain color. For example, violet corresponds to a wavelength of 380-440 nm, green - 500-565 nm, and red - 625-740 nm. In total, 7 primary colors of the visible spectrum are distinguished, they can be observed by looking at the rainbow. But white light is a mixture of all the colors of the spectrum.
Sources of light
The source of light is heated to a certain temperature or excited substance. Light comes to Earth from the Sun, other stars, some heated planets, comets and other celestial bodies. On our planet, the source of light can be fire - a bonfire, a candle flame, a torch or oil lamp, as well as heated matter. Man also invented artificial sources of visible radiation, in particular, an incandescent lamp, where light is emitted by a tungsten coil heated by an electric current, a fluorescent lamp, in which a phosphor layer glows, excited by an electric discharge in the gas filling the flask, a halogen lamp, mercury and others.
properties of light
Reflection
Visible electromagnetic radiation propagates in vacuum and homogeneous transparent media in a straight line with equal to approximately 300,000 km/s. At the same time, light has many other properties. For example, light is reflected from opaque surfaces, and the angle of incidence is equal to the angle of reflection. As a result, the light reflected from objects is perceived by the eye and allows you to see these objects. Also note that the Moon and some planets are not sources of light, but we see them because these celestial bodies reflect the radiation of the Sun.
Refraction
When passing between two media with different optical densities, light can be refracted. For example, when a beam passes from air into water, due to the different optical density of these media, the speed and direction of movement of light in them changes. That is why a spoon in a glass of water seems a little broken, and the pebbles at the bottom of the lake appear closer than they really are.
Interference and diffraction
The wave nature of light is manifested in its properties such as interference and diffraction. The first property is the ability of several waves to add up to the resulting wave, the parameters of which at different points noticeably increase or decrease. The result of light interference can be observed in the form of a game of iridescent stains on soap bubbles, oil stains or insect wings. And diffraction is the ability of a wave of light to bend around an obstacle and fall into the area of its geometric shadow, for example, the scattering of light on water droplets in the form of iridescent clouds.
Light as a stream of particles
At the same time, light has not only wave properties, but in some cases behaves like a stream of particles - photons. In particular, the patterns of the phenomenon of the photoelectric effect, when light incident on a substance pulls out electrons from it, can only be explained from the point of view of the corpuscular theory of light, which represents electromagnetic radiation in the form of a stream of photons. However, the wave and photon theories of light not only do not contradict each other, but complement each other. In the scientific community, they talk about the corpuscular-wave duality of light, which explains what light is, reveals its properties as a wave and as a stream of particles.
It took quite a bit of time from the moment of the discovery of electromagnetic oscillations to understand that light is also a combination of electromagnetic oscillations - only very high-frequency ones. It is no coincidence that the speed of light is equal to the speed of propagation of electromagnetic waves and is characterized by a constant c = 300,000 km/s.
The eye is the main human organ that perceives light. In this case, the wavelength of light vibrations is perceived by the eye as the color of light rays. In a school physics course, a description of the classical experiment on the decomposition of white light is given - it is enough to direct a rather narrow beam of white (for example, sunlight) light onto a glass prism with a triangular cross section, as it immediately splits into many light beams smoothly passing into each other different color. This phenomenon is due varying degrees refraction of light waves of various lengths.
In addition to the wavelength (or frequency), light vibrations are characterized by intensity. From a number of measures of the intensity of light radiation (brightness, luminous flux, illumination, etc.) when describing video devices, the most important is illumination. Without going into the subtleties of determining the light characteristics, we note that illumination is measured in lux and is a measure of the visual assessment of the visibility of objects that is familiar to us. Below are typical light levels:
- Illumination 20 cm from a burning candle 10-15 lux
- Illumination of the room with burning incandescent lamps 100 lux
- Office illumination with fluorescent lamps 300-500 lux
- Illumination generated by 750 lux halogen lamps
- Illumination at bright sunlight 20000lux and above
Light is widely used in communication technology. Suffice it to mention such applications of light as the transmission of information over optical fiber communication lines, the use of optical output for digitized audio signals in modern electroacoustic devices, the use of remote controls remote control infrared light, etc.
The electromagnetic nature of light Light has both wave properties and particle properties. This property of light is called corpuscular-wave dualism. But scientists and physicists of antiquity did not know about this, and initially considered light to be an elastic wave.
Light - waves in the ether But since the propagation of elastic waves requires a medium, a legitimate question arose, in what medium does light propagate? What medium is on the way from the Sun to the Earth? Proponents of the wave theory of light suggested that all space in the universe is filled with some invisible elastic medium. They even came up with a name for it - luminiferous ether. At that time, scientists did not yet know about the existence of any waves other than mechanical ones. Such views on the nature of light were expressed around the 17th century. It was believed that light propagates precisely in this luminiferous ether.
Light is a transverse wave But this assumption raises a number of controversial questions. By the end of the 18th century, it was proved that light is a transverse wave. And elastic transverse waves can arise only in solids, therefore, the luminiferous ether is solid. This caused a severe headache for the scientists of the time. How celestial bodies can move through the solid luminiferous ether, and at the same time not experience any resistance.
Light is an electromagnetic wave 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. Later this assumption was confirmed. But also relevant was the idea that in some cases light behaves like a stream of particles. Maxwell's theory contradicted some experimental facts. But, in 1990, physicist Max Planck hypothesized that atoms emit electromagnetic energy in separate portions - quanta. And in 1905, Albert Einstein put forward the idea that electromagnetic waves with a certain frequency can be considered as a flux of radiation quanta with energy E=p*ν. Currently, a quantum of electromagnetic radiation is called a photon. A photon has neither mass nor charge and always propagates at the speed of light. That is, during radiation and absorption, light exhibits corpuscular properties, and when moving in space, it exhibits wave properties.