Presentation on the topic "light as an electromagnetic wave." Light is like an electromagnetic wave. Speed of light. Interference of light: Jung's experience; thin film colors
At the end of the 17th century, two scientific hypotheses about the nature of light arose - corpuscular And wave.
According to the corpuscular theory, light is a stream of tiny light particles (corpuscles) that fly at enormous speed. Newton believed that the movement of light corpuscles obeys the laws of mechanics. Thus, the reflection of light was understood as similar to the reflection of an elastic ball from a plane. The refraction of light was explained by the change in the speed of particles when moving from one medium to another.
The wave theory viewed light as a wave process similar to mechanical waves.
According to modern ideas, light has a dual nature, i.e. it is simultaneously characterized by both corpuscular and wave properties. In phenomena such as interference and diffraction, the wave properties of light come to the fore, and in the phenomenon of the photoelectric effect, the corpuscular ones.
Light as electromagnetic waves
In optics, light means electromagnetic waves fairly narrow range. Often, light is understood not only as visible light, but also in the broad spectrum regions adjacent to it. Historically, the term “invisible light” appeared - ultraviolet light, infrared light, radio waves. Visible light wavelengths range from 380 to 760 nanometers.
One of the characteristics of light is its color, which is determined by the frequency of the light wave. White light is a mixture of waves of different frequencies. It can be decomposed into colored waves, each of which is characterized by a specific frequency. Such waves are called monochromatic.
Speed of light
According to the newest measurements, the speed of light in a vacuum
Measurements of the speed of light in various transparent substances have shown that it is always less than in a vacuum. For example, in water the speed of light decreases by 4/3 times.
Gymnasium 144
Essay
Speed of light.
Interference of light.
Standing waves.
11th grade student
Korchagin Sergei
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' ideas, light is waves propagating in a special medium - ether, filling all space. The two theories existed in parallel 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 based on the wave theory. However, in early XIX century, such phenomena as diffraction 2 and interference 3 were discovered, which gave rise to the idea that the wave theory finally defeated the corpuscular theory. 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 light is emitted and absorbed, it behaves like a stream of particles.
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 timed 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 (in astronomical terms) further from Jupiter. In the first case, the interval between outbreaks was 48 hours 28 minutes. In the second case, the satellite was 22 minutes late. From this it was concluded that light needed 22 minutes to travel the distance from the previous observation to the present observation. Knowing the distance and delay time of Io, he calculated the speed of light, which turned out to be enormous, approximately 300,000 km/s 4 .
For the first time, the speed of light was measured by a laboratory method by the French physicist Fizeau in 1849. He obtained a value for 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.
Interference of light.
It is quite 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 easy to achieve using light filters. But it is impossible to achieve a constant phase difference, due to the fact that atoms from different sources emit light independently of each other 6 .
Nevertheless, the interference of light can be observed. For example, a rainbow 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 outer surface, and the other is 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 you swing one end of the rope with a correctly selected frequency (its other end is fixed), then a continuous wave will run towards the fixed end, which will then be reflected with the loss of a half-wave. Interference between the incident and reflected waves will result in a standing wave that appears 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 a body. The points where interference is canceled out are called nodes, and the points where interference is enhanced are called antinodes.
Light is an electromagnetic wave……………………………………..2
Speed of light……………………………………………………2
Interference of light…………………………………………………………….3
Standing waves………………………………………………………3
Physics 11 (G.Ya.Myakishev B.B.Bukhovtsev)
Physics 10 (N.M.Shakhmaev S.N.Shakhmaev)
Supporting notes and test tasks(G.D. Luppov)
1 The Latin word “corpuscle” translated into Russian means “particle”.
2 Light bends around obstacles.
3 The phenomenon of strengthening or weakening of light when light beams are superimposed.
4 Roemer himself obtained a value of 215,000 km/s.
5 Waves that have the same lengths and a 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 a time-sustained intensification or weakening of the resulting light vibrations is observed at different points in space.
Light is an electromagnetic wave. At the end of the 17th century, two scientific hypotheses about the nature of light arose - corpuscular And wave. According to the corpuscular theory, light is a stream of tiny light particles (corpuscles) that fly at enormous speed. Newton believed that the movement of light corpuscles obeys the laws of mechanics. Thus, the reflection of light was understood as similar to the reflection of an elastic ball from a plane. The refraction of light was explained by the change in the speed of particles when moving from one medium to another. The wave theory viewed light as a wave process similar to mechanical waves. According to modern ideas, light has a dual nature, i.e. it is simultaneously characterized by both corpuscular and wave properties. In phenomena such as interference and diffraction, the wave properties of light come to the fore, and in the phenomenon of the photoelectric effect, the corpuscular ones. In optics, light refers to electromagnetic waves of a fairly narrow range. Often, light is understood not only as visible light, but also in the broad spectrum regions adjacent to it. Historically, the term “invisible light” appeared - ultraviolet light, infrared light, radio waves. Visible light wavelengths range from 380 to 760 nanometers. One of the characteristics of light is its color, which is determined by the frequency of the light wave. White light is a mixture of waves of different frequencies. It can be decomposed into colored waves, each of which is characterized by a specific frequency. Such waves are called monochromatic. According to the newest measurements, the speed of light in a vacuum. The ratio of the speed of light in a vacuum to the speed of light in matter is called absolute refractive index substances.
When a light wave passes from vacuum to matter, the frequency remains constant (color does not change). Wavelength in a medium with a refractive index n changes:
Interference of light- Jung's experience. Light from a light bulb with a light filter, which creates almost monochromatic light, passes through two narrow, adjacent slits, behind which a screen is installed. A system of light and dark stripes - interference stripes - will be observed on the screen. In this case, a single light wave is split into two, coming from different slits. These two waves are coherent with each other and, when superimposed on each other, give a system of maxima and minima of light intensity in the form of dark and light stripes of the corresponding color.
Interference of light- max and min conditions. Maximum condition: If the optical difference in the wave path contains an even number of half-waves or an integer number of waves, then an increase in light intensity (max) is observed at a given point on the screen. , where is the phase difference of the added waves. Minimum condition: If the optical difference in the wave path contains an odd number of half-waves, then there is a minimum at the point.
In the case of constant currents or charge distributions that slowly change with time, the conclusions from Maxwell's equations are practically no different from the conclusions from those equations of electricity and magnetism that existed before Maxwell introduced the displacement current. However, if the currents or charges change with time, especially if they change very quickly, as in the case of two balls, for example, where the charge rushes from ball to ball (Fig. 351), Maxwell's equations allow solutions that did not exist before.
Consider a magnetic field generated by a current (say, flowing through a wire). Now imagine that the chain is broken. As the current decreases, the magnetic field surrounding the wire also decreases, and therefore an electric field is excited (according to Faraday's law, an alternating magnetic field excites an electric field). When the rate of change magnetic field decreases, the electric field begins to decrease. In accordance with pre-Maxwellian ideas, nothing else happens: the electric and magnetic fields disappear when the current goes to zero, since it was believed that an alternating electric field does not produce any effect.
However, from Maxwell's theory it follows that a falling electric field excites a magnetic field in the same way that a falling magnetic field excites an electric field, and that these fields are combined in such a way that when one of them decreases, the other appears
a little further from the source, and as a result the entire impulse moves through space as a whole. If the value of B is equal to the value of E and these two vectors are mutually perpendicular, then, as follows from Maxwell’s equations, the impulse must propagate in space at a certain speed.
This impulse has all the properties that we previously characterized wave motion. If we have not one, but a lot of impulses caused, for example, by oscillations of electric charges between two balls, then a certain wavelength can be associated with such a set of impulses, i.e., the distance between adjacent ridges. Pulses propagate from point to point just like a wave. And, what is especially important, the main principle is fulfilled, namely the principle of superposition, since electric and magnetic fields have additive properties. Thus, the movement of electric and magnetic pulses is characterized by wave properties.
Let us again consider the planetary system of charged particles (Fig. 352). According to Maxwell's theory, a charged particle (in particular, an electron) moving in a circular orbit (like any particle that has acceleration) excites an electromagnetic wave.
The frequency of this wave is equal to the frequency of the electron's orbital rotation. Using the numerical values obtained in Chap. 19, we find
From the relationship between frequency and wavelength we have
As a result
Let us assume, for example, that the speed of wave propagation is cm/s. Then
This is the wavelength of ultraviolet radiation, which is radiation with a shorter wavelength than violet light. (The minimum wavelength of visible light is on the order of cm.)
A planetary system of charged particles emits electromagnetic waves, i.e., it loses energy (the waves carry energy with them, since they are able to do work on charges located far from the source), and therefore, for its stable existence, additional energy must be pumped in from the outside.
When Maxwell realized that his equations allowed such a solution, he calculated the speed with which the wave must travel through space. He's writing:
“The speed of transverse wave oscillations in our hypothetical environment, calculated from electromagnetic experiments Kohlrausch and Weber, coincides so exactly with the speed of light calculated from Fizeau's optical experiments that we can hardly refuse the conclusion that light consists of transverse vibrations of the same medium that is the cause of electrical and magnetic phenomena."
“I obtained my equations while living in the provinces and not suspecting the proximity of the speed of propagation of magnetic effects I found to the speed of light, so I think that I have every reason to consider the magnetic and luminiferous mediums as the same medium...”.
[It was much more difficult for Maxwell to obtain his famous result than we might think. For convenience, we introduced the letter c, denoting the speed of light, in order to connect changes in the magnetic field with the electric field it excites, replacing a rather arbitrary number with the quantity. Then we used the same quantity c to describe the relationship between the magnetic field and the currents and variables that excite it electric fields. According to Ampere's law, the measured circulation of the magnetic field must be proportional to the measured value of the current flowing through the surface. It turned out, for example, that
where the number in the CGS system is taken from actual measurements of the magnetic field and current flowing through the surface. When Maxwell considered these equations together and found a solution corresponding to the propagation of momentum electromagnetic radiation,
he obtained from these measured numbers another number, which gave the speed of propagation of this impulse. And this number turned out to be approximately cm/s. But the number cm/s is the measured value of the speed of light. That's why Maxwell identified the radiation pulse with light itself. He wrote:
“...we have good reason to conclude that light itself (including radiant heat and other radiations) is an electromagnetic disturbance in the form of waves propagating through an electromagnetic field according to the laws of electromagnetism.”
Fig. 353. The figure shows the solution of Maxwell's equations corresponding to a wave propagating in a vacuum at the speed of light. Vectors E and B are mutually perpendicular and equal in magnitude. Both pulses and periodic solutions corresponding to waves of a given length are possible. Vacuum is a medium without dispersion, i.e. in it all periodic waves propagate at the same speeds.
There was general surprise, but there were also doubters. Thus, one of the letters to Maxwell said:
“The agreement between the observed speed of light and the speed of transverse vibrations in your medium calculated by you seems to be an excellent result. However, it seems to me that such results are not desirable until you convince people that whenever electricity, a small row of particles squeezes between two rows of rotating wheels."
After light was identified with an electromagnetic wave [ various colors correspond to different frequencies (Fig. 354), or wavelengths of radiation, with visible light making up only a small part of the full spectrum of electromagnetic radiation] and since the interactions of electric and magnetic fields with charged particles (Lorentz formula) were known, it was possible for the first time to create a theory of the interaction of light with matter (if we assume that media consist of charged particles). For example, after the publication of Maxwell’s work, Lorentz and Fitzgerald, trying to show the similarity between the behavior of an electromagnetic wave and the behavior of light during its reflection and refraction, calculated the case of transmission
electromagnetic wave across the boundary of two media; It turned out that the behavior of this wave coincides with the observed behavior of light.
Even if Maxwell had failed to identify electromagnetic radiation with light, his discovery would still have had great value. To see this, remember that an electric field can do work on a charge. Consequently, a charge oscillating at one point in space generates an electromagnetic pulse, which is capable of spreading to any desired distance from the moving charge and the electric field of which can do work on another charge there.
Fig. 354. Spectrum electromagnetic vibrations. X-rays, visible light, radio waves, etc. are all electromagnetic waves with different wavelengths. Visible light differs from “invisible” only in that the latter is not perceived by the human eye.
Not much water has passed under the bridge since it was first possible to transmit electrical energy through wires in order to perform work away from the generators that produce the current. Now Maxwell proposed transmitting energy over long distances without the help of any wires, capable of doing work on distant charged bodies. In addition, using controlled changes in such an electromagnetic wave, it is possible to transmit information that can be easily deciphered at any remote point. This conclusion could not but have important practical consequences.
It took very little time since the discovery of electromagnetic oscillations to understand that light is also a set 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 school course physics provides a description of the classical experiment on the decomposition of white light - as soon as a fairly narrow beam of white (for example, solar) light is directed at a glass prism with a triangular cross-section, it immediately stratifies into many light beams smoothly transitioning into each other different color. This phenomenon is due varying degrees refraction of light waves of different lengths.
In addition to wavelength (or frequency), light vibrations are characterized by intensity. Of the 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 intricacies of determining light characteristics, we note that illumination is measured in lux and is a familiar measure for us to visually assess the visibility of objects. Below are typical light levels:
- Illumination 20 cm from a burning candle 10-15 lux
- Room illumination with incandescent lamps burning 100 lux
- Office illumination with fluorescent lamps 300-500 lux
- Illumination created by halogen lamps 750 lux
- Illumination in bright sunlight 20000lux and above
Light is widely used in communication technology. It is enough to note such applications of light as the transmission of information via fiber-optic communication lines, the use of an optical output for digitized audio signals in modern electro-acoustic devices, the use of remote controls using a beam of infrared light, etc.
Electromagnetic nature of light Light has both wave properties and particle properties. This property of light is called wave-particle duality. 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 spreads precisely in this luminiferous ether.
Light is a transverse wave But such an assumption raised a number of controversial questions. By the end of the 18th century, it was proven that light is a transverse wave. And elastic transverse waves can arise only in solid bodies, therefore, the luminiferous ether is solid body. This caused a strong headache among the scientists of that time. How celestial bodies can move through solid luminiferous ether, and at the same time experience no 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. Then 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 flow 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 travels at the speed of light. That is, when emitted and absorbed, light exhibits corpuscular properties, and when moving in space, it exhibits wave properties.