(including light); one of the funds. physical permanent; represents the limiting speed of propagation of any physical. influences (cf. Relativity theory) and is invariant upon transition from one frame of reference to another.
S. s. in the environment With" depends on the refractive index of the medium n, which is different for different frequencies v ( Light dispersion):. This dependence leads to a difference group velocity from phase velocity light in the environment, if we are not talking about monochromatic. light (for S. of page in vacuum these two sizes coincide). Experimentally determining With", always measure group S. with. or so-called. signal speed, or the rate of energy transfer, only in some special. cases not equal to the group.
For the first time S. with. determined in 1676 by O. K. Roemer (O. Ch. Roemer) by changing the time intervals between eclipses of Jupiter's satellites. In 1728, it was established by J. Bradley, based on his observations of the aberration of starlight. In 1849, A. I. L. Fizeau (A. N. L. Fizeau) was the first to measure S. s. by the time it takes the light to pass a precisely known distance (base); since the refractive index of air differs very little from 1, ground-based measurements give a value very close to s. In Fizeau's experiment, a beam of light from a source S(Fig. 1), reflected by a translucent mirror N, periodically interrupted by a rotating toothed disk W, passed the base MN(approx. 8 km) n, reflected from the mirror M, returned to disk. Getting on the tooth, the light did not reach the observer, and the light that fell into the gap between the teeth could be observed through the eyepiece E. From the known speeds of rotation of the disk, the time for light to travel through the base was determined. Fizeau obtained the value c = 313,300 km/s. c) a mirror. Reflecting from the mirror, the beam of light was directed to the base and, upon returning, fell again on the same mirror, which had time to turn through a certain small angle (Fig. 2). With a base of only 20 m, Foucault found that S. s. is equal to 298000 500 km/s. Schemes and basic. the ideas of the experiments of Fizeau and Foucault were repeatedly used in subsequent works to determine S. s. Obtained by A. Michelson (see. michelson experience) in 1926, the value of km / s was then the most accurate and was included in the international. physical tables. quantities.
Rice. 1. Determination of the speed of light by the Fizeau method.
Rice. 2. Determination of the speed of light by the rotating mirror method (Foucault method): S - light source; R - rapidly rotating mirror; C is a fixed concave mirror, the center of which coincides with the axis of rotation R (therefore, the light reflected from C always falls back on R); M - translucent mirror; L - lens; E - eyepiece; RC - accurately measured distance (base). The dotted line shows the position R, which has changed during the time the light travels the path RC and back, and the return path of the beam of rays through the lens L, which collects the reflected beam at point S "and not again at point S, as it would be with a fixed mirror L. Velocity lights are set by measuring the offset SS".
S.'s measurements with. in the 19th century played a big role in, further confirming the wave theory of light. Foucault's 1850 comparison of S. s. the same frequency v in air and water showed that the speed in water is in accordance with the prediction of the wave theory. A connection was also established between optics and the theory of electromagnetism: the measured S. s. coincided with the speed of e-magn. waves calculated from the ratio of e-mag. and e-static. units of electric charge [experiments by W. Weber and F. Kohlrausch in 1856 and subsequent more accurate measurements by J. C. Maxwell]. This coincidence was one of the starting points for the creation by Maxwell in 1864-73 of el-magn. theories of light.
In modern S.'s measurements with. modernized is used. Fizeau's method (modulation. method) with the replacement of a gear wheel with an el-optical, ., interference or to-l. another light modulator that completely interrupts or attenuates the light beam (see. Light modulation). The radiation receiver is a photocell or photomultiplier.Application laser as a light source, ultrasonic modulator with stabilizers. frequency and increased accuracy of measuring the length of the base made it possible to reduce measurement errors and obtain the value of km/s. In addition to direct measurements of S. s. according to the time of passage of a known base, indirect methods are widely used, which give greater accuracy. So, with the help of a microwave vacuum cleaner. [TO. Frum (K. Froome), 1958] at a wavelength of radiation = 4 cm, the value of km/s was obtained. With an even smaller error, S. s is determined. as a quotient of the division of independently found and v atomic or molecular spectral lines. K. Evenson (K. Evenson) and his staff in 1972 on the cesium frequency standard (see. Quantum frequency standards) found, with an accuracy of up to 11 decimal places, the frequency of the CH 4 laser radiation, and according to the krypton frequency standard, its wavelength (about 3.39 μm) and obtained ± 0.8 m / s. By the decision of the General Assembly of the International Committee on Numerical Data for Science and Technology - CODATA (1973), which analyzed all available data, their reliability and error, S. s. in vacuum it is considered to be equal to 299792458 ±1.2 m/s.
The most accurate measurement of c is extremely important not only in general theoretical. plan and to determine the value of other physical. quantities, but also for practical goals. These include, in particular, the determination of distances by the time of passage of radio or light signals in radar, optical location, light ranging, in satellite tracking systems, etc.
Lit.: Vafiadi V. G., Popov Yu. V., speed of light and its importance in science and technology, Minsk, 1970; Taylor W., Parker W., Langenberg D., Fundamental constants and quantum theory, trans. from English, M., 1972. A. M. Bonch-Bruevich.
The speed of light is the most unusual measurement known to date. The first person who tried to explain the phenomenon of light propagation was Albert Einstein. It was he who deduced the well-known formula E = mc² , where E is the total energy of the body, m is the mass, and c is the speed of light in vacuum.
The formula was first published in Annalen der Physik in 1905. Around the same time, Einstein put forward a theory about what would happen to a body moving at absolute speed. Based on the fact that the speed of light is a constant value, he came to the conclusion that space and time must change.
Thus, at the speed of light, an object will shrink indefinitely, its mass will increase indefinitely, and time will practically stop.
In 1977, it was possible to calculate the speed of light, a figure of 299,792,458 ± 1.2 meters per second was named. For more rough calculations, a value of 300,000 km/s is always taken. It is from this value that all other cosmic measurements are repelled. This is how the concept of "light year" and "parsec" (3.26 light years) appeared.
Neither to move at the speed of light, nor, moreover, to overcome it is impossible. At least at this stage of human development. On the other hand, science fiction writers have been trying to solve this problem in the pages of their novels for about 100 years. Perhaps one day fantasy will become a reality, because back in the 19th century, Jules Verne predicted the appearance of a helicopter, an airplane and an electric chair, and then it was pure fantasy!
Really, how? How to measure the highest speed in Universe in our modest, Earthly conditions? We no longer need to puzzle over this - after all, for several centuries so many people have worked on this issue, developing methods for measuring the speed of light. Let's start the story in order.
speed of light is the propagation velocity of electromagnetic waves in vacuum. It is denoted by the Latin letter c. The speed of light is approximately 300,000,000 m/s.
At first, no one thought at all about the question of measuring the speed of light. There is light - that's great. Then, in the era of antiquity, the opinion that the speed of light was infinite, that is, instantaneous, dominated among scientific philosophers. Then it was Middle Ages with the Inquisition, when the main question of thinking and progressive people was the question "How not to get into the fire?" And only in the era Renaissance and Enlightenment the opinions of scientists have bred and, of course, divided.
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So, Descartes, Kepler and Farm were of the same opinion as the scientists of antiquity. But he believed that the speed of light is finite, although very high. Actually, he made the first measurement of the speed of light. More precisely, he made the first attempt to measure it.
Galileo's experience
An experience Galileo Galilei was brilliant in its simplicity. The scientist conducted an experiment to measure the speed of light, armed with simple improvised means. At a great and well-known distance from each other, on different hills, Galileo and his assistant stood with lit lanterns. One of them opened the shutter on the lantern, and the second had to do the same when he saw the light of the first lantern. Knowing the distance and time (the delay before the assistant opens the lantern), Galileo expected to calculate the speed of light. Unfortunately, in order for this experiment to succeed, Galileo and his assistant had to select hills that are several million kilometers apart. I would like to remind you that you can by filling out an application on the site.
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Roemer and Bradley experiments
The first successful and surprisingly accurate experiment in determining the speed of light was the experience of the Danish astronomer Olaf Römer. Roemer applied the astronomical method of measuring the speed of light. In 1676, he observed Jupiter's moon Io through a telescope and found that the time of the satellite's eclipse changes as the Earth moves away from Jupiter. The maximum delay time was 22 minutes. Assuming that the Earth is moving away from Jupiter at a distance of the diameter of the Earth's orbit, Roemer divided the approximate value of the diameter by the delay time, and received a value of 214,000 kilometers per second. Of course, such a calculation was very rough, the distances between the planets were known only approximately, but the result turned out to be relatively close to the truth.
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The Bradley Experience. In 1728 James Bradley estimated the speed of light by observing the aberration of stars. aberration is a change in the apparent position of a star caused by the movement of the earth in its orbit. Knowing the speed of the Earth and measuring the angle of aberration, Bradley got a value of 301,000 kilometers per second.
Fizeau's experience
The result of the experiment of Roemer and Bradley was treated with distrust by the then scientific world. However, Bradley's result was the most accurate for more than a hundred years, right up to 1849. That year the French scientist Armand Fizeau measured the speed of light using the rotating shutter method, without observing celestial bodies, but here on Earth. In fact, this was the first laboratory method after Galileo to measure the speed of light. Below is a diagram of its laboratory setup.
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The light, reflected from the mirror, passed through the teeth of the wheel and was reflected from another mirror, 8.6 kilometers away. The speed of the wheel was increased until the light was visible in the next gap. Fizeau's calculations gave a result of 313,000 kilometers per second. A year later, a similar experiment with a rotating mirror was carried out by Léon Foucault, who obtained the result of 298,000 kilometers per second.
With the advent of masers and lasers, people have new opportunities and ways to measure the speed of light, and the development of the theory also made it possible to calculate the speed of light indirectly, without making direct measurements.
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The most accurate value for the speed of light
Mankind has accumulated vast experience in measuring the speed of light. To date, the most accurate value of the speed of light is considered to be the value 299 792 458 meters per second received in 1983. It is interesting that further, more accurate measurement of the speed of light turned out to be impossible due to errors in the measurement meters. Now the value of the meter is tied to the speed of light and equals the distance that light travels in 1/299,792,458 seconds.
Finally, as always, we suggest watching an informative video. Friends, even if you are faced with such a task as independently measuring the speed of light with improvised means, you can safely turn to our authors for help. you can fill out an application on the website of the Correspondence. We wish you a pleasant and easy study!
Long before scientists measured the speed of light, they had to work hard to define the very concept of "light". One of the first to think about this was Aristotle, who considered light to be a kind of mobile substance that spreads in space. His ancient Roman colleague and follower Lucretius Car insisted on the atomic structure of light.
By the 17th century, two main theories of the nature of light were formed - corpuscular and wave. Newton belonged to the adherents of the first. In his opinion, all light sources emit the smallest particles. In the process of "flight" they form luminous lines - rays. His opponent, the Dutch scientist Christian Huygens, insisted that light is a form of wave motion.
As a result of centuries-old disputes, scientists have come to a consensus: both theories have the right to life, and light is the spectrum of electromagnetic waves visible to the eye.
A bit of history. How was the speed of light measured?
Most scientists of antiquity were convinced that the speed of light is infinite. However, the results of the studies of Galileo and Hooke admitted its limit, which was clearly confirmed in the 17th century by the outstanding Danish astronomer and mathematician Olaf Roemer.
He made his first measurements by observing the eclipses of Io, a satellite of Jupiter, at a time when Jupiter and the Earth were located on opposite sides of the Sun. Roemer recorded that as the Earth moved away from Jupiter at a distance equal to the diameter of the Earth's orbit, the delay time changed. The maximum value was 22 minutes. As a result of calculations, he received a speed of 220,000 km / s.
Fifty years later, in 1728, thanks to the discovery of aberration, the English astronomer J. Bradley "refined" this figure to 308,000 km / s. Later, the speed of light was measured by the French astrophysicists Francois Argo and Leon Foucault, having received 298,000 km / s at the “output”. An even more accurate measurement technique was proposed by the creator of the interferometer, the famous American physicist Albert Michelson.
Michelson's experiment to determine the speed of light
The experiments lasted from 1924 to 1927 and consisted of 5 series of observations. The essence of the experiment was as follows. A light source, a mirror and a rotating octagonal prism were installed on Mount Wilson near Los Angeles, and a reflecting mirror 35 km later on Mount San Antonio. First, light through a lens and a slit fell on a prism rotating with the help of a high-speed rotor (at a speed of 528 rpm).
The participants in the experiments could adjust the rotational speed so that the image of the light source was clearly visible in the eyepiece. Since the distance between the peaks and the frequency of rotation were known, Michelson determined the speed of light - 299796 km / s.
Scientists finally decided on the speed of light in the second half of the 20th century, when masers and lasers were created, which are distinguished by the highest radiation frequency stability. By the beginning of the 1970s, the measurement error had dropped to 1 km/sec. As a result, on the recommendation of the XV General Conference on Weights and Measures, held in 1975, it was decided to consider that the speed of light in vacuum is henceforth equal to 299,792.458 km/sec.
Can we reach the speed of light?
It is obvious that the development of the far corners of the universe is unthinkable without spaceships flying at great speed. Preferably at the speed of light. But is it possible?
The barrier of the speed of light is one of the consequences of the theory of relativity. As you know, an increase in speed requires an increase in energy. The speed of light would require virtually infinite energy.
Alas, the laws of physics are categorically against this. At a speed of a spaceship of 300,000 km/sec, particles flying towards it, for example, hydrogen atoms, turn into a deadly source of powerful radiation equal to 10,000 sieverts/sec. It's about the same as being inside the Large Hadron Collider.
According to scientists at Johns Hopkins University, while in nature there is no adequate protection against such a monstrous cosmic radiation. Erosion from the impact of interstellar dust will complete the destruction of the ship.
Another problem with light speed is time dilation. At the same time, aging will become much longer. The visual field will also be distorted, as a result of which the ship's trajectory will pass as if inside a tunnel, at the end of which the crew will see a shining flash. Behind the ship will remain absolute pitch darkness.
So in the near future, humanity will have to limit its high-speed "appetites" to 10% of the speed of light. This means that it will take about 40 years to fly to the nearest star to the Earth - Proxima Centauri (4.22 light years).
> speed of light
Find out which speed of light in vacuum is a fundamental constant in physics. Read what is the speed of light m / s, the law, the measurement formula.
The speed of light in a vacuum is one of the fundamental constants in physics.
Learning task
- Compare the speed of light with the refractive index of the medium.
Key Points
- The maximum possible indicator of light speed is light in vacuum (constant).
- C is the symbol for the speed of light in a vacuum. Reaches 299,792,458 m/s.
- When light hits a medium, its speed slows down due to refraction. Calculated by the formula v = c/n.
Terms
- Special speed of light: reconciliation of the principle of relativity and constancy of light speed.
- The refractive index is the ratio of the speed of light in air/vacuum to another medium.
speed of light
The speed of light acts as a point of comparison to define something as extremely fast. But what is it?
The light beam moves from the Earth to the Moon in the time interval required for the passage of a light pulse - 1.255 s at an average orbital distance
The answer is simple: we are talking about the speed of a photon and light particles. What is the speed of light? Light speed in vacuum reaches 299,792,458 m/s. This is a universal constant applicable in various fields of physics.
Take the equation E = mc 2 (E is energy and m is mass). It is the equivalent of mass-energy, using the speed of light to link space and time. Here one can find not only an explanation for energy, but also reveal obstacles to speed.
The speed of a wave of light in a vacuum is actively used for various purposes. For example, special relativity states that this is the natural speed limit. But we know that the speed depends on the medium and refraction:
v = c/n (v is the actual speed of light passing through the medium, c is the speed of light in vacuum, and n is the refractive index). The refractive index of air is 1.0003, and the speed of visible light is 90 km/s slower than c.
Lorentz coefficient
Rapidly moving objects show certain characteristics that conflict with the position of classical mechanics. For example, long contacts and time are expanding. These effects are usually minimal, but are more pronounced at such high speeds. The Lorentz coefficient (γ) is the factor where time expansion and length contraction occur:
γ \u003d (1 - v 2 / s 2) -1/2 γ \u003d (1 - v 2 / s 2) -1/2 γ \u003d (1 - v 2 / s 2) -1/2.
At low speeds, v 2 /c 2 approaches 0, and γ is approximately = 1. However, when the speed approaches c, γ increases towards infinity.