The speed of light is the distance that light travels per unit time. This value depends on the medium in which the light propagates.
In vacuum, the speed of light is 299,792,458 m/s. This is the highest speed that can be reached. When solving problems that do not require special accuracy, this value is taken equal to 300,000,000 m/s. It is assumed that all types of electromagnetic radiation propagate at the speed of light in a vacuum: radio waves, infrared radiation, visible light, ultraviolet radiation, x-rays, gamma radiation. Designate it with a letter from .
How is the speed of light determined?
In ancient times, scientists believed that the speed of light was infinite. Later, discussions on this issue began in the scientific community. Kepler, Descartes and Fermat agreed with the opinion of ancient scientists. And Galileo and Hooke believed that, although the speed of light is very high, it still has a finite value.
Galileo Galilei
One of the first to measure the speed of light was the Italian scientist Galileo Galilei. During the experiment, he and his assistant were on different hills. Galileo opened the damper on his lantern. At that moment, when the assistant saw this light, he had to do the same with his lantern. The time during which the light traveled from Galileo to the assistant and back turned out to be so short that Galileo realized that the speed of light is very high, and at such short distance it is impossible to measure it, since light propagates almost instantly. And the time recorded by him shows only the speed of a person's reaction.
The speed of light was first determined in 1676 by the Danish astronomer Olaf Römer using astronomical distances. Observing with a telescope the eclipse of Jupiter's moon Io, he found that as the Earth moves away from Jupiter, each subsequent eclipse comes later than it was calculated. The maximum delay, when the Earth moves to the other side of the Sun and moves away from Jupiter at a distance equal to the diameter of the Earth's orbit, is 22 hours. Although at that time the exact diameter of the Earth was not known, the scientist divided its approximate value by 22 hours and came up with a value of about 220,000 km / s.
Olaf Römer
The result obtained by Römer caused distrust among scientists. But in 1849 the French physicist Armand Hippolyte Louis Fizeau measured the speed of light using the rotating shutter method. In his experiment, light from a source passed between the teeth of a rotating wheel and was directed to a mirror. Reflected from him, he returned back. Wheel speed increased. When it reached a certain value, the beam reflected from the mirror was delayed by the moved tooth, and the observer at that moment did not see anything.
Fizeau's experience
Fizeau calculated the speed of light as follows. Light goes the way L from the wheel to the mirror in a time equal to t1 = 2L/s . The time it takes the wheel to make a ½ slot turn is t 2 \u003d T / 2N , where T - wheel rotation period, N - the number of teeth. Rotation frequency v = 1/T . The moment when the observer does not see the light comes at t1 = t2 . From here we get the formula for determining the speed of light:
c = 4LNv
After calculating this formula, Fizeau determined that from = 313,000,000 m/s. This result was much more accurate.
Armand Hippolyte Louis Fizeau
In 1838, the French physicist and astronomer Dominique François Jean Arago proposed using the method of rotating mirrors to calculate the speed of light. This idea was put into practice by the French physicist, mechanic and astronomer Jean Bernard Léon Foucault, who in 1862 obtained the value of the speed of light (298,000,000 ± 500,000) m/s.
Dominique Francois Jean Arago
In 1891, the result of the American astronomer Simon Newcomb turned out to be an order of magnitude more accurate than Foucault's result. As a result of his calculations from = (99 810 000±50 000) m/s.
The studies of the American physicist Albert Abraham Michelson, who used an installation with a rotating octahedral mirror, made it possible to more accurately determine the speed of light. In 1926, the scientist measured the time during which light traveled the distance between the tops of two mountains, equal to 35.4 km, and received from = (299 796 000±4 000) m/s.
The most accurate measurement was made in 1975. In the same year, the General Conference on Weights and Measures recommended that the speed of light be considered equal to 299,792,458 ± 1.2 m/s.
What determines the speed of light
The speed of light in vacuum does not depend on the frame of reference or on the position of the observer. She stays constant value, equal to 299 792 458 ± 1.2 m/s. But in various transparent media this speed will be lower than its speed in vacuum. Any transparent medium has an optical density. And the higher it is, the slower the light propagates in it. So, for example, the speed of light in air is higher than its speed in water, and in pure optical glass it is less than in water.
If light passes from a less dense medium to a more dense one, its speed decreases. And if the transition occurs from a denser medium to a less dense one, then the speed, on the contrary, increases. This explains why the light beam is deflected at the boundary of the transition of two media.
In the spring of last year, scientific and popular science magazines around the world reported sensational news. American physicists conducted a unique experiment: they managed to lower the speed of light to 17 meters per second.
Everyone knows that light travels at a tremendous speed - almost 300 thousand kilometers per second. The exact value of its value in vacuum = 299792458 m/s is a fundamental physical constant. According to the theory of relativity, this is the maximum possible speed signal transmission.
In any transparent medium, light travels more slowly. Its speed v depends on the refractive index of the medium n: v = c/n. The refractive index of air is 1.0003, water - 1.33, various types of glass - from 1.5 to 1.8. One of the highest refractive index values is diamond - 2.42. Thus, the speed of light in ordinary substances will decrease by no more than 2.5 times.
In early 1999, a group of physicists from the Rowland Institute scientific research at Harvard University (Massachusetts, USA) and from Stanford University (California) investigated the macroscopic quantum effect - the so-called self-induced transparency, by passing laser pulses through a medium that is opaque under normal conditions. This medium was sodium atoms in a special state called a Bose-Einstein condensate. When irradiated with a laser pulse, it acquires optical properties that reduce the group velocity of the pulse by a factor of 20 million compared to the velocity in vacuum. The experimenters managed to bring the speed of light up to 17 m/s!
Before describing the essence of this unique experiment, let us recall the meaning of some physical concepts.
group speed. When light propagates in a medium, two velocities are distinguished - phase and group. The phase velocity vph characterizes the movement of the phase of an ideal monochromatic wave - an infinite sinusoid of strictly one frequency and determines the direction of light propagation. The phase velocity in the medium corresponds to the phase refractive index - the same one whose values are measured for various substances. The phase index of refraction, and hence the phase velocity, depends on the wavelength. This dependence is called dispersion; it leads, in particular, to the decomposition of white light passing through a prism into a spectrum.
But a real light wave consists of a set of waves of different frequencies, grouped in a certain spectral interval. Such a set is called a group of waves, a wave packet, or a light pulse. These waves propagate in a medium with different phase velocities due to dispersion. In this case, the pulse is stretched, and its shape changes. Therefore, to describe the movement of an impulse, a group of waves as a whole, the concept of group velocity is introduced. It makes sense only in the case of a narrow spectrum and in a medium with weak dispersion, when the difference in the phase velocities of the individual components is small. To better understand the situation, we can draw a visual analogy.
Imagine that seven athletes lined up on the start line, dressed in multi-colored T-shirts according to the colors of the spectrum: red, orange, yellow, etc. At the signal of the starting pistol, they start running at the same time, but the "red" athlete runs faster than the "orange" one. , "orange" is faster than "yellow", etc., so that they are stretched into a chain that continuously increases in length. And now imagine that we are looking at them from above from such a height that we cannot distinguish individual runners, but we see just a motley spot. Is it possible to speak about the speed of movement of this spot as a whole? It is possible, but only if it is not very blurry, when the difference in the speeds of different-colored runners is small. Otherwise, the spot may stretch over the entire length of the track, and the question of its speed will lose its meaning. This corresponds to a strong dispersion - a large spread of velocities. If runners are dressed in jerseys of almost the same color, differing only in shades (say, from dark red to light red), this will correspond to the case of a narrow spectrum. Then the velocities of the runners will not differ much, the group will remain quite compact during movement and can be characterized by a well-defined value of speed, which is called the group speed.
Bose-Einstein statistics. This is one of the types of so-called quantum statistics - a theory that describes the state of systems containing very big number particles obeying the laws of quantum mechanics.
All particles - both enclosed in an atom and free - are divided into two classes. For one of them, the Pauli exclusion principle is valid, according to which there cannot be more than one particle at each energy level. Particles of this class are called fermions (these are electrons, protons and neutrons; the same class includes particles consisting of an odd number of fermions), and the law of their distribution is called Fermi-Dirac statistics. Particles of another class are called bosons and do not obey the Pauli principle: an unlimited number of bosons can accumulate at one energy level. In this case one speaks of Bose-Einstein statistics. Bosons include photons, some of which are short-lived elementary particles(for example, pi-mesons), as well as atoms consisting of an even number of fermions. At very low temperatures bosons are collected at the lowest - basic - energy level; Bose-Einstein condensation is then said to occur. The atoms of the condensate lose their individual properties, and several million of them begin to behave as a whole, their wave functions merge, and the behavior is described by one equation. This makes it possible to say that the atoms of the condensate have become coherent, like photons in laser radiation. Researchers at the US National Institute of Standards and Technology have used this property of the Bose-Einstein condensate to create an "atomic laser" (see "Science and Life" No. 10, 1997).
Self-induced transparency. This is one of the effects of nonlinear optics - the optics of powerful light fields. It consists in the fact that a very short and powerful light pulse passes without attenuation through a medium that absorbs continuous radiation or long pulses: an opaque medium becomes transparent to it. Self-induced transparency is observed in rarefied gases with a pulse duration of the order of 10-7 - 10-8 s and in condensed media - less than 10-11 s. In this case, there is a delay in the pulse - its group velocity is greatly reduced. This effect was first demonstrated by McCall and Hahn in 1967 on ruby at a temperature of 4 K. In 1970, delays were obtained in rubidium vapor corresponding to pulse velocities three orders of magnitude (1000 times) lower than the speed of light in vacuum.
Let's turn now to unique experiment 1999. It was carried out by Len Westergaard Howe, Zachary Dutton, Cyrus Berusi (Rowland Institute) and Steve Harris (Stanford University). They cooled a dense cloud of sodium atoms held by a magnetic field until they transitioned to the ground state - to the level with the lowest energy. In this case, only those atoms were isolated, in which the magnetic dipole moment was directed opposite to the direction magnetic field. The researchers then cooled the cloud down to less than 435 nK (nanokelvins, i.e. 0.000000435 K, almost to absolute zero).
After that, the condensate was illuminated with a "binding beam" of linearly polarized laser light with a frequency corresponding to the energy of its weak excitation. Atoms have moved to a higher energy level and stopped absorbing light. As a result, the condensate became transparent to the following laser radiation. And here very strange and unusual effects appeared. Measurements have shown that under certain conditions, a pulse passing through a Bose-Einstein condensate experiences a delay corresponding to light slowing down by more than seven orders of magnitude - 20 million times. The speed of the light pulse slowed down to 17 m/s, and its length decreased several times - up to 43 micrometers.
The researchers believe that by avoiding laser heating of the condensate, they will be able to slow down the light even more - perhaps to a speed of several centimeters per second.
A system with such unusual characteristics will make it possible to study the quantum optical properties of matter, as well as to create various devices for quantum computers of the future, say, single-photon switches.
> 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 to various fields 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, in special theory relativity indicates that this is a natural velocity 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.
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 Carus insisted on the atomic structure of light.
TO XVII 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 visible to the eye. electromagnetic waves.
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 octahedral 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 speed spaceship at 300,000 km / s, particles flying towards it, for example, hydrogen atoms, turn into a deadly source of powerful radiation equal to 10,000 Sieverts / s. 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).
The speed of light in a vacuum - absolute value velocity of propagation of electromagnetic waves in vacuum. In physics, it is denoted by the Latin letter c.
The speed of light in a vacuum is a fundamental constant, independent of choice inertial system reference.
By definition, it is exactly 299 792 458 m / s (approximate value of 300 thousand km / s).
According to the special theory of relativity, is the maximum speed for the propagation of any physical interactions that transmit energy and information.
How is the speed of light determined?
The speed of light was first determined in 1676 O. K. Römer by changing the time intervals between eclipses of Jupiter's satellites.
In 1728 it was installed by J. Bradley, based on his observations of the aberration of stellar light.
In 1849 A. I. L. Fizeau he was the first to measure the speed of light by the time it takes light to travel 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, reflected by a semitransparent mirror N, was periodically interrupted by a rotating toothed disk W, passed the base MN (about 8 km) and, reflected from the mirror M, returned to the disk. When the light hit 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. The time of passage of the light through the base was determined from the known disk rotation speeds. Fizeau obtained the value c = 313,300 km/s.
In 1862 J. B. L. Foucault realized the idea of D. Arago, expressed in 1838, using a rapidly rotating (512 rpm) mirror instead of a toothed disk. 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. With a base of only 20 m, Foucault found that the speed of light is 29800080 ± 500 km/s. The schemes and basic ideas of the experiments of Fizeau and Foucault were repeatedly used in subsequent works to determine s.
