School encyclopedia. Slow light

Many people know about the existence of such a concept as "speed of light" since early childhood. Most people know that light moves very fast. But not everyone knows in detail about the phenomenon.

Many have noticed that during a thunderstorm there is a delay between the flash of lightning and the sound of thunder. The flash usually reaches us faster. This means that it is faster than sound. What is the reason for this? What is the speed of light and how is it measured?

What is the speed of light?

Let's first figure out what the speed of light is. Scientifically, this is such a value that shows how quickly the rays move in a vacuum or in air. You also need to know what light is. This is radiation that is perceived by the human eye. The speed depends on the environmental conditions, as well as other properties, for example, refraction.

Interesting fact: light takes 1.25 seconds to get from the Earth to the satellite - the Moon.

What is the speed of light in your own words?

In simple terms, the speed of light is the time interval during which a light beam travels some distance. Time is usually measured in seconds. However, some scientists use other units of measurement. Distance is also measured in different ways. Basically it is a meter. That is, this value is calculated in m / s. Physics explains this as follows: a phenomenon that moves at a certain speed (constant).

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To make it easier to understand, let's look at the following example. The cyclist moves at a speed of 20 km / h. He wants to catch up with the driver of a car whose speed is 25 km / h. If you count, then a car goes 5 km / h faster than a cyclist. Things are different with rays of light. No matter how fast the first and second people move, the light, relative to them, moves with constant speed.

What is the speed of light?

When not in a vacuum, light is affected by different conditions. The substance through which the rays pass, including. If, without oxygen, the number of meters per second does not change, then in an environment with air access, the value changes.

Light travels more slowly through various materials such as glass, water, and air. This phenomenon is given a refractive index to describe how much they slow down the movement of light. Glass has a refractive index of 1.5, which means that light travels through it at a speed of about 200,000 kilometers per second. The refractive index of water is 1.3 and the refractive index of air is slightly more than 1, which means that air only slightly slows down light.

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Therefore, after passing through air or liquid, the speed slows down, becomes less than in a vacuum. For example, in various water bodies the speed of movement of rays is equal to 0.75 of the speed in space. Also, at a standard pressure of 1.01 bar, the indicator decelerates by 1.5-2%. That is, under terrestrial conditions, the speed of light varies depending on environmental conditions.

For such a phenomenon, a special concept was invented - refraction. That is, the refraction of light. It is widely used in various inventions. For example, a refractor is a telescope with an optical system. Also, with the help of this, binoculars and other equipment are also created, the essence of which is the use of optics.

In general, the least refractive ray is when it passes through normal air. When passing through a specially created optical glass, the speed is approximately 195 thousand kilometers per second. This is almost 105 km / s less than a constant.

The most accurate value for the speed of light

For many years, physicists have accumulated experience in researching the speed of light rays. At the moment, the most accurate value of the speed of light is 299,792 kilometers per second... The constant was established in 1933. The number is still relevant.

However, in the future, difficulties arose with the definition of the indicator. This happened due to errors in the measurement of the meter. Now the very value of the meter directly depends on the speed of light. It is equal to the distance that the rays travel in a certain number of seconds - 1 / the speed of light.

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What is the speed of light in a vacuum?

Since light is not affected by various conditions in a vacuum, its speed does not change as it does on Earth. The speed of light in a vacuum is 299,792 kilometers per second... This indicator is limiting. It is believed that nothing in the world can move faster, not even cosmic bodies that move quite fast.

For example, a fighter, Boeing X-43, which exceeds the speed of sound by almost 10 times (more than 11 thousand km / h), flies slower than the beam. The latter moves more than 96 thousand kilometers per hour faster.

How was the speed of light measured?

The very first scientists tried to measure this value. Various methods were used. In the period of antiquity, people of science believed that it was infinite, therefore it was impossible to measure it. This opinion remained for a long time, right up to the 16-17 century. In those days, other scientists appeared who suggested that the beam had an end, and the speed could be measured.

The renowned Danish astronomer Olaf Römer has taken knowledge of the speed of light to a new level. He noticed that the eclipse of Jupiter's moon was late. Previously, no one paid attention to this. Therefore, he decided to calculate the speed.

The speed of light is the distance that light travels in a unit of time. This value depends on the substance in which the light travels.

In a vacuum, the speed of light is 299,792,458 m / s. This is the fastest speed that can be achieved. 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 with .

How the speed of light was determined

In ancient times, scientists believed that the speed of light is infinite. Later in the scientific community, discussions began on this issue. Kepler, Descartes and Fermat agreed with the opinion of ancient scholars. 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 try 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 shutter on his lantern. The moment the assistant saw this light, he had to do the same with his lantern. The time it took for the light to travel 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 it is impossible to measure it at such a short distance, since light propagates almost instantly. And the time recorded by him shows only the speed of a person's reaction.

For the first time, the speed of light was determined in 1676 by the Danish astronomer Olaf Römer using astronomical distances. Observing with a telescope the eclipses of Jupiter's moon Io, he found that as the Earth moves away from Jupiter, each subsequent eclipse occurs later than expected. 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 the exact diameter of the Earth was not known at that time, the scientist divided its approximate value by 22 hours and obtained a value of about 220,000 km / s.

Olaf Roemer

The result, obtained by Roemer, caused suspicion among scientists. But in 1849 the French physicist Armand Hippolyte Louis Fizeau measured the speed of light by the rotating shutter method. In his experience, light from a source passed between the teeth of a rotating wheel and was directed towards a mirror. Reflected from him, he returned back. The speed of rotation of the wheel increased. When it reached a certain value, the beam reflected from the mirror was delayed by the shifted tooth, and the observer at that moment did not see anything.

Fizeau's Experience

Fizeau calculated the speed of light as follows. Light travels the way L from wheel to mirror in time equal to t 1 = 2L / c ... The time it takes for the wheel to make a ½ notch turn is t 2 = T / 2N , where T - period of rotation of the wheel, N - the number of teeth. Rotation frequency v = 1 / T ... The moment when the observer does not see the light occurs when t 1 = t 2 ... From here we get the formula for determining the speed of light:

c = 4LNv

After performing calculations using this formula, Fizeau determined that with = 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 received 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 with = (99,810,000 ± 50,000) m / s.

Research by the American physicist Albert Abraham Michelson, who used an installation with a rotating octahedral mirror, made it possible to determine the speed of light even more accurately. In 1926, the scientist measured the time it took for light to travel the distance between the tops of two mountains, equal to 35.4 km, and received with = (299,796,000 ± 4,000) m / s.

The most accurate measurement was carried out 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 a vacuum does not depend on either the frame of reference or the position of the observer. It remains constant at 299 792 458 ± 1.2 m / s. But in various transparent media, this speed will be lower than its speed in a vacuum. Any transparent medium has an optical density. And the higher it is, the less speed the light spreads in it. 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 denser 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 interface between the two media.

Doctor of Technical Sciences A. GOLUBEV

The concept of wave propagation speed turns out to be simple only in the absence of dispersion.

Lin Westergard Hau near the installation where a unique experiment was carried out.

Last spring, scientific and popular science magazines around the world reported sensational news. American physicists conducted a unique experiment: they managed to reduce 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 magnitude in vacuum = 299792458 m / s - a fundamental physical constant. According to the theory of relativity, this is the maximum possible signal transmission rate.

In any transparent medium, light travels more slowly. Its velocity v depends on the refractive index of the medium n: v = c / n. The refractive index of air - 1.0003, water - 1.33, various types of glass - from 1.5 to 1.8. Diamond has one of the highest values ​​of the refractive index - 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 for Scientific Research at Harvard University (Massachusetts, USA) and from Stanford University (California) investigated the macroscopic quantum effect - the so-called self-induced transparency, passing laser pulses through an opaque medium under normal conditions. This medium was sodium atoms in a special state called 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 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 speeds are distinguished - phase and group. The phase velocity v f characterizes the phase displacement of an ideal monochromatic wave - an infinite sinusoid of strictly one frequency and determines the direction of propagation of light. Phase velocity in the medium corresponds to the phase refractive index - the same, the values ​​of which are measured for various substances. The phase refractive index, and hence the phase velocity, depend on the wavelength. This relationship is called variance; it leads, in particular, to the decomposition of white light passing through the 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 impulse is stretched, and its shape changes. Therefore, to describe the motion 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. For a better understanding of the situation, a visual analogy can be made.

Imagine that seven athletes are lined up at the start line, dressed in multi-colored shirts according to the colors of the spectrum: red, orange, yellow, etc. At the signal of the starting pistol, they simultaneously start running, but the "red" athlete runs faster than the "orange" , "orange" is faster than "yellow", etc., so that they stretch into a chain, the length of which increases continuously. Now imagine that we are looking at them from above from such a height that we cannot distinguish individual runners, but we just see a motley spot. Can we talk about the speed of movement of this spot as a whole? It is possible, but only if it is not very diffuse, when the difference in the speeds of the multicolored runners is small. Otherwise, the spot can stretch over the entire length of the track, and the question of its speed will be meaningless. 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 speed of the runners will not differ much, the group will remain quite compact when moving and can be characterized by a well-defined value of speed, which is called the group.

Bose-Einstein statistics. This is one of the types of so-called quantum statistics - a theory that describes the state of systems containing a very large number of particles that obey 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; this class also includes particles consisting of an odd number of fermions), and the law of their distribution is called the Fermi-Dirac statistics. Particles of another class are called bosons and do not obey Pauli's 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 short-lived elementary particles (for example, pi-mesons), and atoms consisting of an even number of fermions. At very low temperatures, bosons are collected at the lowest - ground - energy level; then the Bose-Einstein condensation is said to occur. Condensate atoms lose their individual properties, and several million of them begin to behave as a whole, their wave functions merge, and their 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 used this property of 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, a delay occurs 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 corresponding to pulse velocities three orders of magnitude (1000 times) lower than the speed of light in vacuum were obtained in rubidium vapor.

Let us now turn to a unique experiment in 1999. It was carried out by Len Westergard 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, before their transition 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 of the magnetic field. The researchers then cooled the cloud to a temperature of less than 435 nK (nanokelvin, i.e. 0.000000435 K, almost to absolute zero).

After that, the condensate was illuminated with a "binder beam" of linearly polarized laser light with a frequency corresponding to its weak excitation energy. The atoms 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 the slowing down of light 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 - 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 allow one to study the quantum-optical properties of matter, as well as create various devices for quantum computers of the future, say, single-photon switches.

The topic of how to measure, as well as what the speed of light is equal to, has been of interest to scientists since ancient times. This is a very fascinating topic that has been the subject of scientific debate from time immemorial. It is believed that this speed is finite, unattainable and constant. It is unattainable and constant, like infinity. Moreover, it is finite. It turns out to be an interesting physics and mathematics puzzle. There is one of the options for solving this problem. After all, it was possible to measure the speed of light.

In ancient times, thinkers believed that light speed is an infinite quantity. The first estimate of this indicator was given in 1676. Olaf Roemer... According to his calculations, the speed of light was approximately 220 thousand km / s. This was not an entirely accurate value, but close to the true one.

The finiteness and estimate of the speed of light were confirmed after half a century.

In the future, the scientist Fizeau it was possible to determine the speed of light by the time the beam passed the exact distance.

He set up an experiment (see figure), during which a beam of light escaped from the source S, was reflected by a mirror 3, interrupted by a toothed disk 2, and passed the base (8 km). Then it was reflected by mirror 1 and returned to the disk. The light fell into the gap between the teeth and could be observed through the eyepiece 4. The time the beam passed through the base was determined depending on the speed of rotation of the disk. The value obtained by Fizeau was as follows: s = 313300 km / s.

The speed of propagation of a ray in any particular medium is less than this speed in a vacuum. In addition, this indicator takes on different values ​​for different substances. After few years Foucault replaced the disk with a fast-rotating mirror. The followers of these scientists have used their methods and research schemes many times.

Lenses are the backbone of optical instruments. Do you know how it is calculated? You can find out by reading one of our articles.

And you can find information on how to set up an optical sight consisting of such lenses. Read our material and you will not have any questions on the topic.

What is the speed of light in a vacuum?

The most accurate measurement of the speed of light shows the figure 1,079,252,848.8 kilometers per hour or 299 792 458 m / s... This figure is valid only for conditions created in a vacuum.

But to solve problems, the indicator is usually used 300,000,000 m / s... In a vacuum, the speed of light in Planck units is 1. Thus, the energy of light travels 1 Planck unit of length in 1 unit of Planck time. If a vacuum is created in natural conditions, then X-rays, light waves of the visible spectrum and gravitational waves can move at this speed.

There is an unambiguous opinion of scientists that particles with a mass can take a speed that is as close as possible to the speed of light. But they are not able to reach and exceed the indicator. The highest speed, close to the speed of light, was recorded in the study of cosmic rays and during the acceleration of some particles in accelerators.

The value of the speed of light in any medium depends on the refractive index of this medium.

This indicator can be different for different frequencies. Accurate measurement of the quantity is important for calculating other physical parameters. For example, to find out the distance during the passage of light or radio signals in optical location, radar, light range and other areas.

Modern scientists use different methods to determine the speed of light. Some experts use astronomical methods as well as measurement methods using experimental techniques. The improved Fizeau method is often used. In this case, the gearwheel is replaced by a light modulator, which attenuates or interrupts the light beam. The receiver here is a photomultiplier tube or photocell. The light source can be a laser, which helps to reduce the measurement error. Determination of the speed of light the base time can be traversed by direct or indirect methods, which also provide accurate results.

What formulas calculate the speed of light

1. The speed of propagation of light in a vacuum is an absolute value. Physicists designate it with the letter "c". This is a fundamental and constant value that does not depend on the choice of the reporting system and gives a characteristic of time and space as a whole. Scientists suggest that this speed is the limiting speed of the particles.

   Light speed formula in vacuum:

   s = 3 * 10 ^ 8 = 299792458 m / s

   here c is an indicator of the speed of light in a vacuum.

2. Scientists have proven that speed of light in air almost coincides with the speed of light in a vacuum. It can be calculated using the formula: