Generally speaking, the brightness of high-voltage pulsed xenon lamp in artificial light source is the highest, which is equivalent to the brightness of the sun. However, the highest brightness of an artificial light can be more than ten orders of magnitude (billions of times) than that of a xenon lamp, and it is 100 times brighter than the gamma ray burst produced by the most violent star explosion in the universe.

1October 20th,1968,65438+On the moon, which is 300,000 kilometers away from the Earth, the American lunar exploration spacecraft Surveyor 7 aimed its TV camera at the Earth. At this time, the earth is like a crescent moon hanging in the dark space. At this time, the whole North American continent was in darkness. However, in the lens, two appeared on the dark American continent. Or the light of the city?

In fact, one of these two highlights comes from Arizona Observatory and the other from California Observatory. They are laser sources produced by lasers invented several years ago, and their power is only 2W. Interestingly, looking at the earth from the moon 300,000 kilometers away, all the brightly lit cities are dark, and only these two 2W laser sources can be observed by the camera.

The illuminance generated by the beam of 2W laser on the moon is about 0.02 lux (when the luminous flux is 1 lumen area is 1 square meter, the illuminance is 1 lux). If the moon is illuminated by a xenon searchlight with the power of 1000 W, the illumination produced is only about1000,000 lux, which is completely imperceptible to human eyes.

The main reason why laser brightness is so high is that a large number of photons are concentrated in a very small space, so the energy density of laser can increase with power, and there is no upper limit.

The full name of the laser is "optical amplification device generated by stimulated radiation". At first, the Chinese name of laser was literally translated as "laser". 1964, Mr. Qian Xuesen changed it to "laser", which has been used ever since. Laser technology is considered as one of the four great inventions in the 20th century. It is another major scientific and technological breakthrough that affects the world after atomic energy technology, semiconductor technology and computer technology.

Nowadays, laser technology has been applied to all walks of life and is closely related to our lives. However, whether we realize that we use laser technology every day or not, how many people really understand the principle of laser luminescence?

02

Since ancient times, mankind's pursuit of "light" has never stopped. The ancient Greeks believed that light was an invisible "tentacle" extended by the human eye. When these tentacles touch something, the human eye can see something.

The ancients in China had a deep understanding of light: the sun was the main natural light, supplemented by the moon at night. Therefore, the ancients used the word "Ming" as a metaphor for light in Oracle Bone Inscriptions, and the Western Han Dynasty's "Zhou Bian Shu Jing" also pointed out: "The sun shines on the moon, and the moonlight is born, so it becomes a bright moon." The word "light" in Oracle Bone Inscriptions is the way a person holds a fire. The ancients clearly pointed out that "the sun is also a fire."

In the aspect of cold light, whether it is secondary luminescence fluorescence or low-temperature oxidation phosphorescence, the ancients in China also had a deep understanding:

Huainanzi in the Western Han Dynasty first recorded the phenomenon that Fraxinus mandshurica glows: when Fraxinus mandshurica is soaked in water for a whole day, it can be seen that it glows purple and yellowish at night. Interestingly, in Greek mythology, when Zeus created man, man was born from ash tree.

"Huainan Ziyun Lun Xun" also says: "Long-term blood is phosphorus." Records, and vividly described the blood on the ground "exposure for one hundred days is phosphorus, the distance is bright."

Zheng, a famous scientist in Qing Dynasty, summed up "light" wonderfully: "Light and heat are yang, while light and cold are yin. Yang Huo is too lazy to talk. Yin fire is phosphorus, fireflies and seawater, with the light of fire and the warmth without fire. "

As can be seen from the above records, China's understanding of natural phenomena in ancient times was generally deeper than that in the West.

However, by Newton's time (18th century), western science had become the mainstream of the world. Newton thought that light was the propagation of a tiny particle in space. 1704, Newton explained the linear propagation, reflection, refraction and color of light with the particle theory in optics. The particle theory at this time is consistent with the conceptual framework of classical mechanics, which has dominated the optical theory for more than one hundred years.

"Although I admire Newton's name, I don't think he is foolproof. I am sorry to see that he will make mistakes, and his authority may even hinder the progress of science sometimes. "

180 1 year, Thomas Young, a British doctor, bravely published a declaration challenging Newton in his book "Outline of Acousto-optic Experiment and Exploration".

Thomas Young proposed that both sound and light are elastic vibrations-waves propagating in etheric liquid that fills the whole space. He also accurately determined the wavelength values of various colored lights through a large number of experiments.

What's more, Thomas Young also made a famous double-slit interference experiment, which had a far-reaching influence on classical physics: he put a candle in front of a paper with a small hole, thus forming a point light source (the light source comes from a point). There are two parallel slits on the second paper behind this paper. When the candle is projected onto the wall through two slits, a series of light and dark stripes will be formed.

Regrettably, at that time, the doctor's situation in the scientific community was similar to what we call "folk science" today. His paper was ridiculed by the physical authorities at that time and attacked as absurd and illogical.

However, Lenin said: "Truth is often in the hands of a few people."

18 18, a young French civil engineer, Augustine Fresnel, submitted a paper entitled "Light is Wave" to the French Academy of Sciences. He pointed out that light, like water waves and sound waves, will diffract when encountering obstacles and will go around behind them. When different light waves meet, they can interfere with each other and form alternating stripes.

However, a loyal supporter of Newton refuted his view. French mathematician Simon Dennis Poisson believes that according to Fresnel's theory, if a beam of light shines on a small ball in parallel, there will be a bright spot in the shadow of the ball, which is very contrary to common sense, because the shadow should be black. How can there be a bright spot?

At this time, Dominique Francis Jean arago, an English physicist, did an experiment under the witness of everyone, and proved with facts that there was indeed a bright spot in the shadow of the sphere.

This is because although the ball blocks the light shining on it, it also interferes with the light passing by it, thus causing diffraction. When these diffracted rays meet in the middle of the spherical shadow, they all have the same yearning and constructive interference, so there should be a brighter point in the middle.

Since then, the wave theory of light has been concerned by the world.

However, both particle theory and wave theory are only discussing a propagation state of light, so what is the essence of light?

1905 In March, Einstein published a paper entitled "Speculative Views on the Generation and Transformation of Light" in the German Yearbook of Physics:

For the average value of time, light shows fluctuation;

For the instantaneous value of time, light appears as particles.

This is the first time in the history of science to unify the fluctuation and particle nature of light in a microscopic state, that is, the wave-particle duality of light waves. In other words, light has both wave and particle properties.

03

A lot has been said above, just to let everyone know what the essence of "light" is. This is very important for human beings to take light for their own use and deep processing. Next, the focus of human science and technology development will focus on one person.

"Where there is Maxwell, there is light."

When there is a major breakthrough in science, it will not only shock people, but also affect future generations.

1864, British physicist james clerk maxwell integrated the three physical phenomena of electricity, magnetism and light in nature and expressed them in the form of concise, symmetrical and perfect mathematical equations.

It turns out that light is still an "electromagnetic wave" which is intertwined with electric and magnetic fields and propagates in the form of waves.

More than ten years later, German physicist heinrich rudolf hertz proved the existence of electromagnetic waves with the transmission experiment of double inductance coils. The establishment of electromagnetic theory has laid a foundation for the development of communication, broadcasting, computer, information transmission, material science, optical research and other high-tech fields.

So Maxwell's electromagnetic theory was praised by Feynman as the greatest physical discovery in the history of human science and technology, which really had a far-reaching impact on the development of human civilization.

04

Electromagnetic wave is a kind of energy, so it is closely related to temperature. From a scientific point of view, any substance above absolute zero (-273.438+05) will emit electromagnetic waves.

Then, laser is also a state of matter luminescence. To understand the principle of laser generation, we must first understand the structure of the basic unit of matter "atom".

Generally speaking, an atom has a stable structure: a nucleus and electrons moving around it at high speed.

These electrons will move in their own energy level orbits. When an atom passively absorbs a photon, the electrons in the outer orbit will jump to a higher energy level orbit. This is an unstable state that can only last for a short time. When the atom automatically releases this photon, the electron will return to the normal low-level orbit (showing luminescence) and return to the original stable state, which physicists call spontaneous emission.

To put it simply, according to the law of conservation of energy, when the energy of an atom changes, its changed energy does not appear or disappear out of thin air, but propagates in the form of photons.

In 19 17, Einstein pointed out theoretically that atoms at high energy levels can jump to lower energy levels in another way besides spontaneous radiation.

When the photon is incident, it will also cause the electrons in the atom to quickly transition from high energy level to low energy level with a certain probability, and at the same time radiate a photon with the same frequency, phase, polarization state and propagation direction as the foreign photon. This process is called stimulated radiation.

It is conceivable that if a large number of atoms are in the same state, when a photon is incident, one of them is excited to produce stimulated radiation, and two photons with exactly the same characteristics are obtained. These two photons excite two atoms again and make them produce stimulated radiation, so that four photons with the same characteristics can be obtained, and so on, that is to say, the first photon can be multiplied continuously. This kind of light generated and amplified during stimulated radiation is called "laser".

195 1 year, American physicist Charles Harder Towns came up with a brilliant idea to improve stimulated emission: if we can continuously provide high-energy atoms, we can ensure more and more photons.

He put these high-level atoms between two reflective surfaces, so that a photon can shuttle back and forth between high-level atoms, and when enough photons are produced, they are emitted from a slightly light-transmissive reflective surface.

This wonderful idea about the town is a great idea. Since then, all lasers have been applied to his idea.

Seven years later, Downs and Luo Xiao, an American scientist, discovered another magical phenomenon: when they put the light from a neon bulb on a rare earth crystal, the molecules of the crystal will emit bright and intense light, and these lights will always gather together. According to this phenomenon, they put forward the "laser principle":

That is, when a substance is excited by the same energy as the natural oscillation frequency of its molecules, it will produce this kind of undifferentiated strong light-laser. This theory won them the 1964 Nobel Prize in Physics.

1960 May 15, Theodore Mayman, a scientist at Hughes Laboratory in California, announced that he had obtained a laser with a wavelength of 0.6943 micron, which was the first real laser ever obtained by human beings, and thus Mayman became the first scientist in the world to introduce lasers into practical fields.

Maiman's plan is to use high-intensity flash tubes to excite rubies. Because ruby is actually a kind of corundum doped with chromium atoms in physics, ruby will emit red light when stimulated. Drill a hole in the surface of the ruby, and install a reflector on the surface of the ruby to let the red light overflow from the hole, thus producing a fairly concentrated and slender red beam, which can reach a temperature higher than the surface of the sun when hitting a certain point.

label

Man's pursuit of light never stops, and laser is one of the most outstanding inventions of man. Nowadays, laser technology has been applied in six fields, such as military, medical care, industry, commerce, scientific research and information, and it can be called a weapon for the country to keep pace with the times.

# Ignite curiosity #