What is light?

Science shows that light is one of the sources of life on earth. Light is an important foundation of human life; Light is a tool for human beings to know the outside world; Light is an ideal carrier or media of information. So, what is light?

In a narrow sense, glazing is an electromagnetic wave that can be seen by human eyes, which we call the visible spectrum. In the scientific definition, light refers to all the electromagnetic spectrum. Light is made up of a kind of elementary particles called photons. Particles and fluctuations.

Experiments show that light is electromagnetic radiation, and the wavelength range of this part of electromagnetic wave is about 0.77 micron of red light to 0.39 micron of purple light. Electromagnetic waves with a wavelength of more than 0.77 micron to about 1000 micron are called "infrared rays". Below 0.39 micron to about 0.04 micron is called "ultraviolet". Infrared and ultraviolet rays can't cause vision, but the existence of this luminous object can be measured and detected by optical instruments or photography. Therefore, the concept of light in optics can also be extended to infrared and ultraviolet fields. Even X-rays are considered as light, and the spectrum of visible light is only a part of the electromagnetic spectrum.

Scientific experiments show that light has wave-particle duality, which can be regarded as both a high-frequency electromagnetic wave and a particle, that is, photons for short.

Light waves, including infrared rays, have shorter wavelengths and higher frequencies than microwaves. Therefore, it is a natural and inevitable trend to develop from microwave communication in electrical communication to optical communication.

Generally speaking, light consists of many photons. Under fluorescence (ordinary sunlight, light, candlelight, etc.). ), there is no connection between photons, that is, different wavelengths, different phases, different polarization directions and different propagation directions, just like an unorganized and undisciplined photon army. All photons are stragglers and can't act in unison.

When light is reflected, the reflection angle is equal to the incident angle. On the same plane, on both sides of the normal, the light path can be reversed. For humans, the greatest reflection of light occurs on the moon. As we know, the moon itself does not shine, it just reflects sunlight. According to legend, this matter was mentioned in the Book of Songs, which recorded the historical facts of Xia, Shang and Zhou Dynasties. It can be seen that people had the concept of light reflection at that time. Zhou Xie in the Warring States Period clearly pointed out: "The sun shines on the moon, and the moonlight gives birth to a bright moon." In the Western Han Dynasty, people simply said that "the moon is like a mirror", which shows that we have a deeper understanding of the phenomenon of light reflection. A light reflection experiment is specially recorded in the Book of Mohism: reflecting sunlight to the human body with a mirror can make the shadow of the human body between the human body and the sun. This not only demonstrates the reflection of light, but also probably explains the cause of luna.

As we know, when light enters another medium obliquely from one medium, it will be refracted. If the density of the injected medium is greater than that of the original light, the refraction angle is less than the incident angle. On the contrary, if less than, the refraction angle is greater than the incident angle. However, if the incident angle is zero and the refraction angle is zero, no refraction will occur. However, light refraction still occurs in the same inhomogeneous medium. Theoretically, it can be injected from one direction without refraction. However, due to the unclear boundary, it is generally divided into several levels and is not flat, so refraction will occur anyway.

For example, fish can see clearly when swimming in clear water. However, you can't put the harpoon in the direction you see. Experienced fishermen know that the harpoon can only reach the bottom of the fish, and the harpoon is aimed at the real image of the fish.

When you look at objects in transparent media such as water and glass from above, you will feel that the position of the object is higher than the actual position, which is caused by the refraction of light.

Due to the refraction of light, the pool water looks shallower than it really is. Therefore, when you stand on the shore and see waist-deep water, don't rush down to avoid panic and danger because of underestimating the water depth.

Put a thick glass in front of the pen, and the pen holder looks "misplaced", which is also caused by the refraction of light. What is light? This is a problem worth studying and must be studied. Now physics research has reached a bottleneck, which is the conflict between relativity and quantum theory. Whether the essence of light is elementary particles or waves like sound is instructive for future research.

Light is always with us, such as lights, sunlight, starlight and the light emitted by animals themselves, such as fireflies. Before we begin to classify light, we must first understand the meaning of light source.

Objects that can emit light by themselves are called light sources. Scientists divide light sources into cold light sources and hot light sources.

So what is a cold light source? Cold light source refers to a light source that emits light without heating (or generates heat at a very low temperature). Like fireflies.

On the contrary, a thermal light source refers to a light source that emits heat (it must emit high-temperature heat). Like the sun.

In fact, in some cases, light sources can also be divided into the following three types:

The first is the light produced by thermal effect, and sunlight is a good example. In addition, candles and other items are the same. This light will change color with the change of temperature.

The second is atomic luminescence. The fluorescent substance coated on the inner wall of fluorescent tube is excited by electromagnetic wave energy to produce light, and the principle of neon lamp is the same. Atomic luminescence has its own basic color, so we need to make corresponding corrections when shooting colors.

The third is atomic furnace luminescence, which carries powerful energy, but we rarely have the opportunity to contact it in our daily life. chromatic dispersion

As early as in ancient China, there was an understanding of dispersion, which originated from the understanding of natural dispersion phenomenon-rainbow.

Rainbow is a complex dispersion phenomenon caused by the refraction and reflection of sunlight injected into water droplets in the air at a certain angle. China recorded rainbows in Oracle Bone Inscriptions as early as the Yin and Shang Dynasties. There is a record that the colors of rainbow are divided into "five colors" in the Songs of the South during the Warring States Period. Cheng Dachang (A.D.1123 ~1195) described the phenomenon of dew splitting in Fan Yanlu in the Southern Song Dynasty, and pointed out that sunlight can be converted into many colors through a drop of water, which is actually dispersive, and this color is not owned by the drop itself, but sunlight.

Since the Jin Dynasty in China, many ancient books have recorded the phenomenon of crystal dispersion. For example, it is recorded that peacock hair and insect skin will constantly change color in the sun. When sunlight shines on mica, various colors of light can be observed after reflection. Li Shizhen also pointed out that both larger hexagonal crystals and smaller crystal beads can form dispersion. By the end of the Ming Dynasty, Fang Yizhi made a wonderful summary of the dispersion phenomenon in his book "Introduction to Physics", combining the previous research results. He divided white light into five colors by using natural crystals with edges and triangular crystals artificially fired, which were related to natural phenomena such as five-color artificial rainbows made by water spraying from the sun, five-color phenomena produced by flying springs irradiated by sunlight, rainbow colors, sun and moon halos, and five-color clouds. He thought that "all of them are one principle", that is, they are all dispersion of white light. All these indicate that China had a certain understanding of the nature of dispersion phenomenon before the Ming Dynasty, but it also reflects that China's understanding of ancient physics is mostly scattered and empirical.

So, what is dispersion?

The phenomenon that polychromatic light is decomposed into monochromatic light to form spectrum is called optical dispersion. Using prism or grating as "dispersion system" can realize dispersion. After entering the prism, polychromatic light has different refractive indexes for light with various frequencies, and the propagation direction of polychromatic light is deflected to varying degrees, so when leaving the prism, they are dispersed separately to form a spectrum. For example, a thin beam of sunlight can be divided into red, orange, yellow, green, blue, indigo and purple by prism. This is because the refractive index of each color in polychromatic light is different. When they pass through the prism, the propagation direction will be deflected to varying degrees, so when they leave the prism, dispersion will occur.

The refractive index of the medium varies with the frequency of light wave or the wavelength in vacuum. When polychromatic light refracts at the interface of the medium, the medium has different refractive indexes for different wavelengths of light, and the lights of various colors are separated from each other due to different refraction angles. 1672, Newton used a prism to decompose sunlight into color bands, which was the first dispersion experiment. The dispersion of any medium can be divided into normal dispersion and abnormal dispersion.

Let a beam of white light shine on the glass prism. After the light is refracted by the prism, a colored light band is formed on the white screen on the other side. The color arrangement is that the end of the prism near the top corner is red, the end near the bottom edge is purple, and the middle is orange, yellow, green and indigo in turn. This band of light is called spectrum. The light of each color in the spectrum can't be decomposed into other colors. This kind of light is called monochromatic light. The light mixed by monochromatic light is called polychromatic light. The light emitted by sunlight, incandescent lamps and fluorescent lamps in nature is polychromatic light. When light shines on an object, part of the light is reflected by the object and part of the light is absorbed by the object. If the object is transparent, there is still a part passing through it. Different objects reflect, absorb and transmit different colors, so they show different colors.

Propagation of light

Light travels in a straight line in the same uniform medium. Light can propagate in transparent substances such as vacuum, air and water. The premise of light propagation along a straight line is not only in a uniform medium, but also in the same medium. When light meets another medium, the direction of light will change, and it will still travel in a straight line after the change.

In inhomogeneous media, light usually propagates in a curve. Light travels in all directions, front, back, left, right, up and down. The brighter the light, the less easy it is to see. When the light is dark, the light from the luminous body to the lighting reference object will expand, and the farther away it is, the greater the diffusion, and expand from the original shape until it disappears.

Pinhole imaging and solar eclipse found in our life have proved the fact that light travels in a straight line in a uniform medium.

speed of light

When it thunders and rains in summer, some people may be confused. Why do you see lightning first and then hear thunder every thunderstorm? Today we will discuss the speed of light with this question.

The so-called speed of light is the speed at which light travels in unit time. According to scientific calculation, the speed of light in a vacuum is 300,000 km/s. Generally speaking, light can travel 60 Wan Li per second, but we know that the speed of sound is only 335 meters per second. That's why when it thunders and rains, we first see lightning and then hear thunder.

Since the speed of light is so fast, how long will it take us to see the sun1.500 million kilometers away? Scientists have come to the conclusion that it takes about eight minutes, that is, it takes about eight minutes for light to reach the earth from the sun, which is 0.5 billion kilometers away from us.

In fact, as early as17th century ago, astronomers and physicists thought that the speed of light was infinite, and all the light emitted by cosmic stars reached the earth instantly. 1676, Danish astronomer Romer measured the speed of light through astronomical observation. 1849, the French scientist Fizeau successfully measured the speed of light on the ground for the first time in the laboratory with a sophisticated device. 1973, evenson of the American Bureau of Standards measured the speed of light as (299792485+ 1.2) m/s by using the laser method. Confirmed by 1975 15th International Metrology Conference, the above light speed is regarded as the international recommended value. 1983 The new definition of rice adopted at the17th International Metrology Conference is "the length of light propagating in a vacuum within a time interval of 1/299792458 seconds."

After people measured the speed of light, it replaced the platinum watch kept in the International Bureau of Metrology in Paris and was selected as the standard for defining "meter". It was unanimously agreed that the speed of light was strictly equal to 299,792,458 m/s, and the meter was defined as the distance traveled by light in 65,438+0/299,792,458 seconds, and the speed of light was represented by "C".

Superlight speed

Superlight speed will become a topic of discussion, which stems from the inference limit that local objects can't exceed the speed of light C in vacuum in relativity, and the speed of light has become the upper limit of speed on many occasions. Newtonian mechanics before this did not limit superluminal speed. In the theory of relativity, the speed of motion is closely related to other properties of an object, such as mass, and even the passage of time in its reference frame. If an object whose speed is lower than the speed of light (in a vacuum) wants to accelerate to the speed of light, its mass will increase to infinity, so it needs infinite energy, and the time it feels will even stop (if it exceeds the speed of light, there will be a "time reversal"), so it is theoretically impossible to reach or exceed the speed of light (as for photons, it is because they are always at the speed of light, not from below the speed of light to the speed of light). But it also makes physicists (and the general public) particularly interested in some "seemingly" superluminal physical phenomena.

The so-called "time reversal" is the Doppler effect of light, not the real "time reversal", but the feeling of the world. The Doppler effect is basically due to the absolute propagation velocity of waves, which is only related to the medium and has nothing to do with the motion of the sound source and the receiving object. In other words, wave propagation should take the medium as the frame of reference. Breaking through the light speed barrier will lead to "light barrier". Compared with supersonic flight, it is not impossible.

The condition that the speed of light is constant is that the medium is stable. Because in any stable medium, the velocity of any wave is constant, regardless of the reference frame. When the medium of sound wave is stationary relative to the measurer, the speed of sound remains unchanged (only the audio is changed) no matter how the speed of sound source changes. This is a famous Doppler experiment. All other mechanical waves have similar phenomena.

Slow clock, scale contraction, super-light speed and other time reversal phenomena can be done by sound experiments, which can only prove that Einstein's conclusion is flawed. He ignored the problem of speed measurement and regarded the phenomenon as the physical essence.

Current research shows that there is superluminal-some stars explode and throw debris, and the speed of debris has exceeded the speed of light, but the speed is not fixed, some are fast and some are slow.

However, the academic circles still call the speed of light the fastest speed.

light-year

Usually because the distance on the earth is a little short, it is enough to discuss it in kilometers. For example, the earth is 380,000 kilometers away from the moon and the sun is10.50 billion kilometers away from the earth. However, it seems a bit out of place to use kilometers as a measure of the distance between the universes. So, when we measure the distance between us and many stars, we find that we must use a very huge number to represent it. Just as scientists invented a special unit "angstrom" by studying the wavelengths of different colors of light. So scientists invented a special unit for measuring spatial distance, namely light years. A light year is the distance that light travels in a year. This is a considerable number, because light travels 300,000 kilometers a second. A light year is about 10 trillion kilometers. The nearest bright star, Alpha Centauri, is also 4 light years away. It can be seen how far apart galaxies are.

It takes about 8 minutes for light to reach the earth from the sun (the distance from the earth to the sun is 8 "light minutes").

The nearest known star to the solar system is proxima centauri Centauri, which is 4.22 light years away from the solar system.

Our galaxy is about 654.38 million light-years in diameter. Suppose there is a spacecraft approaching the speed of light from one end of the galaxy to the other, which takes more than 65438+ million years. But this is only for static observers (relative to the Milky Way), and the people on the spacecraft feel that the journey is actually only a few minutes. This is due to the time expansion phenomenon of the moving clock in special relativity.

Argument between particles and waves

/kloc-in the 0/7th century, scholars led by Newton believed that light is a particle stream composed of mechanical particles similar to small projectiles, and luminous objects constantly emit high-speed linear light particle streams to the surrounding space. Once these light particles enter people's eyes and impact the retina, they create vision, which is the particle theory of light. Newton easily explained the phenomena of direct light, reflection and refraction with particle theory. Because particle theory is easy to understand and can explain some common optical phenomena, it is quickly recognized and supported by people.

/kloc-in the 9th century, the interference, diffraction and polarization experiments of light proved that light is a kind of wave, and Maxwell put forward the theory that light is electromagnetic wave, which perfected the theory of light wave.

In the 20th century, people discussed whether light is a particle or a wave for a long time. Finally, it is unified that light, like all other microscopic particles, has the duality of particle and fluctuation, and light is an electromagnetic wave with extremely short wavelength. Later, Einstein's photon theory explained the photoelectric effect well, thus establishing the firm position of the particle nature of light. Now, people realize that light is composed of particles called photons, which has wave-particle duality.

After long-term exploration, people's understanding of light is getting deeper and deeper, and since the wave-particle duality of light was discovered, people began to actively explore the mysteries of the micro-world. Learning point

electromagnetic wave

Electromagnetic waves, also known as electromagnetic radiation, are electric and magnetic fields that oscillate in the same phase, are perpendicular to each other and move in the form of waves in space, and their propagation direction is perpendicular to the plane formed by electric and magnetic fields, effectively transmitting energy and momentum. Electromagnetic radiation can be classified by frequency, from low frequency to high frequency, including radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X rays and gamma rays. The wavelength of electromagnetic radiation that human eyes can receive is about 380 nm to 780 nm, which is called visible light. Any object with a temperature higher than absolute zero can emit electromagnetic radiation, but there is no object with a temperature equal to or lower than absolute zero in the world.