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The study of light originated in ancient Greece, where philosophers began to think about how vision works. Thinkers such as Plato and Pythagoras believe that our eyes will emit a faint light to detect. These rays will collect information about the objects around us and bring that information back to us in some way.

About a thousand years later, the Arab mathematician Alhazen proved these philosophers wrong by asking a simple question: if our eyes can shine, why can't we see anything in the dark?

Alha once put forward his own vision theory. He believes that our eyes do not create light but capture it. According to him, we see an image of an object from the light of the sun reflected from the object into our eyes.

Al-Hazen's theory is correct that all objects around us reflect light.

This theory can also explain why we can see different colors. When we shine light on a glass prism, we can see that white light contains light of various colors.

For example, when white light shines on a red apple, the apple absorbs all light except red light. In this way, the red light can be reflected into our eyes and let us see the apple.

I still haven't explained what light is. Let's first take a look at Wikipedia's definition of light:

Light or visible light is electromagnetic radiation that can be perceived by the human eye.

Electromagnetic spectrum the spectrum of all electromagnetic radiation is called the electromagnetic spectrum. Visible light is an electromagnetic wave with a wavelength of about 380-700 nm.

Electromagnetic radiation is the electrical and magnetic vibration that propagates at the speed of light in space. The biggest difference between electromagnetic waves and mechanical waves (sound waves, water waves on the lake surface, etc.) is that electromagnetic waves travel without a medium (such as water or air).

Let's take a look at the mechanical waves of sound waves:

We can see a rod moving back and forth in the air. Accordingly, air particles (black spots) also begin to move. The reason why they move like this is entirely due to the effect of pressure.

The higher the particle density, the greater the pressure; the lower the particle density, the lower the pressure. Particles always move from high pressure to low pressure. Imagine a balloon: for a full balloon, the pressure inside the balloon is higher than that outside the balloon (the number of air particles in the same amount of space). When you release the end of the balloon, the air particles will be pushed out of the balloon and the balloon will contract. This indicates that the air particles move from the high pressure area to the low pressure area.

If you look closely at the moving particles, you may notice that each particle moves back and forth between two areas where high pressure occurs in turn. It is this movement that produces fluctuations. The particle itself does not move from the moving rod to the human ear, but the energy of the rod can. Mechanical waves transmit only energy, not mass. These waves require particles in the medium to transmit energy, and then these particles transfer energy to other particles. Another example of a mechanical wave is a fallen domino.

As mentioned earlier, the propagation of electromagnetic waves does not need a medium, they can propagate in a vacuum.

Electromagnetic waves, electricity and magnetism complement each other. There is even an entire discipline of physics devoted to the study of the relationship between them, that is electromagnetism.

Any moving electric charge will produce a magnetic field. For example, when an electric current moves in a straight wire, an electromagnetic field is formed around the wire.

If you want to increase the strength of the magnetic field, you can wind the wire into a coil. This is the basic way electromagnets work.

A changing magnetic field can also produce an electric field, which can further generate an electric current.

An electric current is generated only when the magnet moves (the magnetic field changes).

Now that we know the basic relationship between electricity and magnetism, we can rethink electromagnetic waves.

Create electromagnetic waves | accelerate electrons an accelerated charged particle (such as an electron) produces an electric field, and its motion can also produce a magnetic field. This magnetic field will further produce another electric field, and the electric field will produce a new magnetic field, and the electric field and magnetic field will excite each other in this way. The result is a high-speed electromagnetic wave in space.

Can't you keep up with your thoughts? Don't worry, let's take a look at the following explanation:

Accelerating electrons can be achieved by placing an alternator in the center of the wire (antenna), which can generate alternating current in the wire. As I mentioned earlier, any movement of the charge will produce a magnetic field. Because there is charge movement in our wire, a magnetic field will be formed around the wire, another electric field will be formed around the magnetic field, and another magnetic field will be formed around the electric field. As a result, there is a spontaneous, time-varying process in which electric and magnetic fields excite each other, and the electric and magnetic fields propagate farther and farther away from the antenna. So we generate electromagnetic waves.

The electric field and magnetic field are always perpendicular to each other.

The picture above shows only one of the electric field lines or magnetic field lines. The real electromagnetic waves can be shown in the following picture. Vector E represents the electric field and vector B represents the magnetic field.

In reality, electromagnetic waves starting from the antenna will propagate in all directions.

By controlling the speed at which the current changes direction in the antenna, we can control the frequency of the electromagnetic wave. The higher the frequency, the higher the compression of the wave (the smaller the wavelength). The frequency of the wave does not affect the propagation velocity of the wave itself. The speed of propagation is always equal to the speed of light.

Creating electromagnetic waves | stimulating electrons to use antennas is not the only way to generate electromagnetic waves. Electrons can also emit electromagnetic waves when they transition from a high-energy state in an atom to a low-energy state. You can see that inside the atom, electrons revolve around the nucleus. The farther away the electron is from the center, the higher its energy level is.

We can inject energy into atoms in many ways to keep electrons away from the nucleus. However, this state is unstable and the excited electrons will inevitably return to their initial energy state. The remaining energy (emitted energy) is then released in the form of photons. Depending on the amount of energy released by electrons, photons also carry more or less energy.

But what are photons? Photons are particles without static mass, which represent the minimum discrete energy of light. When the mass of a particle is small enough, it behaves like a wave. Photons have no static mass, so they behave like waves. However, we used to say that light is made up of electromagnetic waves, but now why do we suddenly regard it as a particle?

To better understand this, we must review the centuries-old debate about whether light is a wave or a flow of particles.

Is it waves or particles? In the 17th century, Isaac Newton further developed the theory of particles. In this theory, light is made up of tiny light particles. These particles move in a straight line at the speed of light. He can use this theory to explain the reflection and refraction of light. However, particle theory cannot explain all the other properties of light. For example, light produces an interference pattern. These patterns occur when two (or more) waves interfere with each other. The particle flow can not form an interference pattern, only the interference of waves can be produced. In addition, waves of different wavelengths can explain why different colors can be seen, which is another reason why light should be a wave rather than a flow of particles. Based on some similar experiments, the wave theory of the Dutch scientist Christian Huygens was widely accepted; in the 19th century, James Maxwell could even prove that light is an electromagnetic wave that travels through space at the speed of light.

However, in the 20th century, scientists like Einstein did some new experiments. These experiments seem to show that light is actually made up of a stream of particles. The most important experiment they do has to do with the photoelectric effect: when we shine light on the metal, it produces a photoelectric effect and electrons are emitted from the metal surface.

What makes these experiments special is that when you increase the intensity of the incident light, the energy of the individual electrons emitted remains the same. The intensity of light only changes the number of electrons emitted. The stronger the light, the more electrons are emitted. This couldn't have happened if it were just waves. Because, when the wave hits the electron, the electron absorbs energy from the light wave and cancels out this part of the light. This means that increasing the intensity of light increases the kinetic energy of each emitted electron.

The results of the experiment puzzled scientists. To better understand what is happening, they keep the intensity of the incident light constant, but change the frequency of the light (different colors). The results show that the higher the frequency, the higher the kinetic energy of the emitted electrons.

Thanks to these experiments, Einstein was able to explain what happened. He said that light is not just a wave, it is made up of discrete wave packets called photons. When a photon has enough energy, it can overflow electrons from the metal. The greater the light intensity, the more photons are shot at the surface of the metal, so the more electrons are emitted from the metal. The higher the frequency of light, the higher the energy of each photon, resulting in a higher kinetic energy of the electrons emitted.

In short, light is not only a wave, but also a flow of particles. In quantum mechanics, this is called wave-particle duality. After explaining the photoelectric effect, Albert Einstein wrote:

It seems that sometimes we have to use one theory, sometimes we have to use another theory, and sometimes we can use any of them. We are faced with a new dilemma. We have two contradictory views of reality; neither of them can fully explain the phenomenon of light, but by combining them, we can explain what light is.

-- Einstein

Original link: https://www.cantorsparadise.com/but-what-is-light-7bd0c3309ffa

For the source of the picture, see the original text.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: M. D. Baecke, translator: Nothing, revision: zhenni

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