What is the electromagnetic wave emitted by?

Many effects can emit electromagnetic waves. What you need to know about the electromagnetic spectrum is the radio long wave with the longest wavelength, medium wave, short wave and microwave, and then the infrared, visible light, ultraviolet, X-ray and gamma ray with the shortest wavelength.

The following lists the currently known methods of emitting electromagnetic waves:

1, thermal radiation.

As long as the temperature is higher than absolute zero (in fact, all objects, so far we think that nothing can reach absolute zero), electromagnetic waves will radiate. But the intensity and wavelength distribution of radiation are related to the temperature of the object. For example, you can't see the electromagnetic wave emitted by iron at room temperature, which is mostly infrared (the so-called infrared temperature measurement principle is to measure the infrared radiation at this time. ), when it burns red, it starts to radiate red, and then turns blue and white when it is heated, indicating that the higher the temperature, the shorter the main wavelength of emission.

Application distance: incandescent lamp is heated to a certain temperature with tungsten wire to emit light. Flashlight, the most primitive lighting tool, mainly relies on this principle.

2. Electromagnetic oscillation and antenna combination

Mobile phones, radio stations, satellite TV stations and other devices that use electromagnetic waves to communicate all rely on the combination of oscillating circuits and antennas to emit electromagnetic waves. As long as the magnetic field or electric field oscillates, electromagnetic waves will be radiated. Only the efficiency of radiation is different. An oscillating circuit is a circuit that can generate an oscillating current with a certain frequency. The oscillation of current will cause the oscillation of electric field or magnetic field generated by current. Since the electric field/magnetic field oscillates, it will emit electromagnetic waves, so why do you need an antenna? This is because the shape of the antenna can improve the efficiency of generating electromagnetic waves.

Application examples: mobile phones, radio stations, communication satellites, satellite TV stations, walkie-talkies, cordless phones and other devices that use electromagnetic waves for communication.

Microwave ovens also emit microwaves by oscillating current, but this oscillation does not occur in wires, but in vacuum tubes. The principle is the same.

3. Migration radiation of outer electrons.

The principle of this electromagnetic wave is that the outer electrons of atoms or molecules radiate electromagnetic waves when they move from high energy level to low energy level. This radiation can range from infrared radiation to ultraviolet radiation. In order to realize this transition, we must first move the outer electrons from the low energy state to the high energy state (that is, atoms or molecules are excited to the high energy state). Here we discuss it separately.

3. 1 Gas molecules/atoms reach high energy level by gas ionization.

In this method, a certain gas is usually filled in a vacuum glass container, and then the gas is ionized by high pressure, so that it is excited to a high energy level.

Application examples: high-pressure mercury lamp (emitting light with mercury vapor), xenon lamp (emitting light with xenon gas) and early arc lamp (emitting light with air) used in searchlights.

3.2 Direct use of current to excite to high energy level

In this method, the current directly passes through the material and excites the material to a high energy level.

Application examples: light emitting diode, liquid crystal.

3.3 Use other light sources to excite it to high energy level.

In this method, the material is excited to a high energy level by using high frequency light emitted by other light sources, and then it moves back to a low energy level to emit light.

Application example: fluorescent lamp (containing low-pressure mercury vapor, which is ionized by current to emit ultraviolet rays) belongs to the principle introduced in 3. 1 However, these ultraviolet rays irradiate the fluorescent material coated on the surface of the fluorescent lamp, and the fluorescent material is excited to a high energy level, and then moves back to a low energy level to emit visible light.

3.4 Use the energy released by chemical reaction to excite the molecules or atoms in the material to a high energy level.

For example: fireflies, cold light sticks (a kind of lighting apparatus that can emit cold light after bending). In addition, as I said just now, the principle of combustion is mainly 1, but combustion will have some additional principles. Flame reaction is produced by exciting the material to a high energy level in combustion and then moving back to a low energy level.

3.5 laser.

In fact, the principle of laser generation is 3. 1-3.4, but as a special light source, we discuss it separately. The characteristic of laser is that because the pumping source excites the substance (here, the pumping source, or the excitation principle is 3. 1-3.4), its substance stays at the high energy level all the time, and when it is excited, it suddenly jumps to the low energy level, thus emitting a powerful pulse and emitting high-quality light under the action of the resonant cavity.

For example, He-Ne laser uses 3. 1 principle, semiconductor laser uses 3.2 principle, many solid-state lasers need other lasers to pump 3.3, and some dye lasers use 3.4 principle.

4. The electrons in the inner layer of the atom are excited and move back to the original position to emit light.

The light emitted by this principle is called x-ray. There are many kinds of excitation methods, and the common one is to bombard atoms with a beam of electrons.

When the nucleus is excited to a high energy level, it returns to a low energy level.

The light emitted by this principle is generally called gamma rays. There are many reasons why the nucleus is excited, such as nuclear fusion, fission and decay in nature. Artificial bombardment of the nucleus with particles will cause excitation, thus emitting gamma rays.

In addition, this process may also excite the inner electrons, or indirectly excite the outer electrons, so that the phenomena described in Principles 3 and 4 occasionally occur.

6. Various microscopic high-energy particles react to emit light.

Such as the annihilation of positive and negative electrons, the disappearance of a particle's life, and the electromagnetic waves emitted. This phenomenon is rare in the atmosphere, but it will occur in physical experiments.

The above is the principle of emitting electromagnetic waves that has been discovered and used by most people.