Research Background
The research history of light is the same as mechanics. It has been noticed in ancient Greece. Before the separation of religion, mankind's understanding of the nature of light has hardly progressed, and just stayed at the level of understanding of the form of the spread and use of light. (In addition, history tells us that as early as the Warring States Period, Mo Zi, the founder of Mohism, discovered the law of reflection of light and established China's optical system.) In the 17th century, wave theory and particles began to exist on this issue. Doctrine of two voices.
In 1925, French physicist De Broglie put forward the theory that all matter has wave-particle duality, that is, all objects are both waves and particles. Then the famous German physicist Several scientists such as Planck established the theory of quantum physics, which has completely expanded the human understanding of the properties of matter. In summary, the essence of light should be considered as photons, which have wave-particle duality. But the meaning of wave here is not a mechanical wave like sound wave and water wave, but a wave in a statistical sense, that is to say, the nature of the wave reflected by the behavior of a large number of photons. At the same time, light has a dynamic mass, and its mass can be calculated according to Einstein's mass-energy equation.
The concept of light
Light is a physics term, and its essence is a stream of photons in a specific frequency band. The light source emits light because the electrons in the light source gain extra energy. If the energy is not enough to make it jump to a more outer orbit, the electrons will accelerate and release energy in the form of waves. If the vacancy of the orbital is just filled after the transition, the electrons will not move from the excited state to the stable state. Otherwise, the electrons will jump back to the previous orbit again and release energy in the form of waves.
Characteristics of light
Light has the following four important characteristics at the same time:
1 In geometric optics, light travels in a straight line. The straight beam of light and the sun's rays all illustrate this point.
2 In wave optics, light travels in the form of waves. Light is like water waves on the surface of water, with different wavelengths of light showing different colors.
3 The speed of light is extremely fast. It is 299792458≈3×10⁸m/s in vacuum, and the speed in air is slower. In a medium with a higher refractive index, such as in water or glass, the propagation speed is slower.
4 In quantum optics, the energy of light is quantized and constitutes the quantum of light (basic particles). We call it light quantum, or photon for short, so it can cause chemical changes in film emulsions and other substances. .
The law of light propagation
Light propagates in a straight line in the same homogeneous medium. Hole imaging, solar and lunar eclipses, and the formation of shadows all prove this fact.
Leave aside the wave nature of light, and based on the straight line propagation of light, the discipline that studies the propagation of light in media and the law of object imaging is called geometric optics. In geometric optics, a geometric line with an arrow represents the direction of light propagation, which is called ray. Geometrical optics treats an object as a combination of countless object points (in the approximate case, the object can also be used to represent the object), the light beam emitted by the object point is regarded as a collection of countless geometric light rays, and the direction of the light rays represents the transmission direction of light energy .
There are three law of light propagation in geometric optics:
(1) The law of light propagation in a straight line is as above. Geodesy is also based on this.
(2) The law of independent propagation of light. When the two beams meet during the propagation process, they do not interfere with each other, and continue to propagate according to their respective paths. When the two beams converge to the same point, the light energy at that point is simply added.
(3) The law of light reflection and refraction.
When light encounters the interface between two different media during its propagation, part of it reflects and part of it refracts.
Reflected light obeys the law of reflection, and refracted light obeys the law of refraction.
Scattering, reflection and absorption of light
Scattering
According to scientists’ determination, blue light and purple light have relatively short wavelengths, which are equivalent to small waves; orange The wavelengths of light and red light are relatively long, which is equivalent to large waves. When encountering obstacles in the air, the blue light and purple light are scattered everywhere, covering the entire sky, because they can't get over those obstacles, and they are scattered into blue in this way. This was discovered by Nobel Prize winner Rayleigh 130 years ago. When the sun sets in the evening, the sky does not appear blue but red, and the setting sun turns dark red. The same is true. It turned out that in the evening the temperature dropped, the humidity increased, and the concentration of particulate matter increased. The light encountered more particles, making the purple and blue parts of the sunlight invisible, leaving only a little orange-red light absorbed by the particulate matter. The light formed by radiation, thus appears red or dark red.
Reflection
When sunlight illuminates the earth, part of the radiation is reflected by the atmosphere, part is reflected by land, water, etc., and part is reflected by ice and snow. Why is the earth's equator so hot and the north and south poles so cold? Analyzed from the distance and angle between the sun's irradiation, the heat energy absorbed by it can not be so different. It is mainly caused by the action of the geomagnetic field. The magnetic field lines of the two poles are very dense, indicating that the magnetic field is relatively large. The lines of magnetic force are straight. Light enters the magnetic field and travels along the lines of magnetic force. In addition, there are few human activities in the two poles, fewer solid particles are emitted, fewer other gas molecules in the air, and less light radiation gas, solid or liquid scattering. Therefore, its temperature is very low, and eventually a cold pole appears.
Absorption
The essence of the interaction between electromagnetic radiation and matter is that the matter undergoes a transition after absorbing light energy. Transition refers to the change in the energy of a substance after absorbing light energy. Because this change is quantized, it is called a transition. Different wavelengths of light have different energies and different transition forms, so there are different spectroscopic analysis methods.
The effect of light
Photoelectric effect
When ultraviolet rays are irradiated on the surface of the metal, the free electrons inside the metal will escape the surface of the metal. Photoelectron emission forms part of the ultraviolet photoelectric effect. The photoelectric effect of ultraviolet light is a way of converting light energy into electrical energy. The photoelectric effect is divided into external photoelectric effect, internal photoelectric effect and photovoltaic effect. In addition to metals and semiconductors, there are certain gases and some chemical substances that can produce photoelectric effects under ultraviolet radiation. People, animals and plants can also produce photoelectric effects after being irradiated.
Photochemical effect
Ultraviolet rays can produce photochemical reactions when irradiating certain substances. The energy (3~6eV) of ultraviolet light with a wavelength of 200~400 nanometers is exactly the energy required for photochemical reaction after absorption by many substances (the chemical bond energy is also in the range of 3~6eV). In particular, the photon energy of short-wave ultraviolet rays is relatively large, which is particularly effective for photochemical reactions, and can directly cause the combination and decomposition of some substances.
Acousto-optic effect
When there is elastic stress or strain in the medium, the optical properties (refractive index) of the medium will change. This is the elasto-optic effect. When the ultrasonic wave propagates in the medium, because the ultrasonic wave is an elastic wave, it will cause the density of the medium to change alternately, or cause elastic deformation. Due to the elastic-optical effect, the optical properties of the medium will change, thereby affecting the light in it. Propagation characteristics. The elastic-optical effect caused by ultrasound is usually called the acousto-optical effect.
Light applications
Light in energy (clean energy), electronics (computers, TVs, projectors, etc.), communications (fiber optics), medical care (γ-ray knife, light wave room, Lightwave sweat room, X-ray machine) and other aspects have a wide range of applications.
Related concepts
Light source
The object that is emitting light is called the light source. This condition must be met. The light source can be natural or artificial. In physics, it refers to an object that can emit electromagnetic waves of a certain wavelength range (including visible light and ultraviolet, infrared, X-ray and other invisible light).
Light sources can be divided into three main categories.
The first category is light generated by thermal effects. Sunlight is a good example, because the surrounding environment is cooler than the sun. In order to achieve thermal equilibrium, the sun will continue to release energy in the form of electromagnetic waves until the surrounding temperature is the same as it.
The second type is atomic transition luminescence. The fluorescent substance coated on the inner wall of the fluorescent lamp tube is excited by electromagnetic wave energy to generate light. In addition, the principle of neon lights is the same. Atomic luminescence has its own characteristic spectral line. Scientists often use this principle to identify element types.
The third type is the light generated when the charged particles inside the substance accelerate. For example, the synchrotron radiation emitted by a synchrotron carries powerful energy at the same time.
In addition, the light blue dim light (Cherenkov radiation) emitted by the atomic furnace (nuclear reactor) also belongs to this kind.
Photons
According to quantum field theory (or quantum electrodynamics), photons are the direct result of the quantization of electromagnetic fields. The particle nature of light reveals that the electromagnetic field, as a substance, has its inherent basic structure (composition particles) just like the physical particles such as molecules and atoms. In the classic electrodynamics theory, there is no concept of photons. In quantum physics, photons are the microscopic components of the electromagnetic field, and the electromagnetic field is the cumulative effect of a large number of photons. Just like the earth's water distribution is the cumulative effect of a large number of water molecules.
Speed of light
Usually refers to the velocity of electromagnetic waves (including light waves) propagating in a vacuum, usually expressed as c. The experiment measured that the speed of electromagnetic waves of various wavelengths in vacuum is a constant, and its value is c=2.99792458×108m/s.
Superluminal speed
Superluminal speed will become a topic of discussion. It originates from the inference limit in the theory of relativity that local objects cannot exceed the speed of light c in vacuum. The speed of light becomes the speed of many occasions. Upper limit. Before this, Newtonian mechanics did not limit the speed of super-light. In the theory of relativity, the speed of motion is closely related to the other properties of the object. If an object with a speed lower than the speed of light (in a vacuum) accelerates to the speed of light, its mass will grow to infinity, which requires infinite energy, and the time it feels passes by. It will even stop, so it is theoretically impossible to reach or exceed the speed of light.
But this theory is not sacred and inviolable. Since 1955, a series of theoretical and experimental studies have attempted to discover superluminal phenomena, and multiple experiments have shown that superluminal speed is possible. An object needs infinite energy to reach the speed of light, but it cannot exceed the speed of light in parallel space. Scientists have now proposed ideas to compress the space in front of the object and expand the space behind the object to exceed the speed of light. It just requires a huge amount of energy, which cannot be achieved with existing technology.