Light is a physical term whose essence is a stream of photons in a specific frequency band. A 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 undergo an accelerated motion and release the energy in the form of a wave. If the transition is followed by just enough to fill the vacancy in the orbit and go from the excited state to the stable state, the electron stops transitioning. Otherwise, the electrons jump back to their previous orbits again and release energy in the form of waves.

Background:

The history of the study of light, like mechanics, has been noted in the ancient Greek era, and the law of reflection of light has been known as early as the time of Euclid, but before the separation of natural science and religion, human understanding of the nature of light has hardly improved, and it only stays at the level of understanding of the propagation and use of light. (In addition, history tells us that as early as the early Warring States period in ancient China, Mozi, the founder of Moxue, discovered the law of reflection of light and established China's optical system.) In the seventeenth century, there were already two voices on this issue: the wave theory and the particle theory.

In 1925, the French physicist de Broglie put forward the theory that all matter has wave-particle duality, that is, all objects are both waves and particles, and then several scientists such as the famous German physicist Planck established quantum physicsThe doctrine has completely expanded man's understanding of the properties of matter. In summary, the essence of light should be considered to be photons, which have wave-particle duality ^[5].

The concept of light

Light is a physical term whose essence is a stream of photons in a specific frequency band. A light source emits light because the electrons in the light source gain extra energy.

Characteristics of light

Light has the following five important characteristics at the same time:

1 In geometric optics, light travels in a straight line.

2 In wave optics, light travels in the form of waves. Different wavelengths of light exhibit different colors.

3 The speed of light is extremely fast. 299792458≈3×10⁸m/s in vacuum,

4 In quantum optics, the energy of light is quantized, and the quanta (elementary particles) that make up light are called light quanta, or photons for short, so they can cause chemical changes in substances such as film photosensitive emulsions.

5 Polarization of lightThe polarization phenomenon of light is different from other properties of light, and the human sensory organs cannot sense the existence of polarization. Polarized light can be divided into partially polarized light, linearly polarized light (planar polarized light), elliptically polarized light, and circularly polarized light.  ^[6]

The law of light propagation

Light travels in a straight line in the same homogeneous medium. Aperture imaging, solar and lunar eclipses, and the formation of shadows all testify to this fact.

In geometrical optics, the direction of light propagation is represented by a geometric line with an arrow, which is called a ray ^[1].

There are three laws of light propagation in geometric optics:

(1) The law of linear propagation of light

(2) The law of independent propagation of light.

(3) The law of reflection and refraction of light.

When light travels through the interface of two different media, part of it is reflected and part of it is refracted.

Reflected rays follow the law of reflection, and refracted rays follow the law of refraction.

Scattering, reflection and absorption of light

scatter

According to scientists' measurements, blue light and violet light have relatively short wavelengths, equivalent to small waves; The wavelengths of orange and red light are relatively long, which is equivalent to large waves. When encountering obstacles in the air, the blue and purple lights are scattered everywhere and the entire sky because they can't get over those obstacles. This was discovered 130 years ago by Nobel laureate Rayleigh. In the evening when the sun sets, the sky does not appear blue but red, and the sun is setting and turns dark red. It turns out that in the evening, when the temperature drops, the humidity increases, the concentration of particulate matter increases, and the light encounters more particles, so that the purple and blue parts of the sunlight are invisible, leaving only a little bit of the orange-red light absorbed by the particles to radiate again to form the light, so that it appears red or dark red.

Formation of reflections

When light hits the interface of two different media, part of the light is reflected back into the original medium from the interface, which is called the reflection of light.

classify

Specular reflection: Parallel rays of light are also parallel when they hit a smooth surface, and this reflection is called specular reflection.

Diffuse reflection: Parallel rays of light hit an uneven surface, and the reflected rays go in all directions, this reflection is called diffuse reflection.

Directional reflection: The reflection between diffuse and specular reflection is called directional reflection, also known as non-Lambert reflection, which is characterized by reflection in all directions and uneven intensity in all directions.

Both specular reflection and diffuse reflection follow the law of reflection of light.

distribution

Incident point: the intersection point of the incident ray and the mirror surface;

Normal: A straight line that passes the point of incidence and is perpendicular to the mirror is called a normal;

Angle of incidence: The angle between the incident ray and the normal is called the angle of incidence;

Reflection angle: The angle between the reflected ray and the normal is called the reflection angle.

The Law of Reflection

1. The angle of reflection is equal to the angle of incidence, and the angle between the incident ray and the plane is equal to the angle between the reflected ray and the plane.

2. The reflected rays and the incident rays are on both sides of the normal.

3. The reflected rays, the incident rays, and the normals are all in the same plane.  ^[2]

Four characteristics

1. The coplanar normal is the straight line where the angular bisector of the reflected and incident rays is located.

2. The sum of the angle between the incident ray and the reflective surface on the opposite side and the angle of incidence is 90°

3．Equiangular The angle of reflection is equal to the angle of incidence. The angle of reflection increases with the increase of the angle of incidence, and decreases as it decreases.

4. The reversible light path is reversible

As shown in Figure 1 (A), the ray BO hits the interface against the direction of the original reflected ray (Fig. B), and the reflected ray OA will shoot out against the direction of the original incident ray AO.

According to the law of reflection of light, the optical path is diagrammed

Specific method: first find the incident point, through the incident point as a normal perpendicular to the interface, then the angle bisector of the angle between the reflected ray and the incident ray is the normal. If the reflected ray corresponding to an incident ray is determined, the incident ray and the normal determine the magnitude of the incident angle and the plane where the reflected ray is located, and then determine the reflected ray according to the characteristics of the reflected ray being on the other side of the normal and the reflection angle equal to the incident angle in the reflection law of light ^[3]. 

study

quality

Reflection is an important property of light. Ancient China had a wealth of knowledge in this area, which was reflected in many practical issues.

For humans, the largest reflection of light occurs on the Moon. We know that the moon itself does not emit light, it only reflects the sun's light. According to legend, this incident is mentioned in the "Book of Books", which records the historical facts of the Xia, Shang, and Third dynasties. It can be seen that at that time, people already had the concept of light reflection. In the work "Zhou Ji" during the Warring States Period, it is clearly pointed out: "The sun shines on the moon, the moonlight is born, and it becomes the bright moon." In the Western Han Dynasty, people simply said that "the moon is like a mirror", which shows that they have a deeper understanding of the reflection phenomenon of light. The "Book of Ink" specifically records a light reflection experiment: using a mirror to reflect sunlight on the human body, so that the shadow of the human body can be between the human body and the sun. This not only demonstrates the reflection of light, but is also likely to explain the cause of the moon.

imaging

Flat mirror imaging is the result of light reflection. Ancient China was very creative in this regard. In the earliest days, people used the still water surface as a reflection surface of light, and used it as a mirror (Fig. 12), and this mirror was called "prison". The word "prison" in the Jin Wen of the Western Zhou Dynasty is written very much like a person bending down to take a picture of himself on a plate filled with water. This shows that three or four thousand years ago, the method of using water surface reflection imaging was popular. In the Ming and Qing dynasties, some poor families also used "water mirrors". Didn't the Hu butcher written in "The Outer History of Confucianism" ask his son-in-law, Fan Jin, who was too hot, to pee according to his own description? Although this is not very elegant, it is still a kind of water mirror, and Hu Butcher is by no means the inventor. In the middle of the Zhou Dynasty, with the progress of smelting technology, the metal reflective surface was gradually replaced by the water mirror, and only then did the word "jian" be added with "gold" next to the word "jian". Or "Jian", it is the so-called bronze mirror unearthed in large quantities. As for glass (reflector) mirrors, it's even later.

Ink Sutra

The study of the reflection imaging law of plane mirrors was carried out in the late Zhou Dynasty. In the "Book of Ink", it is pointed out that there is only one image made of a plane mirror; The shape, color, distance, and inversion of the image are all the same as the object. It also points out that the object moves closer to the mirror, the image also moves closer to the mirror, the object is far away, the image is also far away, and there is a symmetrical relationship. This summary is completely correct.

effect

Spherical mirrors

During the Spring and Autumn Period and the Warring States Period, spherical mirrors, the so-called spherical mirrors, also appeared. The reflective surface of the root captivity is different in concave and convex shape, and it is divided into concave spherical mirror and convex spherical mirror. The object is placed in front of the mirror and can be imaged in the mirror. A concave spherical mirror causes a beam of parallel light rays to be reflected and intersected at a point, which is called focus. The convex spherical mirror is a divergent lens, and the focal point is a virtual focus. Because the sun's rays have heat energy, they are concentrated at one point and thrown into the object, which is not only bright, but also heats a lot, which can make the temperature of the object rise and catch fire. In Europe, legend has it that in ancient Greece, the Romans drove a large fleet of warships to attack Syracuse. The physicist Archimedes (287-212 BC) used a huge concave mirror to point at the sun, concentrated the light on a warship, burned it, and won the war. Of course, this is just a legend. In ancient China, the concave mirror was indeed one of the main fire tools for people.

Translucent lenses

Speaking of reflection, we can't fail to introduce a wonderful mirror in ancient China, that is, the "translucent mirror". The shape of the "translucent mirror" is exactly the same as that of ordinary copper mirrors in ancient times, it is also made of metal, with patterned text on the back, and the reflective surface is polished very bright, which can illuminate people. It stands to reason that when a ray of light hits the mirror surface and reflects it on the wall, it should be a plain circular area of light. The wonderful thing is that in this bright area, there is a pattern on the back of the mirror, as if it is "through", so it is called "through the mirror". A Western Han Dynasty translucent mirror treasured by the Shanghai Museum, with the eight characters "Seeing the light of the sun, the world is bright" on the back, and even the pattern is " "Transparent" is clearly visible in that bright area. It's hard to imagine. Not only have Chinese scientists of all dynasties studied it, but many foreign scientists in modern times have also been amazed and called it a "magic mirror" and have studied it in an attempt to unravel this mystery. For some time in the nineteenth century, there was a heated discussion, but no satisfactory answer was received. In recent years, Chinese scientists have made gratifying achievements in the study of translucent mirrors by using modern scientific and technological means.

Ancient Mirror

A large number of bronze mirrors have been unearthed in China, among which there are many before the Qin Dynasty. However, the "translucent" test was not carried out one by one, so it is not certain that there must be no translucent lenses. In the Song Dynasty, Shen Kuo, the author of "Six Memories of the Floating Life", hid a translucent mirror in his home, and the words behind it were so "ancient" that even he did not know the erudite himself. It can be seen that this is not an ordinary text, it may be something before Qin. One side of the Shanghai Museum's collection has been identified as a relic of the Western Han Dynasty, and after the Western Han Dynasty, the people can gradually make more and more. In the Qing Dynasty, mirrors in Jiangsu and Zhejiang could also be manufactured and spread to Japan. As for the written record, in the novel "The Ancient Mirror" of the Qing Dynasty, it is described that an "ancient mirror" is "inherited by the sun, and the text is painted on the back, and the ink is in the shadow, and there is no loss." "It's obviously a translucent lens. In the Song Dynasty, Shen Kuo recorded the collection of translucent mirrors in great detail, and there were more inscriptions in the records of literati in successive dynasties.

absorb

The essence of the interaction between electromagnetic radiation and matter is that the substance absorbs light energy and then transitions. Transition refers to the change of a substance's own energy after absorbing light energy, and 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 ^[2].

Applications of light

Light in energy (clean energy), electronics (computers, televisions, projectors, etc.), communications (fiber optics), healthcare (γ light knife, light wave room, light wave sweat room, X-ray machine), video camera, microscope, telescope, traffic light, street lamp, neon signs, computers, magnifying glasses, etc. have a wide range of applications.

Related Concepts:

light source

The object that is emitting light is called a light source, and it must be in this condition, and the light source can be natural or artificial. In physics, it refers to an object that can emit electromagnetic waves in a certain wavelength range (including visible light, ultraviolet, infrared, X-rays and other invisible light).

There are three main types of light sources.

The first category is the light produced by the thermal effect. Sunlight is a good example because the surroundings are cooler than the sun, and in order to achieve thermal equilibrium, the sun releases 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 applied to the inner wall of the fluorescent lamp tube is excited by the energy of electromagnetic waves to produce light. In addition, the principle of neon is the same. Atomic luminescence has its own characteristic spectral lines. Scientists often use this principle to identify elemental species.

The third type is the light produced when the charged particles inside the substance are moving at an accelerated pace. For example, synchrotrons emit synchrotron light that carries a lot of energy.

photon

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 an intrinsic basic structure (constituent particles) like physical particles such as molecules and atoms.

velocity of light

It usually refers to the rate at which electromagnetic waves (including light waves) propagate in a vacuum, and is often denoted by C. The velocity of electromagnetic waves of various wavelengths in a vacuum is a constant measured at a value of c = 2.99792458×108 m/s ^[3].