Knowledge about "Light"

Light is an electromagnetic wave, and visible light is an electromagnetic wave with a wavelength of 400-700 nanometers. Electromagnetic waves less than 400 nanometers are ultraviolet rays, such as X-rays; electromagnetic waves greater than 700 nanometers are infrared rays, such as microwaves and broadcast radio waves. The unit of wavelength is nanometers.

What is light?

We have been debating whether "light" belongs to waves or particles. Even Newton, who is world-famous for classical mechanics, also discussed this issue. Physics then developed to quantum theory (1900), quantum mechanics, and then Einstein published the theory of reciprocity in 1904, which gave a new explanation to the definition of light: light is both a wave and a particle. . In other words, neither side of the argument is wrong.

Light is a type of electromagnetic wave and a form of energy. Its propagation speed in vacuum reaches 300,000 kilometers per second, and no matter can travel faster than light - some people say that this cannot be said absolutely. When taking black and white photography, we usually use red or green filters. The principle is to use the filter to absorb light that is different from its own color, and convert the absorbed light energy into heat energy and release it. This is why I often feel that the filter is heating up when I use it. For electromagnetic waves, what human eyes can recognize is called visible light, which is what we usually call "light". Light itself cannot be seen. We can only feel it by looking at the light source and relying on reflections. Some insects use ultraviolet light to identify objects, pit vipers use infrared light, and dogs, cows, cats, and horses cannot identify colors.

Types of light

Light sources can be divided into three types.

The first is the light produced by thermal effect. Sunlight is a good example. In addition, candles and other items are the same. This type of light changes color as the temperature changes.

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

The third type is the synchrotron that emits light and carries powerful energy. This is the light emitted by the atomic furnace, but we have almost no chance to come into contact with this kind of light in our daily life, so remember the previous Two are enough.

Impression of light

Light travels in a straight line. When it hits something, it will be reflected. If it is a transparent object, it will still pass through. Depending on the density of the material, there will be twists and turns. ——This is the principle of the lens. In addition, when light encounters a semi-transparent material (such as a soft light plate), there is still a scattering phenomenon, that is, it loses its parallelism and scatters in any direction. The result we see is that the intensity of the light decreases during the propagation process. On the other hand, if light remains in a non-dispersed state, it can travel very far. We know that lasers have such characteristics, and the most common example around us is searchlights, which we will talk about later. The specific types of light used in shooting include astigmatism, direct light or a mixture of the two. Knowing these differences will be of great help when taking photos.

Direct light and reflected light

Astigmatism refers to scattered light. Think about the sunlight that spreads into the room through the curtains after an afternoon, and you will have a rough impression. There are two types of astigmatism, one is formed by transmitted light, and the other is formed by reflected light (in actual shooting, we use a soft light plate to obtain astigmatism, and the reflected light is reflected by the reflector).

If the sunlight is allowed to pass through the diffuser, the light will be scattered in all directions by the diffuser paper. At this time, the light in the dark parts of the nearby subject is enhanced, while the light in the highlight parts is weakened, and the photos taken will appear very soft. At this time, the main light source is the soft light plate - to be precise, it should be the part of the soft light plate that is illuminated by the sun. If the entire soft light panel is a square with a side length of 10 meters, and the part illuminated by the light is a square of 1 square meter, then the size of the main light source should be within the range of 1 meter side length.

When the model is close to the soft light board, the main light source becomes relatively larger, and the astigmatism effect will be brighter than before. In addition, using white paper and white cloth has the same effect. Astigmatism is a method of disrupting the parallelism of light. Therefore, in an astigmatic environment, it is difficult to have an obvious shadow part, and the outline of the shadow will be blurred or even invisible. If you want to get clear shadow edges, you usually use direct light.

Let’s discuss direct light. You can imagine what happens when sunlight directly hits a character’s face. Compared with the surrounding reflected light, the sunlight at this time is very strong, and the difference between light and dark is also quite large, giving people a clear and contrasting impression. We start from the shadow of a plurality of objects and draw straight lines to the opposite points of the objects that cause the shadow. After the straight lines are extended, they will intersect at one point. The light source exists at the intersection point. The number of intersection points and the number of light sources should be equal. The light of the sun and moon is parallel (we can hardly prove that it is not parallel by physical means), so there will be no intersection. This phenomenon can be demonstrated using geometry.

Contrast

Contrast refers to the difference between light and dark. Simply put, it is the difference in the amount of light between highlights and shadows. When we say strong contrast, we mean that there is a large difference in the amount of light between highlights and shadows; when contrast is small, it's just the opposite.

It can be seen that, under the same conditions, the contrast of photos taken with astigmatism should be relatively low - giving the impression that the light is very smooth and soft, creating a Luxurious atmosphere. However, due to insufficient contrast, this kind of photo may appear unclear. On the other hand, the advantage of small brightness difference is that it helps color film reproduce various colors.

Contrary to astigmatism, images taken under direct light give people a vivid feeling. If the ratio of light and dark is moderate, it can also play a role in emphasizing the three-dimensional sense of the subject. At the same time, the edges of the image in the photo look more distinct. This kind of light makes it difficult to correctly display the color of the subject.

Astigmatism is more suitable for Japanese paintings, especially those that emphasize subtle color differences, emotions, and subjectivity. Direct light is suitable for Western painting, or when you want to give the impression of objectivity. In terms of printing, direct light is suitable for black and white, and astigmatism is suitable for color. We will continue to introduce it in the future.

The combination of telephoto lens and astigmatism is more suitable for Japanese paintings and decorative photography; the combination of direct light and telephoto lens is suitable for expressing powerful images - such as sports scenes. The combination of wide-angle lens and direct light is very objective and gives a Western impression; the combination of astigmatism and wide-angle lens is in the middle and is the most difficult to control. There is no concept of light and shadow in Eastern painting techniques.

Sometimes when you encounter physical nouns, you can look them up in the Modern Chinese Dictionary. If you don’t know them at all, you can also read the explanations in the text first. (See Chapter 1 of the revised version of the Modern Chinese Dictionary. Page 468)

Light: usually refers to the substance that shines on an object so that people can see it, such as sunlight, lamplight, moonlight, etc. Visible light is an electromagnetic wave with a wavelength of 0.77-0.39 microns. In addition, Including invisible infrared light and ultraviolet light. Because light is a type of electromagnetic wave, it is also called light wave; in general, light propagates along straight lines, so it is also called light.

Light knowledge

In a narrow sense, optics is the science of light and vision. In the early days, the word optics was only used for things related to eyes and vision. Today, optics is often referred to in a broad sense. It is the science that studies the generation, propagation, reception and display of electromagnetic radiation, as well as the interaction with matter, in a wide range of wavelengths from microwaves, infrared, visible light, ultraviolet to X-rays. .

A brief history of the development of optics

Optics is a subject with a long history, and its development history can be traced back to more than 2,000 years ago.

Human research on light initially focused on trying to answer questions such as "How can people see the objects around them?" About 400 BC (pre-Qin Dynasty), China's "Mo Jing" recorded the world's earliest optical knowledge. It contains eight records about optics, describing the definition and generation of shadows, linear propagation of light and pinhole imaging, and discusses the relationship between objects and images in plane mirrors, concave spherical mirrors and convex spherical mirrors in rigorous writing.

Starting from the "Mo Jing", the Arab Ibn al-Hasham invented the lens in the 11th century AD; from 1590 AD to the beginning of the 17th century, Jensen and Lipsch independently invented the microscope at the same time; until the 17th century In the first half of the century, Snell and Descartes summarized the observations of light reflection and refraction into the laws of reflection and refraction that are commonly used today.

In 1665, Newton conducted experiments on sunlight, which decomposed sunlight into simple components. These components formed a light distribution with colors arranged in a certain order - a spectrum. It brought people into contact with the objective and quantitative characteristics of light for the first time. The spatial separation of each monochromatic light is determined by the nature of light.

Newton also discovered that a convex lens with a large radius of curvature was placed on an optical flat glass plate. When illuminated with white light, a group of colored concentric annular stripes appeared at the contact point between the lens and the glass plate; When illuminated by a certain monochromatic light, a group of concentric ring stripes of alternating light and dark appear. Later generations called this phenomenon Newton's rings. With this phenomenon, the thickness of the air gap of the first dark ring can be used to quantitatively characterize the corresponding monochromatic light.

While discovering these important phenomena, Newton believed that light is a flow of particles based on the linear propagation of light. The particles fly out from the light source and follow the laws of mechanics to move in a straight line at a constant speed in the uniform medium. Newton used this point of view to explain the phenomena of refraction and reflection.

Huygens was an opponent of the particle theory of light. He founded the wave theory of light. It is proposed that "light, like sound, propagates in a spherical wave surface." And it is pointed out that every point reached by light vibration can be regarded as the vibration center of the secondary wave, and the envelope surface of the secondary wave is the wave front (wave front) of the propagating wave. Throughout the 18th century, the particle flow theory of light and the wave theory of light were roughly proposed, but neither was complete.

At the beginning of the 19th century, wave optics was initially formed, in which Thomas Young successfully explained the "thin film color" and double-slit interference phenomena. Fresnel supplemented Huygens' principle with Young's interference principle in 1818, thus forming the Huygens-Fresnel principle that is well-known today, which can perfectly explain the interference and diffraction phenomena of light. It can also explain the linear propagation of light.

In further studies, polarization of light and interference of polarized light were observed.

To explain these phenomena, Fresnel assumed that light is a transverse wave propagating in a continuous medium (ether). In order to explain the different speeds of light in different media, it must be assumed that the properties of ether are different in different materials; more complex assumptions are required in anisotropic media. In addition, more special properties must be given to the ether to explain that light is not a longitudinal wave. Aether of this nature is unimaginable.

In 1846, Faraday discovered that the vibration surface of light rotates in a magnetic field; in 1856, Weber discovered that the speed of light in a vacuum is equal to the ratio of the electromagnetic unit and the electrostatic unit of current intensity. Their findings indicate that there is a certain intrinsic relationship between optical phenomena, magnetism, and electrical phenomena.

Around 1860, Maxwell pointed out that changes in electric and magnetic fields cannot be limited to a certain part of space, but propagate at a speed equal to the ratio of the electromagnetic unit of the current to the electrostatic unit. This is the case with light. An electromagnetic phenomenon. This conclusion was confirmed by Hertz's experiment in 1888.

However, such a theory cannot explain the properties of electric oscillators that can produce high frequencies like light, nor can it explain the dispersion phenomenon of light. It was not until Lorentz founded the electron theory in 1896 that he explained the phenomena of luminescence and light absorption by matter, as well as the various characteristics of light propagation in matter, including the explanation of dispersion. In Lorentz's theory, ether is a vast and infinite immovable medium, and its only characteristic is that light vibrations in this medium have a certain propagation speed.

For important issues such as the wavelength distribution of energy in the radiation of a hot black body, Lorentz theory cannot yet give a satisfactory explanation. Moreover, if Lorentz's concept of the ether is considered correct, the stationary ether can be chosen as the frame of reference, allowing people to distinguish absolute motion. In fact, in 1887, Michelson used an interferometer to measure the "ether wind" and obtained negative results. This shows that in the period of Lorenz's electron theory, people's understanding of the nature of light was still very one-sided.

In 1900, Planck borrowed the concept of discontinuity from the molecular structure theory of matter and proposed the quantum theory of radiation. He believed that electromagnetic waves of various frequencies, including light, can only be emitted from the oscillator with their own definite components of energy. Such energy particles are called quanta, and the quanta of light are called photons.

Quantum theory not only naturally explains the distribution of energy radiated by hot bodies according to wavelength, but also raises the entire issue of the interaction between light and matter in a completely new way. Quantum theory provides new concepts not only to optics, but also to the entire physics, so its birth is usually regarded as the starting point of modern physics.

In 1905, Einstein used quantum theory to explain the photoelectric effect. He gave a very clear expression to photons, specifically pointing out that when light interacts with matter, light also interacts with photons as the smallest unit.

In September 1905, the German "Annals of Physics" published Einstein's article "On the Electrodynamics of Moving Media". The basic principles of special relativity were proposed for the first time. The article pointed out that the scope of application of classical physics, which has dominated since the era of Galileo and Newton, is limited to situations where the speed is much smaller than the speed of light, while his new theory can explain a large number of The characteristics of the process related to the speed of motion completely gave up the concept of ether, and satisfactorily explained the optical phenomena of moving objects.

In this way, at the beginning of the 20th century, on the one hand, it was confirmed that light is an electromagnetic wave from the optical phenomena of light interference, diffraction, polarization and moving objects; on the other hand, it was confirmed from the thermal radiation, photoelectric effect, light pressure As well as the chemical effects of light, etc., there is no doubt that the quantum nature of light - the particulate nature of light.

The Compton effect discovered in 1922, the Raman effect discovered in 1928, and the ultra-fine structure of atomic spectra that could be obtained experimentally at that time all show that the development of optics is closely related to quantum physics. closely related. The history of the development of optics shows that the two most important basic theories in modern physics, quantum mechanics and special relativity, were both born and developed in the study of light.

Since then, optics has entered a new era, becoming an important part of modern physics and the frontier of modern science and technology. One of the most important achievements is the discovery of stimulated radiation of atoms and molecules predicted by Einstein in 1916, and the creation of many specific technologies for generating stimulated radiation.

When Einstein studied radiation, he pointed out that under certain conditions, if stimulated radiation can continue to excite other particles, causing a chain reaction and an avalanche of amplification effects, monochromaticity can finally be obtained. Extremely powerful radiation, namely laser. In 1960, Maiman made the first visible light laser from ruby; in the same year, he made a helium-neon laser; in 1962, a semiconductor laser was produced; in 1963, a tunable dye laser was produced. Since laser has excellent monochromaticity, high brightness and good directionality, it has been rapidly developed and widely used since its discovery in 1958, causing major changes in science and technology.

Another important branch of optics is composed of imaging optics, holography and optical information processing.

This branch can be traced back to the microscope imaging theory proposed by Abbe in 1873, and the experimental verification completed by Porter in 1906; in 1935, Zelnick proposed the phase contrast observation method, and based on this, the phase contrast was made by the Zeiss factory. Microscope, for which he won the 1953 Nobel Prize in Physics; in 1948, Gabor proposed the principle of wave front reproduction, the predecessor of modern holography, for which Gabor won the 1971 Nobel Prize in Physics.

Since the 1950s, people have begun to combine mathematics, electronic technology and communication theory with optics, introducing concepts such as spectrum, spatial filtering, carrier wave, linear transformation and related operations to optics, updating Classic imaging optics formed the so-called "Borier optics". Coupled with the coherent light provided by lasers and the holography improved by Leith and Apatnex, a new subject field-optical information processing was formed. Optical fiber communication is an important achievement based on this theory. It provides a new technology for information transmission and processing.

In modern optics itself, more and more people are paying attention to the nonlinear optical phenomena produced by strong lasers. Laser spectroscopy, including laser Raman spectroscopy, high-resolution spectroscopy and picosecond ultrashort pulses, as well as the emergence of tunable laser technology, have caused great changes in traditional spectroscopy and have become an in-depth study of the microstructure of matter. An important means of movement rules and energy conversion mechanism. It provides unprecedented technology for the study of dynamic processes in condensed matter physics, molecular biology and chemistry.

Optics research content

We usually divide optics into geometric optics, physical optics and quantum optics.

Geometric optics is a discipline that studies the propagation of light based on several basic principles obtained from experiments. It uses the concept of light and the laws of refraction and reflection to describe the path of light propagation in various media. The results it derives are usually always approximations or limits of wave optics under certain conditions.

Physical optics is a discipline that studies the phenomena that occur during the propagation of light based on the wave nature of light, so it is also called wave optics. It can more conveniently study the interference of light, diffraction of light, polarization of light, and the phenomena displayed when light propagates and inserts in anisotropic media.

The basis of wave optics is Maxwell’s equations of classical electrodynamics. Wave optics does not discuss the relationship between dielectric constant, magnetic permeability and material structure, but focuses on explaining the behavior of light waves. Wave optics can explain the phenomenon of light propagating in scattering media and anisotropic media, as well as the behavior of light near the media interface; it can also explain the phenomenon of dispersion and the effects of pressure, temperature, sound fields, electric fields and magnetic fields on light in various media. influence.

Quantum Optics

When Planck was studying blackbody radiation in 1900, in order to theoretically derive an empirical formula that was consistent with reality, he boldly proposed the A completely different assumption from the classical concept, that is, "the energy of the oscillators that make up the black body cannot change continuously and can only take on discrete values."

In 1905, Einstein promoted Planck's above-mentioned quantum theory when studying the photoelectric effect, and then proposed the concept of photons. He believed that light energy was not distributed on the wave front as described in the electromagnetic wave theory, but was concentrated on particles called photons. In the photoelectric effect, when photons hit the metal surface, they are absorbed by the electrons in the metal at once, without the time required to accumulate energy as predicted by electromagnetic theory. The electrons use part of this energy to overcome the impact of the metal surface on it. The suction force is the work function, and the rest becomes the kinetic energy of the electrons after they leave the metal surface.

This discipline that studies the interaction between light and matter based on the properties of photons is quantum optics. Its basis is mainly quantum mechanics and quantum electrodynamics.

The phenomenon that light exhibits both wave and particle properties is the wave-particle duality of light. Later research proved indisputably theoretically and experimentally: Not only does light have this duality, but all matter in the world, including electrons, protons, neutrons and atoms, as well as all macroscopic things, also have their own mass and speed. associated wave characteristics.

Applied Optics

Optics is composed of many sub-disciplines closely related to physics; because it has a wide range of applications, there are also a series of sub-disciplines with strong application backgrounds that also belong to Optical range. For example, photometry and radiometry are related to the measurement of physical quantities of electromagnetic radiation; colorimetry, which uses the normal average human eye as a receiver to study color vision caused by electromagnetic radiation and the measurement of psychophysical quantities; and numerous Technical optics: optical system design and optical instrument theory, optical manufacturing and optical testing, interferometry, thin film optics, fiber optics and integrated optics, etc.; there are also branches that intersect with other disciplines, such as astronomical optics, ocean optics, remote sensing optics, Atmospheric optics, physiological optics and weapon optics, etc.