Interference is? - Education and Training Encyclopedia

Interference is?

Interference:

ɡàn· Xie

(1) Asking or stopping, mostly refers to not rigidly controlling the internal affairs of the other party. 2 correlation; Relationship: two are gone ~.

Interference phenomenon:

Chea Sim Xi ɡàn·.

1. When two or more waves meet, they will affect each other under certain circumstances. This phenomenon is called interference phenomenon. This phenomenon exists in electromagnetic waves such as sound waves and light waves.

Young's interference experiment;

Y á n Xie Xiyang

Thomas Young, a British physicist, did the experiment of light interference. 1802, he let the sunlight pass through the small hole s (later changed to a slit) as shown in the figure, and the light emitted passed through two adjacent small holes s? 1 and s? 2. The light emitted by these two holes forms bright and dark stripes on the back screen ac due to the interference of light. This experiment not only proves the fluctuation of light, but also can be used to measure the wavelength of light.

optical interference

First, the discovery of light interference phenomenon

In the history of optical development, Italian scientist grimma was the first person to discover the phenomenon of light interference. He did a simple experiment, and the experimental design was as follows: two small holes were punched in the window board of the house, and the sunlight entered the room through this small hole to form two conical beams. Put a screen where the two beams overlap. At this time, you can find that, on the contrary, the brightness of some places on the screen is darker than when only one beam is used for illumination. Grimma first draws a conclusion from this experiment: when two beams of light are superimposed together, the brightness does not always increase. By analogy, he compared the interference phenomenon of light to the phenomenon that stones are thrown into water and cause water waves. However, he did not make a profound theoretical analysis of this interference phenomenon.

Hooke, a British physicist (1635- 1703), studied another coherent phenomenon. He studied the interference of light on the film made of soapy water and mica through experiments. Hooke pointed out that light is a kind of fast vibration with small amplitude, and tried to analyze the causes of color produced by thin film interference. He proposed that three conditions must be met to observe the color on the film: the thickness of the film has a certain limit; The film must be transparent; There must be a good reflective film on the back of the film. He believes that a light pulse with the weakest component leading, and then the strongest component inclining and mixing will produce a blue impression on the film. A light pulse with the strongest component leading and then the weakest component obliquely mixing is red on the retina. Although this explanation is incorrect, it contains some viewpoints of modern interference theory such as the phase difference between two beams of light. Hooke's theoretical study of light played an important role in the transition from Descartes' model that light is etheric pressure to light fluctuation.

Newton designed and carried out Newton's ring experiment to study the interference of thin films and found Newton's ring phenomenon. Newton himself made an instrument to carry out the experiment. He put a plano-convex lens on the biconvex lens, so that the plane of the plano-convex lens faces down, and then slowly pressed it, and rings of various colors appeared around the center. If the plano-convex lens above is slowly lifted away from the biconvex lens below, the colored rings in the center will disappear one by one, which is the famous phenomenon of "Newton's Ring". Newton also found that the colors of color rings have a certain arrangement order; When the two lenses are pressed together, the diameter of the color ring will increase and the width of its periphery will decrease. If the upper lens is lifted, the diameter of the color ring will decrease and the width of its periphery will increase. Newton also measured the radius of the circular ring and found that it was related to the radius of curvature of the lens and the thickness of the air film. The phenomenon of "Newton's ring" is actually the result of "interference" between two beams of light. However, because Newton tends to be light particles, so

Second, the establishment of optical interference theory

Grimm, Hooke, Newton and others discovered the phenomenon of light interference one after another, but they didn't get a correct explanation and didn't establish a correct theory of light interference. By the beginning of the19th century, after the work of Thomas Young, Fresnel and others, the interference theory of light was finally formed and established.

Thomas Young introduced the concept of optical interference into physics for the first time by analogy.

1800, Thomas Young questioned the dominant light particle theory according to his own experiments, and defended Huygens' wave theory. Through the study of sound waves, he put forward the phenomenon of strengthening and weakening when sound waves overlap each other, that is, the interference phenomenon of sound waves. He abandoned the concept that overlapping waves can only be strengthened, and put forward the view that overlapping waves can also be weakened or even cancelled out under certain conditions. Inspired by observing the phenomenon of water wave interference, Yang thought of the interference of parallel light. He put forward the interference phenomenon of light by analogy of water wave interference phenomenon. He said, imagine a group of water waves moving along the calm lake surface at a certain constant speed and entering a narrow waterway, which is connected with the lake. Now let's assume that another group is formed under the action of a certain factor. It reaches the waterway at the same speed as the first set of waves. These two groups of waves do not interfere with each other, but their functions will be combined. If the peaks of one group of waves coincide with the peaks of another group of waves after reaching the channel, a group of waves with higher peaks will be formed. But if the peaks of one set of waves coincide with the valleys of another set of waves, the peaks will just fill the valleys and the water surface will remain calm. I suppose that if two beams of light are combined in the same way.

2. Yang proposed the coherence condition.

Young's research on interference phenomenon puts forward that the basic condition of interference phenomenon is that only two parts of the same beam can interfere, while according to modern theory, only two coherent beams can interfere.

3. Yang discovered the law of interference

In 180 1, Yang put forward the interference law of light that he found in a paper: "Whenever two parts of the same light propagate along different paths, and the directions are accurate or nearly parallel, then when the path difference of light is equal to an integer multiple of the wavelength, the light strengthens each other, while in the middle state of the coherent part, the light is the strongest, and this wavelength is different for different colors of light. Yang was also the first to come here.

Thomas Young also made the famous Young's double-slit interference experiment, and he was the first to put forward the close relationship between interference phenomenon and diffraction phenomenon. Although Yang made outstanding contributions to light wave theory, his work was not recognized by the scientific community at that time, and he was also attacked maliciously. His paper was dismissed as "worthless", and the interference principle he discovered was described as "absurd" and "illogical", which led to Yang's discovery being buried.

4. Further development of light interference theory.

Fresnel (1788-1827)1815 independently obtained the laws of interference and diffraction. At the same time, he praised Thomas Young's outstanding work, which made Young's interference theory recognized by the scientific community and restored his research work on optics. Fresnel's idea of light interference complements Huygens' principle. The famous huygens-fresnel principle was put forward, and the famous double-mirror and double-prism interference experiment was carried out, which made the interference theory of light more perfect. Fresnel applied more mathematical analysis and quantitative calculation methods in optical research. He combined correct physical ideas with superb experimental skills. He has made many profound and accurate quantitative achievements in optical research. He also studied the influence of polarization on interference phenomenon by arago experiment, and obtained the principle of non-interference between two mutually perpendicular polarized lights in 18 19, thus further enriching and developing the theory of light interference.

Third, the modern expression of the interference law of light

1. Interference phenomenon of light

When two (or more) beams are superimposed under certain conditions, they show a stable mutual enhancement or weakening phenomenon at different places in the overlapping area, which is called interference phenomenon. Usually, two independent light sources or two different parts on the same light source will not interfere. By some methods such as reflection or refraction of light, the light emitted by the same light source can be divided into two beams, and when the two beams recombine through different paths in space, interference can be realized.

The enhancement and attenuation of two beams in the overlapping region form an interference pattern. The light that can produce interference phenomenon is called coherent light, the light source that produces coherent light is called coherent light, and the condition that produces interference is called coherent condition.

2. Coherence conditions of light

Necessary conditions for optical interference:

① The vibration frequencies of the two light waves are the same;

② The two light waves have a fixed phase difference at the intersection;

In order to obtain the interference phenomenon with zero amplitude at the weakest point of vibration, in addition to the above two conditions, the following two conditions must be met:

③ The two light waves vibrate in the same direction at the intersection;

④ The amplitudes of the two light waves are the same.

If the vibration produced by two light waves at the intersection is not in the same direction, then the vibration synthesized at that point is not simple harmonic vibration, so it is impossible to interfere. If the phase difference between two light waves at the intersection is not fixed, but changes rapidly and irregularly at any time, then the number of light intensity changes caused by this change is almost infinite within the time interval t required for observation or measurement. At the intersection, only the average light intensity in the t interval can be obtained. This is no different from the sum of the light intensities produced by two light waves alone at this point, so there is no interference phenomenon. Therefore, only when the above three conditions ①, ② and ③ are met can interference occur. However, in order to produce obvious interference phenomenon, the sufficient conditions of optical interference must be met.

Sufficient conditions for optical interference:

(1) The vibration amplitude produced by the two light waves at the intersection is not much different;

② The optical path difference between two light waves at the intersection should not be too big.

If the amplitudes of two light waves are very different at the intersection, there will be no obvious difference between the amplitude of the synthesized vibration at that point and the amplitude of the vibration generated by a single light wave at that point, so no interference phenomenon can be observed. If the optical path difference between two light waves at the intersection is large, when the wave train of one light wave passes by, the wave train corresponding to the other light wave has not yet arrived, and there is no overlap between the two corresponding wave trains, so there will be no interference phenomenon. If the optical path difference is moderate, there will be no obvious interference when two corresponding wave trains partially overlap. Therefore, only when the amplitude difference between the two light waves is very small and the optical path difference between the two light waves is very small can obvious interference be observed. When the amplitudes of the two light waves are equal, a clear interference pattern with zero light intensity can also be observed in the darkest part of the interference fringe.

3. Types of light interference

The interference of light can be divided into two categories. One is wavefront interference, that is, two or more parts are separated from the wavefront of the light wave emitted by the same light source, and then two or more optical devices are combined to produce interference phenomena in the intersection area, such as Young's double-slit interference experiment, Fresnel double-prism interference experiment, Fresnel double-mirror interference experiment and so on.

The other is framing interference. The amplitude of the incident light is decomposed into several parts by the sequential reflection of the two surfaces of the transparent thin plate, and the light wave interference produced by the meeting of these parts is the fractional amplitude interference, such as Newton's ring experiment and Michelson interferometer.

In fractional amplitude interference, if the incident angles corresponding to the interference fringes are the same, it is called equal inclination interference; Interference fringes corresponding to the same thickness are called equal thickness interference.

Using optical interference, we can measure tiny angles, tiny lengths, check surface quality and measure tiny changes in length.

4. Analysis of several famous interference experimental methods

According to the interference law of light, it is necessary to create special conditions in order to observe the stable interference phenomenon of light. These conditions can be summarized as follows: at any moment, it should be two waves emitted by the same batch of atoms, but they pass through different optical paths (such as reflection or refraction). Although the luminescence of each atom changes rapidly, any phase change always occurs in two waves at the same time. Therefore, when they reach the same observation point, they always keep a constant phase difference, and only two beams of light passing through such a special device are coherent. Therefore, some famous optical interference experiments in history are designed by using the above experimental design principles.

1. Young's interference experiment

180 1 year, Thomas Young realized the interference between light and ordinary light sources with extremely simple devices and ingenious ideas. This experimental design is not only the prototype of many other optical interference experiments, but also many important concepts and inspirations can be obtained from it theoretically.

Young's interference experimental device is shown in the figure. Let the light source shine on an opaque light shielding plate (called diaphragm) with a small hole S and a diaphragm behind it with two small holes S 1 and S2. Two groups of spherical light waves emitted by pinhole S 1 and S2 interfere with each other, resulting in symmetrical patterns with different intensities on the screen D. If monochromatic light is used as the light source, it can be used on the screen.

Later, in order to improve the brightness of interference, S, S 1 and S2 used three parallel slits in the experiment, which is called Young's double-slit interference. The eyepiece can directly observe instead of the screen. After the appearance of laser, experiments were carried out with laser as light source, and stable and clear interference fringes could be observed on the screen. You can also zoom in with an eyepiece or take pictures with a camera. In Young's experimental device, it is generally required that the distance d between two holes is between 0. 1mm ~ 1mm, the horizontal observation range is between 1cm ~ 10cm, and the distance between the screen and the double-hole screen is between 1cm ~ 10cm.

From Young's experiment, it can be concluded that two sub-wave sources from the same wave front are always coherent, and then all the design ideas of wavefront interference devices come from Young's ingenious ideas. When analyzing the characteristics of interference fringes, Yang deduced that the distance between interference fringes of two holes is:

That is, the wavelength of light is:

Yang calculated the wavelength of light from this formula, which is the first time in human history to measure the wavelength of light by experiment. Moreover, Young's interference experiment is a decisive experiment in the development of optics, which leads to the revival of light wave theory.

2. Fresnel biprism experiment

18 17, the French physicist Fresnel designed and conducted the famous Fresnel biprism experiment, as shown in the figure: the light emitted by the slit light source S passes through the thin prism P with a very small vertex angle α (about 1), and the light beam emitted from S is divided into ac and bd by the refraction of the prism, with different propagation directions, as if from the virtual light source S 66 shown in the figure. Interference fringes are produced in bc region where two beams of light overlap. When measuring the width of interference fringes produced by biprism, the wavelength of light wave can be calculated by the following formula.

Where L 1 and L2 are the distances p between the light source and the screen and the prism, d is the distance between two virtual light sources S 1 and S2, and △x is the distance between two adjacent stripes on the screen. Then Fresnel completed the experiment of double-mirror interference.

3. Michelson interferometer

Michelson, an American physicist, proposed a precision optical instrument for producing double-beam interference by beam splitting in 188 1 year, hence the name. As shown in the figure, the light emitted by the monochromatic wide light source S is coated with a thin silver semi-reflective surface A on the parallel plane glass plate G 1, and the light is divided into two beams with almost equal intensity. They are reflected from the plane mirrors M 1 and M2, respectively, and then recombined to form isoclinic fringes on the focal plane f of the lens L2. M2 is fixed, and M 1 is installed on a base that can be adjusted in translation. Generally, a compensation plate G2, which is the same as and parallel to G 1, is placed between G 1 and G2.

Therefore, the isoclinic fringe on f can be considered to be generated on the parallel air layer between the reflection surface M 1 and the imaginary reflection surface M'2. The shape of the stripes is concentric circles with the focus as the center. When M 1 moves closer to λ/2, it contracts a stripe towards the center. On the other hand, if it is far away from λ/2, stripes will appear in the center. By measuring the number of fringe changes, the moving distance of M 1 can be calculated. Michelson made the following three important experiments with this instrument: Michelson-Morey experiment for measuring ether drift in 1887; 1892 systematically studied the fine structure of spectral lines for the first time. 1895, the wavelength of light was directly compared with the standard table for the first time.

The design of Michelson interferometer is a typical application of optical interference phenomenon.

Interference (positive body)

Interference is the phenomenon that two waves overlap to form a new composite wave.

catalogue

Overlapping principle of 1 wave

Two types of interference

3 standing wave