Relationship between light fluctuation and wavelength

Red Star Fei Da Juren Level 4 (2405) ||| My Know | My Message (0/0) | My Space | Baidu Home | Exit.

I really know my problem.

My answer

Knowledge supervisor

Post it on the news web page, and know the MP3 picture video encyclopedia.

Help settings

Baidu knows that > science and engineering disciplines > search to increase physics-related content has been solved.

What is the fluctuation of light?

Reward score: 0- Solution time: April 2008-1 1 20: 47.

Questioner: LANDLYAAA- the best answer during the probation period

First, the variability of light.

1. Interference of light: When two light waves meet in air, they overlap, always strengthen in some areas, weaken in some areas, and have alternating stripes or color stripes.

Conditions of optical interference: there are two wave sources whose vibration conditions are always the same, namely coherent wave source (the frequencies of coherent wave sources must be the same).

There are two ways to form a coherent wave source:

Use lasers (because lasers emit excellent monochromatic light).

(2) Try to divide the same beam into two beams (so that both beams come from the same light source, so the frequencies must be equal).

(3) Young's double-slit experiment:

Bright line: the optical path difference from a certain point on the screen to the double slit is equal to an integer multiple of the wavelength, that is, δ = n λ (n = 0, 1, 2, ...).

Dark line: the optical path difference from a certain point on the screen to the double slit is equal to an odd multiple of half wavelength, that is, δ = (n = 0, 1, 2, ...).

Distance between adjacent bright lines (dark lines). The wavelength of monochromatic light can be determined by this formula. When doing double-slit interference experiment with white light, because the wavelengths of various colors in white light are different, the spacing of interference fringes is also different, so there are white bright lines in the center of the screen and colored stripes on both sides.

(4) thin film interference:

Application:

A thin layer of air is formed between the detection plane and the standard model and irradiated with monochromatic light. The incident light reflects two light waves on the upper and lower surfaces of the thin air layer, which are superimposed in space. Uniform interference fringes: smooth surface; Uneven: The detected plane is uneven.

Anti-reflection film: The thickness of the transparent film plated on the lens surface is the wavelength of the incident light in the film, and the optical path difference of the reflected light on both sides of the film is exactly equal to half a wavelength, which cancels out each other, thus reducing the reflected light and increasing the transmitted light intensity.

Other phenomena: the color of soap bubbles in the sun.

Example 1. In the double-slit interference experiment with green light, green and dark stripes appear on the screen, and the distance between two adjacent green stripes is δ X. The following statement is correct.

A. If the distance between single seam and double seam is increased, Δ x will increase.

B. If the distance between double seams increases, Δ x will increase.

C. If the distance between the double slit and the light screen increases, δx will also increase.

D. If the width of each double seam is reduced without changing the distance between the double seams, Δ x will increase.

Solution: In the formula, L represents the distance between the double seams and the screen, and D represents the distance between the double seams. So δx has nothing to do with the distance from single seam to double seam, and has nothing to do with the width of the seam itself.

Example 2. Mountaineers should pay attention to prevent excessive exposure to ultraviolet rays when climbing snow-capped mountains, especially their eyes should not be exposed to ultraviolet rays for a long time, otherwise their eyesight will be seriously damaged. Some people want to use the principle of thin film interference to design glasses that can greatly reduce the harm of ultraviolet rays to the eyes. The refractive index of the film material he chose is n= 1.5, and the ultraviolet frequency to be eliminated is 8. 1× 1068.

Solution: In order to reduce the ultraviolet rays entering the eyes, the incident light reflected from the front and back sides of the film should be superimposed and strengthened, so the optical path difference should be an integer multiple of the wavelength, so the thickness of the film should be at least 1/2 of the ultraviolet wavelength in the film. The wavelength of ultraviolet in vacuum is λ=c/ν=3.7× 10-7m, which is in the thin film.

2. Diffraction of light:

Note that the expression of diffraction must be accurate.

All kinds of obstacles with different shapes can diffract light.

⑵ The condition for obvious diffraction is that the size of obstacles (or holes) can be even smaller than the wavelength.

(3) Diffraction phenomenon: alternating light and dark stripes or colored stripes.

(Compared with interference fringes, the central bright fringe is uneven because it is wide on both sides and narrow on both sides. If it is white, there is a white central bright stripe. )

Example 3. Compared with Poisson's bright spot, the diffraction pattern obtained by parallel light passing through a small hole is correct.

A. there are bright spots in the center of diffraction pattern.

B. The dark ring around the central bright spot of Poisson bright spot is wider.

C. The center of pinhole diffraction diffraction pattern is dark spot, and the center of Poisson bright spot pattern is bright spot.

D. The distance between light and dark stripes in pinhole diffraction pattern is uniform, while the distance between light and dark stripes in Poisson bright spot pattern is uneven.

Solution: As can be seen from the pictures in the textbook, options A and B are correct and options C and D are wrong.

3. Spectrum:

Spectral analysis can be carried out by atomic spectrum or absorption spectrum. The solar spectrum is an absorption spectrum, and the components of the solar atmosphere can be found in the dark lines of the solar spectrum.

4. Electromagnetic theory of light:

(1) According to the fact that electromagnetic waves and light travel at the same speed in vacuum, Maxwell proposed that light is essentially electromagnetic waves, which is the electromagnetic theory of light. Hertz proved the correctness of the electromagnetic theory of light through experiments.

(2) electromagnetic spectrum. The order of wavelength from big to small is: radio wave, infrared ray, visible light, ultraviolet ray, X ray and gamma ray. In all kinds of electromagnetic waves, except visible light, there is overlap between two adjacent bands.

The generation mechanism of various electromagnetic waves is as follows: radio waves are generated by the periodic motion of free electrons in an oscillating circuit; The electrons in the outer layer of atoms are excited to produce infrared light, visible light and ultraviolet light; The electrons inside the atom are excited to produce roentgen rays; Gamma rays are produced after the nucleus is excited.

⑶ The main characteristics of infrared rays, ultraviolet rays and X-rays and their application examples.

kind

produce

Main attributes

Application example

infrared ray

Everything can be launched.

heat effect

Remote sensing, remote control, heating

ultraviolet ray

All high-temperature objects will emit

chemical effect

Fluorescence, sterilization

roentgen rays

Cathode rays hit a solid surface.

Strong permeability

Human body perspective, metal flaw detection

In order to broadcast the live broadcast of the rocket launch site, the launch site has set up a launch pad to transmit radio and television signals. The wavelength of electromagnetic wave for radio is 550m, and that for TV is 0.566m m. In order to prevent the hill near the launch site from blocking the signal, it is necessary to build a relay station on the hill to relay the _ _ _ _ _ _ _ signal, because the wavelength of the signal is too _ _ _ _.

Solution: The longer the wavelength of electromagnetic wave is, the easier it is to diffract obviously. The shorter the wavelength is, the less obvious the diffraction is, and it propagates linearly. At this time, it is necessary to build a relay station at the top of the mountain. So the relay station in this problem must relay the TV signal, because its wavelength is too short.

Example 5. The picture on the right shows the structure of Roentgen X-ray tube. The power supply E heats the filament K, thus emitting hot electrons, which fly to the opposite cathode A at high speed under the strong electric field between K and A, and the electron flow hits the surface of the A pole, exciting high-frequency electromagnetic waves, that is, X-rays. The following statement is correct.

A, p and q should be connected with high-voltage direct current, and q should be connected with positive pole.

B.p, q and q should be connected with high-voltage alternating current.

Between C.K., A.K. and A is a high-speed electron flow, that is, cathode rays, X-rays, that is, high-frequency electromagnetic waves, emitted from A.

D. the frequency of x-rays emitted from a is the same as that of alternating current between p and q.

Solution: The direction of the electric field between K and A should always be to the left, so P and Q should be connected with high-voltage DC, and Q should be connected with the positive pole. X-rays are emitted from A, and their frequency is determined by photon energy. If the voltage between P and Q is U, the frequency of X-ray can reach Ue/h at the highest.

Second, the particle nature of light.

1. photoelectric effect

(1) The phenomenon that an object emits electrons under the irradiation of light is called photoelectric effect. (In the device on the right, the zinc plate is irradiated with an arc lamp, and electrons fly out from the surface of the zinc plate, so that the originally uncharged electroscope is positively charged. )

(2) Einstein's photon theory. Light is discontinuous, and each photon is called a photon. Photon energy e is proportional to the frequency of light v: e = h v.

(3) photoelectric effect law:

All metals have a limit frequency ν0, and only ν≥ν0 can photoelectric effect occur;

Instantaneity (photoelectron generation does not exceed 10-9s).

③ The maximum initial kinetic energy of photons has nothing to do with the intensity of incident light, but only increases with the increase of incident light frequency;

④ When the frequency of incident light is greater than the limit frequency, the intensity of photocurrent is directly proportional to the intensity of incident light.

⑷ Einstein photoelectric effect equation: Ek= hν-W(Ek is the maximum initial kinetic energy of photoelectrons; W is the work function, that is, the work done by photoelectrons flying directly from the metal surface to overcome the attraction of positive charges. )

Example 6. For Einstein's photoelectric effect equation EK= hν-W, the following understanding is correct.

A. As long as the same metal is irradiated with light with the same frequency, all photoelectrons escaping from the metal will have the same initial kinetic energy EK.

B w in the formula represents the work done by each photoelectron to overcome the positive charge attraction in the metal when it flies out of the metal.

The relation w = h ν0 should be satisfied between the work function w and the limit frequency ν 0.

The maximum initial kinetic energy of photoelectrons is proportional to the frequency of incident light.

Solution: W EK = hν-W in Einstein's photoelectric effect equation represents the work that photoelectrons directly escape from the metal surface to overcome the positive charge attraction in the metal, so it is the minimum value of the work that all escaped photoelectrons do to overcome the attraction. The initial kinetic energy of photoelectrons is the largest among all photoelectrons. The initial kinetic energy of other photoelectrons is less than this value. If the frequency of incident light happens to be the limit frequency, just a part of photoelectrons can escape. It can be understood that the maximum initial kinetic energy of the escaped photoelectron is 0, so there is W= hν0. From EK= hν-W, it can be seen that the relationship between EK and ν is a linear function, but it is not proportional. This question should be C.

Third, the wave-particle duality of light

1. Wave-particle duality of light

Interference, diffraction and polarization show that light is a wave with irrefutable facts; The photoelectric effect and Compton effect show that light is a particle with irrefutable facts; So modern physics thinks that light has wave-particle duality.

2. Correct understanding of wave-particle duality

The wave mentioned in wave-particle duality is a kind of probability wave, which is meaningful to a large number of photons. The particle mentioned in wave-particle duality refers to its discontinuity and is a kind of energy.

(1) The effect of a single photon is often particle; The action of a large number of photons often appears as fluctuation.

(2) High photon easily visible particles; Photons with low v tend to show fluctuations.

⑶ Light often shows fluctuation in the process of propagation; When it interacts with matter, it often appears in the form of particles.

⑷ From the expression of photon energy E=hν and photon momentum, it can be seen that the fluctuation of light is not contradictory to the particle nature: the calculation formulas of particle energy and momentum representing the particle nature all contain physical quantities representing the characteristics of waves-frequency ν and wavelength λ.

Interviewee: 6 15336- magic apprentice level 1 2008-4-5 19:28

Let me comment >>

The questioner's evaluation of the answer: thank you