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Optical Windows
The Application Principle Of Optical Coating Is Briefly Described
May 31, 2018
Principle of membrane interference

1.The fluctuation of light.
The wave-particle duality of light is known to be the same as radio waves and x-rays.The rays are electromagnetic waves, but they have different frequencies.The relationship between the wavelength of electromagnetic wave, frequency u and propagation rate V is as follows:
V = lambda u
Since electromagnetic waves of various frequencies travel at equal speed in a vacuum, they have different wavelengths.High frequency with short wavelength and long wavelength.For comparison, can according to the radio waves, infrared, visible light, ultraviolet rays, X rays, and gamma rays, the size of the wavelength (or frequency), such as put them in turn arranged in a spectrum, the spectrum of electromagnetic spectrum.
In the electromagnetic spectrum, the longest wavelength is the radio wave, which is divided into long wave, medium wave, short wave, ultrashort wave and microwave due to the different wavelength.The second is infra-red, visible and ultraviolet light, which are collectively called light radiation.Of all the electromagnetic waves, only visible light can be seen by the human eye.The wavelength of visible light is about 0.76 microns to 0.40 microns, which is only a small part of the electromagnetic spectrum.Again, x-rays.The shortest wavelength of electromagnetic waves is y rays.
Since light is a kind of electromagnetic wave, it should be realized in the process of transmission, such as interference, diffraction, polarization and so on.

2.Thin-film interference
The film can be a transparent solid, liquid or a thin layer of gas sandwiched between two pieces of glass.The incident light reflected thin film on the surface after the first beam of light, reflection, refraction of light through the film surface and after surface refraction on the second beam of light, the light on the same side of the membrane, separated by the same incident vibration, is a coherent light, belongs to the amplitude of interference.If the light source is an extended light source (surface light source), the interference can only be observed in the specific overlapping area of the two coherent beams, so it is a localized interference.The two surfaces are parallel to each other, and the interference fringes are infinitely far away, usually by means of convergent lens in their image.For the wedge film, the interference fringes are localized near the thin film. Both experiments and theories have proved that interference fringes can be produced only when two light waves have certain relations, which are called coherence conditions.The coherence conditions of the film include three points: the frequency of the two beams is the same;The beam of light waves vibrates in the same direction.The phase difference between two light waves remains constant.
The optical path difference between the two coherent light interfered by thin film is:
Where n is the refractive index of the film;T is the thickness of the film at the incident point;Q is the refractive Angle in the film;Direction /2 is the additional optical path difference caused by the reflection of two coherent beams of light at two interfaces of different properties (one is the optical defiled medium to the optical dense medium, the other is the optical dense medium to the optical defiled medium).The principle of thin film interference is widely used in optical surface inspection, precise measurement of small Angle or linearity, preparation of anti-reflection film and interference filter.

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