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ANALYSIS OF THE BASIC PROPERTIES OF OPTICAL LENS MATERIALS

        Optical properties are the basic properties of optical lens materials, which are consistent with various optical phenomena seen in the daily life of the lens, mainly including refraction and reflection of light on the lens surface, absorption of the material itself, and scattering and diffraction phenomena.


        We may analyze the following from the refraction properties of light:


        The light passing through the optical lens will refract or deviate on the front and back surfaces of the lens. The deviation range of the light is determined by the refractive index of the material and the incident angle of the incident light on the lens surface.


        1. Refractive index

The refractive index of transparent medium is the ratio of the velocity C of light in vacuum to the velocity V in medium, n = C / v. The ratio has no units and is always greater than 1.


        The refractive index reflects the refractive ability of the medium. The higher the refractive index is, the more deviated the beam from the air into the medium. The deviation or refraction from the air to the transparent medium with refractive index n can be calculated according to the Snell Descartes law as follows:


        The refraction ray is located in the same plane as the incident ray and the normal. The incident angle I and the refraction angle R are composed of the normal, the incident ray and the refraction ray respectively. Calculation formula: sin I = n sin R.


        Because the speed of light in transparent media changes with the wavelength, the refractive index value is always expressed by reference to a specific wavelength: the reference wavelength is 546.07nm (mercury green spectral line) for e-line, but 587.56nm (helium yellow spectral line) for D-line in the United States and other countries. But this difference has no practical effect, because it is only reflected in the third decimal place of refractive index value.


        At present, the refractive index range of lens materials used in the market is from 1.5 to 1.9.


        2. Dispersion coefficient Abbe number

        The change of refractive index caused by light wave will make white light produce dispersion phenomenon according to different refraction. In fact, the shorter the wavelength is, the higher the refractive index is, and the refraction of visible light extends from the red region to the blue region. The dispersion ability of materials can be described by Abbe number. The Abbe number is inversely proportional to the dispersion force of the material. The specified range of lens material is usually 30-60. The larger the value is, the less the dispersion is. Generally speaking, the higher the refractive index, the greater the dispersion force, and the lower the Abbe number. Although all lenses have dispersion, this factor can be ignored in the center of the lens. Only in the periphery of the lens made of high dispersion materials, the dispersion phenomenon can be easily detected. In this case, the dispersion phenomenon shows that the edge of the off-axis object has color stripes.


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