The optical purpose of the lens is to restore clear vision to the eyes with refractive errors by wearing corrective lenses. Therefore, the following factors closely related to the refractive function of the lens need to be considered when selecting lens materials:

　　 Since the speed of light of a transparent medium varies with wavelength, the value of refractive index is always expressed with reference to a specific wavelength: in Europe and Japan, the reference wavelength is e-line 546.07nm (mercury-green spectrum Line), but in other countries such as the United States, it is d-line 587.56nm (helium-yellow spectral line). But this difference has no actual effect, because its difference is only reflected in the third decimal place of the refractive index value.

　　 The refractive index of lens materials currently used in the market ranges from 1.5 to 1.9.

Optical lens 2) Dispersion coefficient: Abbe number

　　 The change in refractive index caused by light waves will cause white light to produce dispersion based on different refraction. In fact, the shorter the wavelength, the higher the refractive index, and the refraction of visible light extends from the red region to the blue region of the spectrum. The dispersion capability of a material can be described by Abbe number, e-line is prescribed in Europe and Japan, and d-line is prescribed in the United States and other countries.

　　 The Abbe number is inversely proportional to the dispersion force of the material. The lens material usually ranges from 30 to 60. The larger the value, the less the dispersion. Generally speaking, the higher the refractive index, the greater the dispersion power and the lower the Abbe number. Although all lenses have chromatic dispersion, at the center of the lens, this factor can be ignored. Only at the periphery of the lens made of high-dispersion materials, the chromatic optical lens astigmatism can be easily noticed. In this case, the chromatic dispersion phenomenon is represented by colored fringes on the edges of off-axis objects.

　　(2) Light reflection

　　 When the light is refracted on the surface of the lens, it will also reflect. The reflection of light will affect the clarity of the lens, and interfering reflected light will be produced on the surface of the lens. Generally, the higher the refractive index of the lens material, the more light is lost due to reflection. Of course, the interfering reflected light can be counteracted by plating multiple anti-reflection coatings on the surface of the lens.

　　 Lens: A piece of glass or other transparent materials with one or more curved surfaces, through which things appear clear, larger or smaller. Usually used in glasses, cameras, telescopes, etc.

Focusing lens lenses can be divided into the following five types according to different materials: resin lenses, special lenses, space lenses, glass lenses, pvc lenses

　　 Myopia glasses are concave lenses, and hyperopia glasses are convex lenses; lenses with deep power are thick, and lenses with light power are thin;

　　能, measure the power of its two lenses separately: take one lens directly facing the sun, and then put a piece of paper on the other side of it Change the distance between the lens and the paper until the light spot on the paper becomes the smallest and brightest. Measure the distance from the smallest and brightest light spot to the convex lens. This is the focal length of this presbyopic lens. The reciprocal of the focal length is the lens power. The value of multiplying the lens power by 100 is the power of the spectacle lens, and the power of the other lens is also measured in this way. Measure the power of the two lenses to know that they are not the same aspheric lens power.

What is the lens of myopia glasses?

　　 is a concave lens. Myopia refers to the state in which the human eye is relaxed and has no adjustment, and the focus of parallel rays falls in front of the retina after being refracted by the entire refractive system of the eye. To correct this situation, it is generally necessary to let the light entering the eye diverge first, so the common choice is a concave lens.

The image formed by a concave lens is always smaller than an upright virtual image of an object. The concave lens is mainly used to correct myopia. Myopia is mainly due to the deformation of the lens, which causes the light to gather in front of the retina prematurely.

　　Concave lens plays the role of divergent light. The concave lens becomes an upright and reduced virtual image, which makes the image distance longer and just falls on the retina. Hyperopia (presbyopia) uses convex lenses.