Because the eye is a biological sensory organ of vision, from an optical point of view, the eye can be regarded as an optical apparatus, containing complex optical principles. Refraction refers to the phenomenon that light will be deflected at the interface when it enters from one medium, to another medium with a different refractive index, called refraction. The refractive system of the eye, mainly refers to the organization of the eye from the outside to the inside consisting of the cornea, atrial fluid, lens and vitreous humor. So from the cornea all the way to the retina at the base of the eye, each interface is a component of a complex optical system.