The refraction of light by the eye is called refraction, and the ability to refract light in terms of optical focus is called refraction, which is the unit of the size of the refractive force. Light from an object into another optical density of different substances when the direction of propagation of light produced by the phenomenon of deflection is called refraction, and the size of this refractive phenomenon is the unit of refractive power, expressed in D. The inability to accurately image the retina due to an incorrect refraction of the eye can lead to the more common eye diseases, which usually include hyperopia, myopia, and astigmatism. The most common principle of refraction determination is to shoot a light beam into the pupil of the examined eye to form an illuminated area on the retina. Changing the direction of the beam causes the illuminated area to move across the fundus of the eye while observing the movement of the light and shadow in the pupil to determine the position. After the determination, the refractive error of the eye itself can be supplemented and corrected by making lenses for proper imaging of the retina. In case of refractive error, it is recommended to go to a regular eye hospital for examination and prescription of lenses as soon as possible to avoid inconvenience as well as delay in life.