Astigmatism: The cornea could have a larger curvature in the vertical plane than in the horizontal plane or vice-versa. If a person with such a defect in eye-lens looks at a wire mesh or a grid of lines, focusing in either the vertical or the horizontal plane may not be as sharp as in the other plane. Astigmatism results in lines in one direction being well focused while those in a perpendicular direction may appear distorted.Astigmatism can be corrected by using a cylindrical lens of desired radius of curvature with an appropriately directed axis. This defect can occur along with myopia or hypermetropia.Similarly, if the eye-lens focuses the incoming light at a point behind the retina, a convergent lens is needed to compensate for the defect in vision. This defect is called far sightedness or hypermetropia.RAY OPTICS AND OPTICAL INSTRUMENTS: THE EYE: The light from a distant object arriving at the eye lens may get converged at a point in front of the retina. This type of defect is called near sightedness or myopia. This means that the eye is producing too much convergence in the incident beam. To compensate this, we interpose a concave lens between the eye and the object, with the diverging effect desired to get the image focused on the retina.Put Lens

ที่มา : http://www.youtube.com/channel/UCflmr7Tyyy38MaAENetim_g อัพโหลดโดย 7activestudio