Image Formation and Accommodation

Power of Accommodation of Eye : On account of elasticity of muscles, the eye lens has the ability to change its focal length, which enables us to see nearby and distant objects. The power of eye lens is called power of accommodation. The maximum distance at which an

object can be seen distinctly by the eye is infinity. When the object is at infinity (or very far) from the eye, the parallel rays emitting from it and after refraction through eye lens are focussed on the retina and the object is clearly seen. In this state there is no tension in muscles, i.e., the eye is in relaxed state and the focal length of eye-lens is the maximum. When the object is near the eye-lens, the muscles contract to increase the curvature (or decrease the radius of curvature) of the refracting surfaces of the lens. This causes decrease in focal length of the lens and again a clear image is formed on the retina. Thus eye can clearly see the nearby and distant objects. When the object is brought continuously from infinity towards the eye, then in order to form its image on retina the pressure on muscles is increased to decrease the focal length of the eye lens. But there is a limit of elasticity of muscles or limit of exerting pressure on muscles. Therefore a stage comes when the image of the object is not formed on retina. The minimum distance upto which an eye can see clearly by the maximum possible contractions of ciliary muscles is called the least distance of distinct vision. For normal eye this distance is 25 cm and is denoted by D. For normal eye the near point D is 25 cm and far point is infinity.