The simplest model of the human eye is a single lens with an adjustable focal length that forms an image on the retina, or the light-sensitive bed of nerves which lines the back of the eyeball. The eye is either relaxed in its normal state in which rays from infinity are focused on the retina, or it is accommodating adjusting the focal length by flexing the eye muscles to image closer objects. For a normal eye, the near point is located 25 cm from the eye far point. The far point of the eye is the location of the farthest object on which the fully relaxed eye can focus.