Physics · Ch 6 — Optics
The Eye
The Eye
The eye is a natural optical instrument. As the eye's own lens is flexible, its focal length can be varied to some extent by muscular effort (accommodation): fully relaxed, the focal length is at its maximum; fully strained, it is at its minimum. For clear vision, the image must form exactly on the retina; since the diameter of a normal adult eye (and hence the eye-lens-to-retina distance) is fixed at about 2.5 cm, applying the lens equation with cm shows a person with normal vision sees objects at infinity in the relaxed condition with maximum focal length cm, and objects at the near point (25 cm) in the strained condition with minimu …
What this figure shows. Panel (a) shows the relaxed eye focusing on a distant object at infinity, with the eye lens at its own maximum focal length fmax; panel (b) shows the same eye instead straining to focus on a close object at the standard near point, 25 cm away, with the eye lens now shortened to its minimum focal length fmin. Because the retina-to-lens image distance stays fixed at 2.5 cm in both cases, the eye's entire accommodation range from infinity down to the near point is accomplished purely by this small change in the lens's own focal length, from fmax = 2.5 cm down to f …
What this figure shows. Panel (a) shows a myopic (nearsighted) eye: parallel rays from a distant object are brought to a focus in front of the retina rather than exactly on it, blurring distant vision. Panel (b) shows that this same eye can still focus clearly, but only on objects within its own reduced far point x. Panel (c) shows the correction: a diverging (concave) spectacle lens is placed in front of the eye, slightly spreading the parallel rays from a truly distant object apart before they enter the eye, so they now appear (to the eye) to come from the person's own far poi …
What this figure shows. Panel (a) shows a hypermetropic (farsighted) eye: rays from an object at the normal near point (25 cm) would be brought to a focus behind the retina rather than exactly on it, blurring near vision. Panel (b) shows that this same eye can only focus clearly on objects beyond its own increased near point y. Panel (c) shows the correction: a converging (convex) spectacle lens is placed in front of the eye, slightly converging the rays from an object actually at the normal near point (25 cm) before they enter the eye, so they now appear (to the eye) to come from the person's own far-out near poi …