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The Human Eye; Corrective Lenses

The Human Eye; Corrective Lenses. The Human Eye; Corrective Lenses. The Human Eye Resembles a camera in its basic functioning, with an adjustable lens, the iris, &the retina. In the eye, most of the refraction is done at the surface of the cornea.

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The Human Eye; Corrective Lenses

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  1. The Human Eye; Corrective Lenses

  2. The Human Eye; Corrective Lenses The Human EyeResembles a camera in its basic functioning, with an adjustable lens, the iris, &the retina.

  3. In the eye, most of the refraction is done at the surface of the cornea. The lens makes small adjustments to focus at different distances.

  4. Some Vision Terminology Near PointThe closest distance at which eye can focus clearly. For a normal eye, this is about 25 cm. Far PointThe farthest distance at which object can be seen clearly. For a normal eye, this is close to infinity. NearsightednessMyopia:The far point is too close for the eye to function properly. FarsightednessHyperopia:The near point is too far away for the eye to function properly.

  5. NearsightednessCan be corrected with a diverging lens.

  6. FarsightednessCan be corrected with a converging lens.

  7. Example:Farsighted eye. Sue is farsighted with a near point of 100 cm. What lens power should her reading glasses must have so that she can read a newspaper at a distance of 25 cm? Assume the lens is very close to the eye.

  8. Example Nearsighted eye. A nearsighted person has near and far points of 12 cm and 17 cm, respectively. (a) What lens power is needed for this person to see distant objects clearly? (b) What then will be the near point? Assume that the lens is 2.0 cm from the eye (typical for glasses).

  9. Vision is blurry under water because light rays are bent much less than they would be if entering the eye from air. This can be avoided by wearing goggles.

  10. Magnifying Glass A simple Magnifying Glass is a converging lens. It allows us to focus on objects closer than the near point, so that they make a larger, and therefore clearer, image on the retina.

  11. See figures above! The Power of a magnifying glass is defined by its Angular Magnification:   In the figures, N is the Near Point distance &fthe Focal Length. If the eye is relaxed this becomes (assuming small angles!): If the eye is focused at the Near Point this can be written:

  12. Example :A jeweler’s “loupe.” An 8-cm focal-length converging lens is used as a “jeweler’s loupe,” which is a magnifying glass. Estimate:(a) The magnification when the eye is relaxed, (b) The magnification if the eye is focused at its near point N = 25 cm.

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