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12.3 - Lens Technologies and The Human Eye

12.3 - Lens Technologies and The Human Eye. The Human Eye. The Human Eye. Cornea: Tissue that forms a transparent, curved structure in front of the eye Refracts light before it enters the eye Retina:

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12.3 - Lens Technologies and The Human Eye

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  1. 12.3 - Lens Technologies and The Human Eye

  2. The Human Eye

  3. The Human Eye • Cornea: • Tissue that forms a transparent, curved structure in front of the eye • Refracts light before it enters the eye • Retina: • A layer of cells (known as rods and cones) that respond to light and initiate nerve responses • Rod cells are very sensitive to light but cannot distinguish between colours • Cone cells detect colour

  4. How the Eye Focuses • Recall: normally, when an object moves in front of a lens, its image moves; moving the lens has the same effect • This won’t work in the eye – images from different distances need to focus on the retina, but the distance between the retina and the lens is always the same • So what happens?

  5. How the Eye Focuses • The cornea refracts light in the same way regardless of the location of the object • The lens, however, can change shape and refract light to a different extent • This allows it to focus light from both nearby and distant objects onto the retina • The ciliary muscles (or ciliary bodies) make the lens shorter and thicker by pulling on it

  6. How the Eye Focuses • This process is called accomodation • To focus on a nearby object, the curvature of the lens needs to be greater because it needs to refract light more

  7. The Eye vs. The Camera • Cameras are designed very much like eyes • Both have lenses that focus light on a light-sensitive material • Retina in the eye • Film or CCD in a camera • Lens of the eye changes shape in order to focus on objects at different distances • Lens of the camera must be moved in and out to focus on objects at different distances • Eye has an iris (or pupil) that controls the amount of light entering it • Camera also has an iris (or aperture) to control the amount of light

  8. Correcting Vision Using Lenses - Myopia • Myopia is near-sightedness • The eyes cannot focus on distant objects • Caused by the eyeball being too long • The image forms in front of the retina, not on it • By the time light rays reach the retina they have begun to diverge, and the image looks fuzzy

  9. Correcting Vision Using Lenses - Myopia • Adding a diverging lens spreads out the parallel light rays before they reach the eye • The rays that are separating from each other appear to be coming from an object that is closer to the eye • Due to the higher angle of incidence, when the eye refracts the light, it is focused correctly on the retina

  10. Correcting Vision Using Lenses - Hyperopia • Hyperopia is far-sightedness • The eyes cannot focus on nearby objects • Caused by the eyeball being too short • Light rays entering the eye reach the retina before they converge at a point, causing the image to be blurry

  11. Correcting Vision Using Lenses - Hyperopia • Adding a converging lens bends the rays slightly inward before they enter the eye • The lens of the eye then refracts them more, and causes the rays to be correctly focused on the retina

  12. Correcting Vision Using Lenses - Presbyopia • As a person ages, the lenses of the eye become stiffer, and the ciliary muscles can no longer make them change shape • This condition is called presbyopia • People with presbyopia cannot focus on nearby objects, but this is not farsightedness (which is caused by an incorrect eyeball length) • If someone is already nearsighted and gets presbyopia, they cannot focus on distant OR nearby objects! • To correct this condition, people wear bifocals

  13. Correcting Vision Using Lenses - Presbyopia • Bifocals are lenses with two parts • The top part of the lens corrects for nearsightedness • A small section of the lower part with a different curvature helps the eye focus on nearby objects • Bifocal contact lenses are also available

  14. Correcting Vision Using Lenses - Astigmatism • Astigmatismis blurred or distorted vision that is usually caused by an incorrectly shaped cornea • Instead of being rounded, the cornea is oval-shaped • Part of an image might be in focus, but the rest of the image is blurry • This is corrected by using lenses that have a cylindrical curvature • Refracts incident light rays along one axis only

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