5. Vision - Retina I and II The eye is a receptor that...

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Retina I and II The eye is a receptor that responds to electromagnetic radiation in the range of 400-700 nm. The retina actually responds to shorter wavelengths (UV) as well, but the cornea and lens filter out this band. The eye acts like a camera in many ways, transferring the image of the real world onto the retinal surface (film). Many parts of the eye have counterparts in the camera. Focusing: The focusing power of a lens depends on the radius of curvature of the surfaces and the difference in indices of refraction between the lens and the surrounding medium. The focusing power and the image and object distances are related by: 111 fI O =+ In a simple camera, the focusing power, f , is fixed. Therefore, compensation for changing object distance, O , is made by changing the image distance, I , the distance between the lens and the film. In the eye the image distance, I (the effective length of the eye) is fixed. Therefore, accommodation to changing object distance is accomplished by changing the focal length of the lens. In the eye, the optical lens is a combination of the cornea and the lens. Most of the refraction occurs at the air-cornea interface. The lens, because it is immersed in a medium of high refractive index, makes only a small adjustment to corneal bending. However, because the lens focusing power is variable, it has a crucial role in accommodation.
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Without neuronal innervation the lens is usually flattened by the tension in the ligaments of the zonular fibers. In this condition, the eye usually is focused on far objects. When the ciliary muscle is contracted the tension is released, the lens rounds up, and the eye focuses on closer objects. Both eyes work together. As we get older the ability of the lens to round by itself diminishes. Therefore, we have trouble seeing close objects. This condition is called presbyopia . Variability in accommodation : EMMETROPIA: Normal match of the eye’s refractive power to axial length. MYOPIA: (cannot see distant objects), axial length too long for lens. Correctable with concave lens. HYPEROPIA: (cannot see close objects), axial length too short for lens. Correctable with convex lens. ASTIGMATISM: lens has different refractive power along the vertical axis than along the horizontal axis. Sometimes known as barrel vision. Correctable with contact lenses or appropriate cylindrical lens.
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This note was uploaded on 04/18/2008 for the course PHYS 1010 taught by Professor Thompson during the Fall '07 term at New York Medical College.

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5. Vision - Retina I and II The eye is a receptor that...

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