Psychology in Action

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II. HOW WE SEE AND HEAR A. Vision - The physical stimulus for vision is light, a form of energy that is part of the electromagnetic spectrum. The wavelength of light determines its hue, or color; and the amplitude, or height, of the light wave determines its brightness. The function of the eye is to capture light and focus it on the visual receptors that convert light energy into neural impulses. The major parts of the eye include the cornea, the pupil, the iris, the lens, and the retina. All light enters the eye through the cornea which protects the eye and bends incoming light rays toward the lens. Light then enters through an opening called the pupil. The pupil is surrounded by the iris which dilates or constricts to vary the amount of light entering the eye. Behind the pupil and iris is the lens, a clear elastic structure that can change its shape to Instructor’s Resource Guide                              Chapter 4                                            Page   113                                                                            
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focus an image on the retina at the back of the eye. The lens thins to focus light on the retina from distant objects and bulges to focus light from near objects. The retina is the back layer of the eye that contains the visual receptor cells. The visual receptors, called photoreceptors, are the rods and cones. The rods are very sensitive to light and enable individuals to see at night. The cones are specialized for bright light conditions and enable individuals to see close and fine detail. Nearsighted can be caused by a longer than normal eyeball or a too sharply curved cornea. Farsightedness is caused by a shorter than normal eyeball. Both conditions are easily remedied with corrective lenses. Two traditional color theories: the trichromatic theory and the opponent-process theory are proposed. The trichromatic theory proposes that three kinds of color systems are maximally sensitive to blue, green, and red. The opponent-process theory proposes that, indeed, three color systems exist, but that each is sensitive to two opposing colors—blue and yellow, red and green, and black and white. These systems work in an on-off fashion (for example, a person may see blue or yellow, but not both at the same time). According to the modern dual process theory, the trichromatic system operates at the level of the retina while the opponent- process system occurs at the level of the brain. Some people have a small genetic deficiency and may perceive only two colors and are called dichromats. Those who are sensitive to only the black-white system is called monochromats, and they are totally color blind.
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