ex. 24 [visual tests and experiments]

• refraction: light bending
• accommodation: ability to focus for close vision (less than 20ft)
• emmetropia: normal vision
• hyperopia: farsightedness, the light rays focus behind the retina
• myopia: short sightedness, the light rays are focused in front of the retina
• astigmatism: blurred vision due to unequal curvatures of lens or cornea
• optic disc test: blind spot test
• lens refractive index can be varied by changing lens shape
• greater the lens convexity, or bulge, the more light is refracted
• the less convex the lens (flatter it is) the less it bends light
• distance vision, ciliary muscles relax, zonule is tense, convexity and refraction are reduced (flat lens)
• close vision, ciliary muscles contracted, zonule is loose, convexity and refraction is increased (bulged lens)
• visual acuity: sharpness of vision, tested by Snellen eye chart
• corrective lenses

  concave for myopic eye

  convex for hyperopic eye
• near point vision tests for accommodation
• astigmatism chart test for unusual curvatures in the eye
• Ishihara's color plates test for color photoreceptor cell deficiencies
• three types of cone: red, blue and green
• overlapping input from more than one cone type makes our brain interpret intermediate colors
• binocular vision: 170 degree visual field with overlap of considerable extent; primary visual cortex fuses different images to provide depth perception
• eyes in front like to hunt; eyes to the side like to hide
• depth perception test with pencil and test tube
• convergence: medial eye movements
• intrinsic muscles of eye controlled by ANS like ciliary body, sphincter and dilator pupillae muscles of iris
• extrinsic eye muscles are rectus muscles and responsible for convergence
• ophthalmoscopic exam checks the fundus to determine condition of retina, optic disc, and internal blood vessels

  • diabetes mellitus, arteriosclerosis and degenerative optic nerve and retina changes detected by this test
• presbyopia: age-related loss of accommodation, resulting in difficulty focusing on near objects (far-sightedness)
• 20/20 vision is the ability to see objects clearly at a distance of 20 feet
• 20/40 visual acuity: person stands 20 feet from chart and read letters that are discernable to normal eye at 40 feet; less than normal vision/myopia
• 20/15 visual acuity: vision is better than normal as person can read at 20 feet letters that are discernible to normal eye at 15 feet
• fundus of retina

  A) optic disc

  B) macula lutea