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Chapter 2

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Matthias Niemeier

Chapter 2 First Steps in Vision: Seeing Stars A Little Light Physics Light is form of electromagnetic radiation energy produced by vibrations of electrically charged material 2 ways to conceptualize light o Wave: oscillation that travels through medium by transferring energy from one particle or point to another without causing any permanent displacement of medium o Photon: quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties Visible light waves have wavelengths between 400 and 700 nanometers o Colour we observe changes from violent at about 400 nm through rainbow up to red at 650 nm In empty space, electromagnetic radiation from a star travels in straight line at speed of light (186 000 miles per second) Once reaches atmosphere, some of starlights photons will be absorbed by encounters with dust, vaporized water, and so on; and some of light will be scattered by particles Absorbed: to take up light, noise, or energy and not transmit it at all Scatter: disperse light in irregular fashion Reflected: to redirect something that strikes surface especially light, sound or heat usually back toward its point of origin Transmit: to convey something from one place to another. Refracted: 1. To alter course of wave of energy that passes into something from another medium, as water does to light entering it from air. 2. To measure degree to refraction in lens or eye Eyes That See Light In order to see stars or anything else, need some type of physiological mechanism for sensing light An eye can form an image of outside world, enabling animals that possess eyes to use light to recognize objects, not just to determine whether light is present and what direction it is coming from Image: picture or likeness Cornea: transparent window into eyeball. Firs tissue light from star will encounter Transparent: allowing light to pass through no interruption so that objects on other side can be clearly seen Cornea is transparent because its made of highly ordered arrangement of fibers and because it contains no blood vessels or blood Cornea has high supply of transparent sensory nerve endings, which are there to force eyes to close and produce tears if cornea is scratched, preserving its transparency External layers of cornea regenerate every 24 hours Aqueous humor: watery fluid in anterior chamber of eye. Fluid derived from blood, fills space immediately behind cornea, supplying oxygen and nutrients to, and removing waste from cornea and crystalline lens Crystalline lens: lens inside eye that enables changing focus Pupil: dark circular opening at center of iris in eye, where light enters eye. Controls amount of light that reaches retina Iris: colored part of eye, consisting of muscular diaphragm surrounding pupil and regulating light entering eye by expanding and contracting pupil Francis Bacon showed that stepping to sun with eyes closed and didnt sneeze. o Guessed that suns light makes eyes water and that moisture seeps into and irritates nose o Current thinking suggests that photic sneeze reflex is result of crossed wires in brain After passing through lens, light enters vitreous chamber, where it will be refracted for 4 and final time by vitreous humor Vitreous humor: transparent fluid that fills vitreous chamber in posterior part of eye o Longest part of journey through eyeball; this chamber comprises 80% of internal volume of eye Retina: light sensitive membrane in back of eye that contains rods and cones, which receive an image from lens and send it to brain through optic nerve Good deal of light becomes lost in eyeball, so only about half a starlight that arrives at the cornea actually reaches retina o Role of retina is to detect light and tell brain abut aspects of light that are related to objects in world Shining Starlight onto Retina Because cornea is highly curved and has higher refractive index than air (1.376 vs. 1), it forms most powerful refractive surface in eye Accommodation: process by which eye changes focus (in which lens gets fatter as gaze is directed toward nearer objects) o Accomplished through contraction of ciliary muscle o When ciliary muscle is relaxed, zonules are stretched and lens is relatively flat eye focused on distant objects o To focus on closer things, ciliary must contract as it reduces tension on zonules and enables lens to bulge o Enables power of lens to vary by as much as 15 diopters Ability to accommodate declines with age starting at 8 and lose 1 diopter of accommodation every 5 years up to age 30 By 40-50, find arms are too short because they can no longer easily accommodate 2.5 diopters Presbyopia: literally old sight. Loss of near vision because of insufficient accommodationo Main reason to look forward to presbyopia is because lesn becomes sclerotic (harder) and capsule that ecircles lens (enabling I to change shape) loses its elasticity Lens is normally transparent because crystallins are packed together very densely and therefore very regular Cataracts: opacity of crystalline lens. Caused by irregularity of cyrstallins o Congenital cataracts are relatively rare; but if are dense, can have devastating effecs on normal visual development if not treated as early as possible Most cataracts discovered by 50, and prevalence of cataracts increases with age so that by70 almost everyone has some loss of transparency To focus on retinal, refractive power of 4 optical components of eye must be perfectly matched to length of eyeball Emmetropia: condition in which there isnt refractive error, because refractive power of eye Is perfectly matched to length of eyeball Refractive errors occur when eyeball is too long or too short relative to power of optical components o If eyeball is too long for optics image of star will be focused in front of retina, and star will thus be see as blur rather than a spot of light o Myopia: condition in which light entering eye is focused in front of retina and distant objects cant be seen sharply
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