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Chapter 2

PSYB51H3 Chapter Notes - Chapter 2: Photic Sneeze Reflex, Vitreous Body, Refractive Error

Course Code
Matthias Niemeier

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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 starlight’s 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
Eyes That See Light
In order to see stars or anything else, need some type of physiological mechanism for sensing
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
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 it’s 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

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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 didn’t sneeze.
o Guessed that sun’s light makes eyes water and that moisture seeps into and irritates
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 4th 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 accommodation

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o 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 isn’t 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
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 can’t be seen sharply
o Corrected with negative (minus) lenses, which diverge rays of starlight before they enter
o If too short, image will be focused behind retina
o Hyperopia: condition in which light entering eye is focused behind retina
o Corrected by increasing power of eye and positive (plus) lenses, which converge rays of
starlight before enter eye
Astigmatism: visual defect caused by unequal curving of one or more refractive surfaces of eye,
usually cornea
The Retina
Process of seeing begins with retina where light from star is traduced into neural energy that
can be interpreted by brain
Transduced: referring to conversion from one form of energy to another
Opthalmoscope used to look at back surface of patients’ eyes
Fundus: back layer of retina what eye doctor sees through an ophthalmoscope
White circle is known as optic disc where arteries and veins hat feed retina enter eye, and where
axons of ganglion cells leave eye via optic nerve
o Contains no photoreceptors, and consequently it’s blind
o Only place in body where arteries and veins directly seen
Reason don’t normally notice this large blind spot in visual field is that visual system “fills it in”
with info from surrounding area
Photomicrograph used to get good structure of retina
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