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Lecture

Chapter 2


Department
Psychology
Course Code
PSYB51H3
Professor
Matthias Niemeier

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Chapter Two – First Steps in Vision
A Little Light Physics
Light is a form of electromagnetic radiation (energy produced by vibrations of electrically charged
material)
oOnly make up tiny portion of this spectrum ( gamma rays w/ shortest wavelength to radio/TV
waves with longest wavelength
Visible light is from 400 – 700 nm (10 ^ -9 m) in wavelength = 400 violet & 650 red
2 ways to conceptualize : As wave when it moves around the world & As photos when its being
absorbed
Once star- light enters the atmospheresome photos will be absorbed & some will be scattered –
BUT most will hit the surface of an object
Light STRIKE a LIGHT-COLOURED surface most of light will be REFLECTEDto re-direct
something that strikes a surfaceespecially light, sound, or heat usually back toward its point of
origin
oSTRIKE a DARK-SURFACE is ABSORBED
Light neither reflected or absorbed is TRANSMITTEDto convey something form one place or
thing to another
Light is REFRACTEDto alter the course of a wave of energy that passes into something from
another medium (light in air that strike a water)
oDo this in the eye-exam – determine how much light must be BENT by eye-glasses for it to
properly focused on retina
Eyes that See Light
Single-celled organism such as Amboea detect light & change direction accordingly to it
BUT eyeball of humans (& other) can form imageto recognize objects
Light FIRST encounters CORNEAa transparent window into the world
oReason why its transparent because fibres w/ no blood vessels or blood which could absorb light
oSince its transparent most of light photos are transmitted (not reflected or absorbed)
oHave rich supply of transparent sensory nerve ending which force eye to close & produce tears of
when its scratched (preserving its transparency)
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These external layers regenerate quickly so even it scratched heals w/in 24 H
AQUEOUS HUMORfluid derived from blood that fills the space immediately behind the
cornea & supply oxygen, nutrients and removes waste from cornea & crystalline lens
CRYSTALLINE LENSalso transparent since it has no blood supply & its shape can be
controlled by CILIARY MUSCLE
Light must pass through the PUPIL to get to the LENS
PUPILis a hole in a muscular structure called the IRIS (which gives the eye its distinct colour)
- it controls the amount of light that reaches the RETINA via PUPILLARY LIGHT REFLEX
oWhen light level increase/ decrease the IRIS contracts/ expand automatically to allow more/less
light into eye
ENTER A light room from dark roomeyes contricts & have photonic sneeze reflex
After passing via LENSlight enters the VITEROUS CHMABER (space b/w lens & retina)
where it will refracted for the fourth & final time by the VITREOUS HUMOR
oCHAMBERthere is floaters which are bio debris flat around – this why you think you see
debris in sunny day when you look up
oHUMOR is comprises 80% of the internal volume of eye its the longest journey
Light is brought into FOCUS at the RETINAwhere the seeing actually beingsthis where light
is detect aspects of light that are related to objects in the world
Shining Starlight onto the Retina
Refraction is necessary to focus light rays
CORNEA is highly CURVED & has higher REFRACTIVE INDEX than air - it forms the most
POWERFUL REFRACTIVE surface in the eye
oAQUEOUS & VITREOUS CHAMBER also help refract light
oRefractive power of these 3 structure is FIXEDthus, they CANNOT be used to bring CLOSE
OBJECTS into FOCUS
This job performed by LENScan ALTER the REFRACTIVE POWER by changing its shape -
a process called ACCOMMODATION
Accommodation ( change in focus) accomplished via CONTRACTION of the CILIARY
MUSCLE - the LENS is attached to this muscle via the tiny fibres (ZONULES OF ZINN)
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oCILIARY MUSCLE is RELAXED , the ZONULES are STRECTHED & the LENS is relatively
FLAT in this state eye will FOCUSED on very DISTANT objects
oTo FOCUS on CLOSE objectneed the CILIARY MUSCLE to be CONSTRACTthis
REDUCE the TENSION on the ZONULES & enables the LENS to BULGE
FALTER THE LENS IS = MORE PWER IT HAS
Accommodation enables the power of lens to vary by much as 15 DIOPTERS -> this ability
DECLINE w/ AGE, starting w/ age 8 we LOSE about 1 DIOPTER/ 5 YEARS up to AGE
30 (EVEN MORE AFTER THIS AGE)
PRESBYOPIAliterally means old sight the loss of near vision because of INSUFFICIENT
ACCOMADATION
oFind their arms too short
WHY is this DECLINE? Main reason is that LENS become SCLEROTIC (harder) & the
CAPSULE that encircles the lens (enabling it to change shape) LOSES its ELASTICITY
oFranklin invented the BIFOCALS - have one power at top (see distant) & different power at
bottom (see close up)
LENS is normally TRANSPARENT because the CRYSTALLINS ( a class of proteins that make
up lens) are PACKED together very DENSELYthus, very REGULAR
oAnything that INTERFERES w/ the REGULARITY of crystallins will result in LOSS of
TRANSPARENCY (OPAQUE LENS - is known as CATARACTS )
CATARACT - can occur at DIFFERENT AGE & take many DIFFERENT FORMS
oInterfere w/ vision becausethey ABSORB & SCATTER more light than normal lens do but
is treatable by w/ lens extracted & putting a plastic/ silicone implants
CONGENITAL cataractare present at BIRTH & is relatively RARE - if you have this , will
INTERFERE w/ REINTAL IMAGE QUALITY & can have devastating effects on normal visual
development if not treated early
BUT most cataracts are discovered after AGE 50 & PREVELANE INCREASE w/ AGE – by age
70, almost everyone has some loss of transparency
To FOCUS on RETINA –REFRACTIVE POWER of the 4 OPTICAL COMPONENT OF EYE
must PERFECTLY MATCH to LENGTH OF THE EYEBALL = EMMETROPIA
oHave REFRACTIVE ERROR when eyeball is TOO SHORT/LONG relative to the POWER OF
OPTICAL COMPONENTS
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