6 The eye

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Habre, Nadim , of Martini, Marianna and Hanna,
Razan
Visual perception
By Habre Nadim, Marianna DeMartini and Razan
Hanna
École La Dauversière, Montreal, June 2000
Content validation and linguistic revision
Stéphane Lamarche ?Science animée, 2000
Translated from French by Nigel Ward
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Table of Contents
Structure of the eye
Visual perception
Perception by the eye
Vision defects
Seeing Colours
Circuit of the eye to the brain
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The structure of the eye
The eyeball possesses three membranes. -the
sclera the most external membrane, white,
fibrous and very resistant, protects and gives
shape to the eye. The front part of the sclera
has a hole in which the transparent, dome-shaped
cornea is located.
-the choroid intermediate membrane,
pigmented, richly vascularised (many blood
vessels). It is this membrane which nourishes the
eye. The ciliary muscle and the iris are located
in a hole in the front of the choroid.-the
retina the most internal membrane, it covers the
back third of the eye. It supports the
light-sensitive nerve cells. The light rays
converge on the retina where the images are
formed.
click here
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Blind Spot
click on the structures
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Sclera
A white, fibrous membrane which surrounds the
eyeball. The front part of the sclera has a hole
in which the transparent, dome-shaped cornea is
located.
click to return to the eye.
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The Cornea
The eyes first lens, deprived of blood vessels
in order to ensure its transparency to light.
the yellow mark represents the part concerned.
click to return to the eye.
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The Aqueous Humour
A liquid that fills the anterior (front) chamber
of the eye, between the cornea and the iris.
click to return to the eye.
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Choroid
A thin (less than 1 mm) richly vascularised (many
blood vessels) layer or membrane between the
sclera and the retina. It is this membrane which
nourishes the eye.
the yellow mark represents the part concerned.
click to return to the eye.
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Ciliary muscle
A ring of muscle tissue surrounding the
crystalline lens. When the ciliary muscle
contracts it squeezes the lens, making it rounder
and allowing the eye to focus on objects that are
close.
click to return to the eye.
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The Iris
A ring of smooth, coloured muscle whose
constriction and dilation controls the size of
the pupil which is its central opening.
the yellow mark represents the part concerned.
click to return to the eye.
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The Pupil
The central opening in the iris. The pupil widens
in dim light to let more light through.
the yellow mark represents the part concerned.
click to return to the eye.
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The Crystalline Lens
The eyes internal lens. By adjusting its
curvature, accommodation (focusing) is possible.
The eye also possesses an external lens, the
cornea.
The yellow mark represents the part concerned.
click to return to the eye.
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The Retina
Membrane sensitive to light covering the inner
surface of the back of the eye, which converts
the optical images into nerve signals which the
optic nerve carries to the brain.
the mark yellow represents the part concerned.
click to return to the eye.
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The Macula
A yellow patch, 3 to 4 mm in diameter, situated
at the centre of the back part of the retina. At
the centre of the macula is the fovea. It is on
the fovea that the sharpest images are formed.
The majority of the cones are concentrated at
this spot. The cones are the nerve cells
responsible for colour vision and for the
sharpness of images. The rods are the neurons
which allow you to see when the light is dim but
only in shades of gray.
click to return to the eye.
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The Fovea
The centre of the macula, characterised by a high
concentration of cones. The cones are the cells
responsible for the vision of details and colours.
the yellow mark represents the part concerned.
click to return to the eye.
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The Optic Nerve
Nerve of vision (one for each eye). Each one has
about 1 million fibres carrying visual
information from the retina to the brain.
the yellow mark represents the part concerned.
click to return to the eye.
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The Blind Spot
A zone on the retina where the nerve fibres from
the 800 000 light-sensitive cells (rods and
cones) join together to form the optic nerve.
This zone has no light-sensitive cells so it is
called the blind spot.
click to return to the eye.
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Vitreous humour
A viscous gel occupying the principal cavity of
the eye, between the crystalline lens and the
retina. Vitreous means non-crystalline.
The yellow mark represents the part concerned.
click to return to the eye.
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Perception by the eye
Perception of a distant object When one looks at
a distant object, the ciliary muscles relax
causing the lens to become flatter and thinner.
Light rays passing through the lens are refracted
(bent) only slightly.
Image
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Looking at a distant object
The ciliary muscles are relaxed and the
crystalline lens is relatively flat.
Click here to see how the eye focuses on a close
object
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Perception of a close object To enable us to see
objects that are close, the ciliary muscles
contract, squeezing the crystalline lens and
making it more rounded.The closest point that
the object can be without appearing blurred
corresponds to the maximum curvature of the lens.
Image
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Looking at a close object
The ciliary muscles contract, squeezing the lens
and making it more rounded.
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Defects of vision
Image of the short sight
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light rays
cornea
crystalline lens
to correct short sight
concave lens to correct short sight
to long sight
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Long sight Long sight is due to the eyeball
being too short.
The image
forms behind the retina instead of on it.
Solutions Convex glasses or contact lenses
make it possible to correct the focus.
Image of long sight
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Use a convex lens to correct long sight.
Correction of the anomalies of the eye
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Seeing Colours
The purest colours are the colours you see in a
rainbow (the ones you see on the back of this
slide). We often say there are seven colours in
the rainbow red, orange, yellow, green, blue
indigo and violet (Richard of York gave battle
in vain!) but in fact there are many more shades
than that. Also, many other colours exist that do
not appear in a rainbow (gray, brown etc.). Your
eyes can distinguish more than a million
different colours but actually contain only three
types of colour-sensitive cells. How can that be?
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Recall that CONE CELLS are the cells on the
retina that are responsible for sharp colour
vision in bright light (as opposed to the ROD
cells which give less sharp greyscale vision in
dim light). There are in fact three kinds of cone
cells one kind detects mainly short wavelength
light (the blue end of the spectrum), one detects
intermediate wavelengths (the green part of the
spectrum) and one detects longer wavelengths (the
red end of the spectrum). We can perceive more
than a million different colours because each
colour stimulates the three types of cones in a
different way. Pure yellow light, for example,
stimulates the red and green detectors. This
means that by combining red, green and blue light
together in different combinations we can trick
our eye into seeing any colour that it is capable
of seeing. For example, if the eye receives a
combination of red and green light then the eye
will see yellow it is impossible for the eye to
tell the difference between this compound
yellow and the pure yellow found in a rainbow,
for example, since they both stimulate the red
and green receptors in the same way.
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Since humans eyes have cells that respond to red,
green and blue light, scientists call these
colours the PRIMARY COLOURS (in art class you may
have been given a different set of primary
colours but that is because art class uses paint
pigments which absorb (subtract) colours whereas
scientists are more interested in adding coloured
lights together). The image below shows what you
would see if you projected circular red, blue and
green light beams onto a white screen such that
they overlap.
By adding pairs of primary colours together we
obtain the SECONDARY COLOURS yellow, cyan and
magenta. Pairs of colours on opposite sides of
the picture, such as yellow and blue, are known
as COMPLEMENTARY COLOURS. If you are looking at
this on a computer screen then look very closely
and you will see that the screen consists of red,
green and blue dots only if you look at the
yellow part of this picture you will see that the
red and green
dots are glowing in that area this combination
gives yellow. Any colour you have ever seen can
be reproduced by mixing red, blue and green in
the right proportions.