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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Structure of the eye
Visual perception
Perception by the eye
Vision defects
Seeing Colours
Circuit of the eye to the brain
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.
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Blind Spot
click on
the structures
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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.
The eye’s 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.
A liquid that fills the anterior (front)
chamber of the eye, between the cornea
and the iris.
click to return to the eye.
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.
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.
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.
The central opening in the iris. The pupil
widens in dim light to let more light
through.
the mark en
yellow
represents the
part concerned.
click to return to the eye.
The eye’s 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.
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.
A yellow patch situated at the centre of the back part of the retina,
on the optical axis. It is a non-vascularised zone (no blood vessels),
of 3 to 4 mm of diameter, at the centre of which 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.
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.
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.
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.
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.
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
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
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
Looking at a close object
The ciliary muscles contract, squeezing the
lens and making it more rounded.
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Short sight:
Short sight is due to the eyeball being too long.
The image forms in front of the retina instead of on it.
Solutions:
Concave glasses or contact lenses can be used to
correct the focus.
Image of the short sight
light rays
cornea
crystalline lens
to correct
short sight:
concave lens to
correct short sight
to long sight
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
Use a convex lens to
correct long sight.
Correction of the anomalies of the eye
In ‘radial keratotomy’, it is possible of make incisions
in the cornea with a scalpel or a laser beam in order to
modify the refraction of light rays.
The laser can also be used to reduce the thickness of
the cornea to change the shape of the eyeball.
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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 coloursensitive cells. How can that be?
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.
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.
This apple appears red.
Here’s why…
Light beam
Explanation
When you look at an apple that appears red, is it really the
apple that is red, or is rather the light coming from the
apple to your eye which is red? If it is the apple that is red
then would it still be a red apple in a completely dark
room? Or in a room lit only by a blue light?
Best answer: When we say that an apple is red we mean
that when illuminated by white light (a mixture of all
colours) it will reflect only red light.
We understand that if the apple is illuminated by light
that is not white then the apple may no longer appear red.
For example, the same apple illuminated by green light,
will probably appear black (the apple can only reflect red
light but the green light does not contain any red light).
Example
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Skin of the apple
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the colours of white light
The reflected rays (red)
arrive in our eyes.
This is how we perceive
the colours of all objects.
Reflection of the
light red
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Absorption of all
other colours of
light
The light passes through the transparent parts of the
eye and is captured by the light-sensitive cells of the
retina. It is transformed into nerve signals which are
carried by the optic nerve to the visual zone of the
brain which interprets the signals. Most of the
signals from the right eye are sent to the left side of
the brain for processing and most signals from the
left eye are sent to the right side of the brain.
Field of binocular vision
right eye
Field of binocular vision
left eye
retina
click here
optic nerve
optic chiasm
(crossing point)
optic
voie
right
visual
cortex
nerve
signals
left visual
cortex
General Information:
- Marieb N., Elaine and Laurendeau, Guy. Anatomie and
physiologie humaines, Quebec, Éditions du Renouveau
pédagogique Inc, 1993, 1014 pp.
- Mader S., Sylvia, Biologie, Ottawa, Éditions Reynald
Goulet Inc., 1987, 767pp.
- Union des opticiens de France. (Page
consulted 16 April 2000). La vision sous
tous ses angles, [En ligne]. Adresse URL :
www.udo.org/
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