THE EYE
Our visual systems perform all kinds of amazing jobs, from finding stars in the night sky,
to picking out just the right strawberry in the supermarket, to following a ball flying into a
glove in a baseball game. How do our eyes and brains recognize shape, movement, depth,
and colour? How do we so easily pick a friend's face out of a crowd, yet get fooled by
optical illusions?
1. Our eyes allow us to see electromagnetic radiation reflected from objects
Most animals and many plants are photosensitive; that is, they can detect different light
intensities. Some organisms do this with single cells or with simple eyes that do not form
images but do allow the organism to react to light by moving toward or away from it. In
order for an eye to collect more information about the world it must be able to form an
image.
Our eyes, like those of many animals, detect a just narrow range of all the wavelengths
of electromagnetic radiation. This range of light is called the visible spectrum. Figure 1
shows how the visible spectrum fits into the entire electromagnetic spectrum.
Figure 1. The electromagnetic spectrum and the visible spectrum.
2. The eyeball is an optical device for focusing light
The mammalian eyeball (Figure 2) is an organ that focuses an image onto the retina, which
lines the back of the eye. Light from a scene passes through the cornea, pupil, and lens on
its way to the retina. The cornea and lens focus light from objects onto photoreceptors
(rods and cones), which absorb and then convert it into electrical signals that carry
information to the brain through the optical nerve.
Two rooms of transparent fluid feed eye tissues and maintain constant eye shape. The
lens projects an inverted image onto the retina in the same way a camera lens projects an
inverted image onto film; the brain adjusts this inversion so we see the world in its correct
orientation.
To control the images that fall upon our retinas, we can either turn our heads or turn our
eyes independently of our heads by contracting the extraocular muscles.
The cornea and lens bend or refract light rays as they enter the eye, in order to focus
images on the retina. The eye can change the way the rays are bent and thus can focus
images of objects that are close by or far away, by changing the shape of the lens. The
ciliary muscle (ciliary body) does this by relaxing to pull at the lens and allowing it to
flatten up so it can bend light rays less, or contracting for the opposite effect (Fig. 2B, C).
blind spot
A
blind spot
B
C
Figure 2. The mammalian eyeball.
3. Mistakes in the eye cause focusing problems
Mistakes occur when the bending of light rays by the cornea and lens does not focus the
image correctly onto the retina. An eyeball that is too long or too short for the optics of
the cornea and lens or an irregularly shaped cornea can cause refractive errors. Near
sightedness (bijziendheid) results either when the eyeball is too short or when the cornea
is curved too much, and the focused image falls in front of the retina. Far sightedness
(verziendheid) is the opposite, with the image falling behind the retina. Fortunately, most
mistakes can be corrected with prescription lenses (Figure 3).
Figure 3.
nearsightedness
farsightedness
4. The retina contains photoreceptors for detecting light
The photoreceptors in the retina are of two types: rods and cones, so named because of
their shapes (see Fig. 2a). These cells detect light by molecules that absorb certain
wavelengths of light. These molecules are called photopigments which absorb light; in so
doing they undergo a shape change. This shape change leads to a change in the electrical
state of a rod or cone cell membrane. This change in the rod or cone cell membrane is
passed on to nerve cells in the retina, and from there to the brain.
5. Rods function in dim light
In dim light, we use our rods, which cannot work in bright light. Rods outnumber cones (120
million rods and about 6 million cones in each retina). A little bit of light can trigger a
group of rods, leading to an electrical signal sent to the brain. Cones, on the other hand,
must each absorb a lot of light in order to send signals.
Many rods (up to 150) work together to form one signal. This way, where cones can
make an image consisting of 1500 dots, rods can make an image of only 10 dots. Therefore,
we cannot see fine detail using rods and cones we can.
6. Cones give us day vision
Our vision in bright is completely done by cones, which give us colour vision, black and
white vision, and the ability to see fine detail. Like rods, cones contain photopigments
which are sensitive to one of three colours: red, green or blue. Cones are spread
throughout the retina but are especially concentrated in a central area called the fovea.
When we want to read or inspect fine detail, we move our heads and eyes until the image
of interest falls onto the fovea. Because the fovea lacks rods, it is easier to see in dim
light by looking to the side of an object instead of directly at it (Figure 4). You can test
this by looking to the side of a faint star so that its image falls on rods, rather than on
the fovea where it probably will not register. When you look directly at the faint star, it
disappears.
fovea
rods
cones
nose
ear
Figure 4.
Thus, cones give us day vision and rods take over in dim light and at night. Both rods and
cones can operate at the same time under some conditions: in dim or dark conditions, rods
are most sensitive, but cones respond to stimuli that are sufficiently bright. This is why
we can see the colours of neon lights on dark nights.
TABLE 1. PARTS OF THE EYE
STRUCTURE
FUNCTION
Blind spot
small area of the retina where the optic nerve leaves the eye: any
image falling here will not be seen
Ciliary muscles
(ciliary body)
involuntary muscles that change the lens shape to allow focusing
images of objects at different distances
Cornea
transparent tissue covering the front of the eye: does not have
blood vessels; does have nerves
Cones
photoreceptors responsive to colour and in bright conditions; used
for fine detail
Rods
photoreceptors responsive in low light conditions; not useful for fine
detail
Fovea
central part of the macula that provides sharpest vision; contains
only cones
Iris
circular band of muscles that controls the size of the pupil. The
pigmentation of the iris gives "color" to the eye. Blue eyes have the
least amount of pigment; brown eyes have the most
Lens
transparent tissue that bends light passing through the eye: to
focus light, the lens can change shape
Optic nerve
bundle of over one million axons from ganglion cells that carry visual
signals from the eye to the brain
Pupil
hole in the centre of the eye where light passes through
Choroid
Thin tissue layer containing blood vessels, lying between the sclera
and retina; also, because of the high melanocytes content, the
choroid acts as a light-absorbing layer.
Retina
layer of tissue on the back portion of the eye that contains cells
responsive to light (photoreceptors, cones and rods)
Sclera
tough, white outer covering of the eyeball; extraocular muscles
attach here to move the eye
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