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We are able to see because light from an object can move through space and reach our eyes. Once light reaches our eyes, signals are sent to our brain, and our brain deciphers the information in order to detect the appearance, location and movement of the objects we are sighting at.
The whole process, as complex as it is, would not be possible if it were not for the presence of light. Without light, there would be no sight.
The human eye is the organ which gives us the sense of sight, allowing us to
learn more about the surrounding world than any of the other five senses.
The eyeball is set in a protective cone-shaped cavity in the skull called
the orbit or socket and measures approximately one inch in diameter. The orbit
is surrounded by layers of soft, fatty tissue which protect the eye and enable
it to turn easily.
In general the eyes of all animals resemble simple cameras in that the lens
of the eye forms an inverted image of objects in front of it and projects it
onto the sensitive retina, which corresponds to the film in a camera.
The front of the eye mainly contains an elaborate array of structures which
are mainly concerned with the refraction
(ie bending) of light rays and bringing them into focus on the retina.
The structure most directly involved is the lens.
The eye changes light rays into electrical signals then sends them to the brain, which interprets these electrical signals as visual images via the optic nerve.
Focusing the eye is accomplished by a flattening or thickening (rounding) of
the lens. By altering its shape, the lens can focus on near and far objects.
The lens becomes flatter for distant objects and rounder for closer objects.
The transparent, double convex (outward curve on both sides) structure suspended between the aqueous and vitreous humours helps to focus light on the retina.
These are elastic-like structures present in the eye that suspend the lens and pull it into shape for focusing distant objects onto the retina. This process is known as accommodation.
For nearer objects the lens is increasingly rounded by ciliary
muscle contraction, which relaxes the suspensory ligament.
When the ciliary muscles are relaxed, the perimeter of the lens is pulled outwards
giving it a flattened shape. The ciliary body projections are responsible for
the secretion of the aqueous humour.
As opposed to the vitreous humour, the aqueous humour (as its name suggests) is a clear watery fluid that occupies the front chamber of the eye and nourishes the lens and the cornea. It also helps to give the eye shape, and protection. The aqueous humour is secreted by the ciliary processes
The spherical shape of the eyeball is maintained by this jelly-like substance. It is the transparent, colorless mass of gel that lies behind lens and in front of retina.
The front of the eye is protected by the thick transparent cornea, which like
the lens, also refracts (bends) light
rays.
The front of the cornea is covered by a thin layer of epithelium
which is continuous further out with the conjunctiva
The sclera is the tough, white, outer layer (coat) of the eyeball, and is continous with the cornea, it protects the entire eyeball.
The iris contains circular and radial muscles.
Different contractions of these 2 sets of muscles has the effect of varying
the size of the pupil hole.
The iris is also what determines our eye color.
Eye colour depends on the amount of pigment on the back of the iris, which is
constant, and the amount at the front, which is variable. The more melanin
pigment found in the iris, the darker the eye colour will be.
Most new-born white babies have no pigment cells at the front, and light is reflected from the rear of the iris and scattered in such a way that it appears blue (a similar scattering of light makes blue skies). After a few months, the eventual colour of the eye emerges as cells bearing melanin build up on the front of the iris. Some people have more melanin than others. A large amount gives brown or black eyes, less gives green and the least amount, blue or grey. Albinos have pink eyes because their irises have no pigment on either the front or back and therefore the 'pinkness' occurs due to reflection of the choroid capillaries behind the retina .
By the age of six, most people's eye colour is fixed, and in most of us, it fades with old age, along with our hair.
Melanin's function in the iris is to help absorb excess light that might otherwise overwhelm our vision. Albinos who lack melanin, for instance, often have eyesight problems associated with an oversensitivity to light.
Is the opening in the iris which permits entry of light into eye. Through the opening and closing of the iris' circular muscles excessive light is prevented from falling on the retina and damaging the light-sensitive photoreceptors.
The retina is nourished by a layer rich in blood vessels called the choroid which is found immediately beneath it and protected by a thick connective tissue coat called the sclera. Heavy pigmentation in the choroid layer and in the epithelium on its inner side, shields the retina and prevents light being reflected within the eye.
All photoreceptors (light sensitive) cells are concentrated in the retina, which lines most of the interior of the eye ball.
Macula
The small, sensitive area of the central retina which provides vision for
fine work and reading.
The macula is responsible for the sharp,
clear vision that occurs when we look
directly at something (as it lies roughly in the center of the retina).
The macula is unique in that it has a high denisty of cones
which are the photoreceptors concerned with colour and enable high quality
vision. The most sensitive region of the macula is the fovea
(it has an even higher density of cones)
The rest of the retina's surface handles our peripheral
vision, and this is where most of the rod
photoreceptors of the retina are located.
Have you ever noticed how we can often see things more sharply in faint
light if we do not look at them directly? This is because the rods are designed
for vision in dim light. However for images seen in bright light, we use
the central retinal region (the macula) for a sharper focus when we look
directly at the image, as the cones detect detail brilliantly.
In people over sixty, the most common cause of blindness is macular
degeneration, in which the macula becomes covered with scar tissue and
vision is obscured.
Fovea
The fovea is the most central part of
the macula and provides the sharpest of sharp vision. It boasts the highest
concentration of cones and therefore the highest acuity.
Because of the fovea you achieve your sharpest day vision however by looking
straight at the object of interest. Humans actually move their eyes so that
images of interest are projected onto their foveas.
Blind Spot
The blindspot is a small area of the
retina where the optic nerve actually enters the eye, this occurs normally
in all eyes. It results in a gap in the visual
field which corresponds to an area of the retina where no visual cells
are present.
Photoreceptors are the unique cell types that are the very crux of our ability
to see.
The photoreceptors are light sensitive cell types that reside in the retina
- there are 2 types:
For a great animated flash movie about the eye and it's components, visit brainpop.com - vision
For full url list for optics click here