~@_@~Human eye structure~ @_@~

This is our eye structure~it does look odd to some people, but it is what you and me see through whenever you open your eyes~

''Laser Surgery'' #_# @_@%_%=$_$

Laser are used to re-attach retinas, remove contaracts, stop bleeding, and reshape corneas.
Re-attach retinas: A laser can sometimes be used to weld the piece of retina back in place, if it's detached from inside of the eye. It can also prevent detachment from happening again.
Remove contaracts: This can occur with age. ( naturally ) It's when the lens has becaome cloudy. The laser can cut throught the cornea and replace the lens with a synthtic lens.
Stop bleeding: The power from th elaser is very intense, that is can stop the bleeding from blood vessels, and helps reduce the bleeding when operating the surgery.
Reshape cornea: The laser can also correct the person's vision by reshaping the cornea. The laser allows the surface of the cornea to weaken, so it would be foled back. The surgeon will then change the shape inside of the cornea. After the correction, the outer surface can be restored to it's original place. It's not painful during the surgery, and are often completed only a few minutes. ( '' Like all types of medical precedures, there are some risks involved. Because laser eye surgery is only a few decades old, long term risks and benefits are not yet known." )

~Cameras Have Similarities to Human Eyes~ *.* ~

'' There are many structural similarities between a camera and the human eye." Eg: compare the lens cap from the camera to the eyelids of human. The iris limits the amounts of light from entering our eyes. For cameras, the iris is represented using a device name the diaphragm. It's made of many opaque circles, which are placed in a circle. The circle can be moved to let the central hole increase larger or decrease smaller. When light transparent through the lens and forms an inverted image in both human eye and the camera.

Yet, not all the functions in the camera work in the same way as the human eye. Eg: Controlling the distance between the lens & the detector does the focussing in the camera. For human eyes, the lens changes shape instead moving closer to the retina.

A detector called a charge-coupled device (CCD) is at the back of the camera. It absorbs light, then produce the electrical signals needed to make a digital image. The CCD also has many regions called pixels, they record a tiny part of a whole image. The pixels relates to the rods and cones cells, which detect light in our eyes. ( the picture provide a comparison of the camera and the human eye. )

Correcting Astigmatism

Astigmatism is when a person's cornea has a distorted shape. As you know that a normal cornea is shaped round like a ball, but the astigmatic eye has an irregularly shaped cornea. This situation causes an image to be focus on more than only a point on the retina, this will bring to blurred / fuzzy vision. It can be corrected by using eyeglasses or contact lenses, or even with laser surgery to reshape the cornea.

~Blindness~ @_@

A blind person can't detect light. Most people, who are blind can recieve some light or maybe a limited amount of vision. In some blindness, a person may only see a small part of the whole scene. In other blindness, a person sees the opposite. They can only see the surrounding edges of the whole scene. Some others can only determine light or darkness.
blindness is often occur as a disease or malnutrition. About 40 million blind people, and about 80% could have their sight restored through treatments.

Correcting Focus Problems

Correcting Far-sighted vision: " Some people can see distant objects clearly, but find that nearby objects remain fuzzy no matter how hard they try to focus on them." It's called far-sighted vision. Distant objects from light rays are almost parallel, and needs less refraction to converge them than the light from close objects. Light rays are disverging from close objects as they enter the eye. " A convex lens is needed for the light rays to come into focus exactly on the retina." Far-sighted vision: ( eye has a shorter shape than a normal eye.)

Correcting Focus Problems

Correcting near-sighted vision: " People who can see nearby objects clearly, but who cannot bring distant objects into focus are near-sighted." This happen, because the lens gather the light rays to make an image in front of the retina. When the light rays hit the retina, they are already spread out. ''A concave lens is used to diverge the parallel rays slightly, so that the image forms farther back, on the retina." ( Near-sighted vision : eye has longer shape than a normal eye. )

Correcting Focus Problems

Normal Vision: When light rays enter the eye from a distant object, the rays are parallel. The lens ( convex ) can make the rays, which come from different directions to meet at the retina, then form a sharp image. The light rays from a close by object are separated and go in different directions when they enter the eye. So, then the muscles in the eye will make the lens to change shape, it makes the lens thicker. The result of this gives the light rays to form a clear image of the object.

''Correcting Focus Problems''

People have difficulties focussing clearly on objects some time in their lives. When children are growing in their teen years, the shape of their eye can change. This will affect their ability to focus on many different things and can also affect their learning too. They may need eyeglasses (concave lens)for a while. As adult age, their eyes'lenses flexibility will decrease, that makes it harder to focus on close objects. Adults may need eyeglasses too( convex lens ).

BLACK&WHITE / COLOUR VISION

When the light rays are all focussed correctly on the retina. The cells which can absorb the light begin their work. In the retina, some cells detects the low levels of light, and some cells detect bright light. The cells that absorb light are shaped in two shapes: rod cells are shaped: longer cylindrical /colour appears white, while cone cells are rounder in shape/ colour appears yellow.

Rod cells: It can absorb any colour of light , especially '' green'' light. Our brain does not use any signals from rod cells to identify colour, only shades of light and dark. It is called the black & white vision system, also in low light it helps us see the movements and shapes of our surroundings.

Cone cells: It can let us detect colours. Human have three kinds of cone cells, each of them has a different pigment. Using lony red, green & blue, it can see all the colours of a rainbow. If our brain detect the equal amount of those colours, then we can see the object as white. eg: the white page of paper appears white.

Blind Spot

O X
If the images are too close, then try it with objects. place them about 15cm apart, then follow the steps carefully, and you will soon discover your own ''blind spot''

''Blind spot''

An area where the optic nerve pass through the retina does not have any light sensing cells at all. It is called the blind spot, you can identify the blind spot by testing it with the picture below.

''STEPS towards your BLIND SPOT'' [ Cover your right eye with your hand, and look at only the '' X ''. and move your face towards it slowly. the '' O '' should disappear then reappear, while image moves onto your blind spot, then went on & off again.]

O X

Forming an image

The light rays that enters the eye are from one spot at the base of an object come together again in a place at the top of the retina. All the light rays that enters the eye from a spot on the top of the object come together at a place on the bottom of the retina. ( the image is inverted, formed by the lens, but the brain translate the image to be upright.)

The Cornea-Lens-Retina System

All light rays has to pass through a focussing system that involves the cornea, the lens, and spaces in the eye filled with watery fluid.The conea and the lens are supported by the fluid, which lies between the cornea and the lens. It gives nutrients to the cornea, theat does not have any blood vessels. The fluid supports the lens and gives shape to the eye.
When light rays pass into the cornea, they are focussed right away. The cornea changes the direction on the incoming light rays, so they could be focussed and meet towards the retina. The cornea gives most of the focussing to our eyes, and the lens does the left over focussing. Most people think that the lens does all the focussing, which is not true, it is the cornea that does the most.
Lens: Lens has the ability to fine-tune our focus. It just automatically change the shape. When some certain muscles in the eye decreases, there is less tension for the lens,which allows the lens to be thicker.
Looking at distant objects, the same muscles relax, which will increase tension on the lens. This allows the lens to become thinner. When your eyes feel it's very hard to focus when you look at an object up close, and try to see it clearly.

6.1 Human Vision

Using optical systems, human vision can be corrected and extended.
What is human vision? Human eyes can focus on objects that are near or far and can also adapt to both sunlight at day or darkness at night.
How light enters the eye: Light enters the eye through the pupil. It is in the center and it appears dark in order to let light passes through the opening without reflecting back. The iris is a circle of muscle surrounding the pupil. It is the part with colour on one's eye being: brown, blue, green, hazel, or grey....etc. The iris also controls the amount of light from entering the eye. ex: When in very dim light, the iris expands in order to let more light to enter the pupil.
ex: When in very bright light, the iris contracts the pupil and reduce the amount of light from entering the eye.
Covering over the pupil and iris is a transparent tissue ( clear, allow light to pass through) called the cornea. It is made of cells that are transparent enough to allow light to pass through, but it's also tough enough to hold the eye together. Around the cornea is the sclera, an opaque tissue. It is the white part surrounding the iris.
Beneath the pupil lies a flexible convex lens. All the light rays must pass through the lens and are focused on a screen behind the eye called the retina, then an image of the object is formed. Some other cells in the retina convert the light rays into electrical signals which are sent to the brain through the optic nerve (the pair of nerves passing impulses from the eye to the brain)