User:Mansour94

HUMAN EYE
[[File:http://www.pasadenaeye.com/faq/faq15/eyelabeled.jpg] The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the mammalian eye allows vision.

1-Is a Human Eye Similar to a Camera?
Light Control

A camera functions very much like the human eye. Both see light that is reflected from the object being viewed. It transmits itself to the eye as sight or the camera as a picture in film or digital form. Light rays diffuse and bounce randomly from the myriad of objects around us so in order to see or form a picture, the rays need to be focused into a coherent form. To begin, the light enters the eye through the cornea and, like a camera's aperture, the pupil enlarges or contracts to ensure that the right amount of light enters the eye.

Focus

When light enters the eye, it focuses on cells on an area at the back of the eye that is sensitive to light, which is known as the retina. Just as a camera responds to light entering it to form an image on film or digitally, the retina forms an image by transmitting the light to the brain by means of the optic nerve.

Distinctions

The human eye, like a camera, can view tones of gray and the various shades of color (unless the person happens to be color blind, of course.) Both the eye and the camera can see near and far, judge size, register depth perception and see movement. However, the human eye can only see in visible light. Specialized cameras, films and digital photography can go much further by capturing images that are far beyond the eye's capability. The camera can show heat images and X-ray views, stop motion or capture images too fast for the human eye, such as a bullet in mid-flight, and cameras can be designed to view everything from the smallest particle to the planets at extreme distances.

2-Describe three vision defects: Nearsightedness, Farsightedness and Astigmatism
- Myopia or nearsightedness means you can see better up close than you can at far distances



- Hyperopia or farsightedness means that you can see better far away than you can close up



- Astigmatism Patients with astigmatism have distortion of vision in addition to blurring of vision.



3. Explain which kinds of lenses are needed to correct the previous vision defects.
Contact lenses are thin transparent plastic discs that sit on the cornea. Just like eyeglasses, they correct refractive errors such as myopia (nearsightedness) and hyperopia (farsightedness). With these conditions, the eye doesn't focus light directly on the retina as it should, leading to blurry vision. Contact lenses are shaped based on the vision problem to help the eye focus light directly on the retina.

- Several different types of soft contact lenses are available:


 * Daily-wear lenses
 * Extended-wear lenses
 * Disposable lenses
 * Color-tinted or cosmetic lenses
 * Ultraviolet (UV) protection lenses

to the millions of americans with vision problems, the world can be a blurry place. For people who need vision correction but don't want to wear glasses, contact lenses offer an easy, virtually invisible solution.

Although nearly 36 million Americans wear contact lenses, not everyone wears them to correct vision problems. Celebrities use contact lenses to change the color of their eyes, athletes put them on to give them extra-sharp vision on the field, and others use them to make Halloween costumes more realistic.