Components Of An Eye
Cornea - The eye has a nearly spherical shape of diameter about an inch. The front portion is more sharply curved and is covered by the cornea. The Cornea is a transparent protective membrane. It is this portion which is visible from outside.
Aqueous Humor - Behind the cornea, we have a space filled with a liquid called the aqueous humor and behind that a crystalline lens.
Iris -
Between the aqueous humor and the lens, we have a muscular diaphragm called Iris, Iris is the coloured part that we see in an eye.
Pupil - It has a small hole in it called pupil. The pupil appears black because any light falling on it goes into the eye and there is almost no chance of light coming back to the outside.
The amount of light entering the eye may be controlled by varying the aperture of the pupil with the help of the iris. In low-light condition, the iris expands the pupil to allow more light to go in. In good light conditions, it contracts the pupil.
Lens -
The lens is hard in the middle and gradually becomes soft towards the outer edge. The curvature of the lens may be altered by the ciliary muscles to which it is attached. The light entering the eye forms an image on the retina which covers the inside of the rear part of the eyeball. The retina contains about 125 million receptors called rods and cones which receive the light signal and about one million optic-nerve fibres which transmit the information to the brain.
Vitreous humor -
The space between the lens and the retina is filled with another liquid called the vitreous humor. The aqueous humor and the vitreous humor have almost same refractive index 1.336. The refractive index of the material of the lens is different in different portions but on the average, it is about 1.396. Then light enters the eye from air, most of the bending occurs at the cornea itself because there is a sharp change in the refractive index. Some additional bending is done by the lens which is surrounded by a fluid of somewhat lower refractive index. In normal conditions, the light should be focused on the retina.
Accommodation-
When the eye is focused on a distant object, the ciliary muscles are relaxed so that the focal length of
the eye lens has its maximum value which is equal to its distance from the retina. The parallel rays coming
into the eye are then focused on the retina and we see the object clearly. When the eye is focused on a closer object, the ciliary muscles are strained and the focal length of the eye-lens decreases. The ciliary muscles adjust the focal
length in such a way that the image is again formed on the retina and we see the object clearly. This process of adjusting the focal length is called accommodation.
Near point of the Eye - If the object is brought too close to the eye, the focal length cannot be adjusted to form the image on the retina. Thus, there is a minimum distance for the clear vision of an object The nearest point for which the image can be focused on the retina, is called the near point of the eye.
The distance of the near point from the eye is called the least distance for clear vision.
The average value of the least distance for the clear vision for a normal eye is generally taken to be 25 cm.
Iris -
Between the aqueous humor and the lens, we have a muscular diaphragm called Iris, Iris is the coloured part that we see in an eye.
Pupil - It has a small hole in it called pupil. The pupil appears black because any light falling on it goes into the eye and there is almost no chance of light coming back to the outside.
The amount of light entering the eye may be controlled by varying the aperture of the pupil with the help of the iris. In low-light condition, the iris expands the pupil to allow more light to go in. In good light conditions, it contracts the pupil.
Lens -
The lens is hard in the middle and gradually becomes soft towards the outer edge. The curvature of the lens may be altered by the ciliary muscles to which it is attached. The light entering the eye forms an image on the retina which covers the inside of the rear part of the eyeball. The retina contains about 125 million receptors called rods and cones which receive the light signal and about one million optic-nerve fibres which transmit the information to the brain.
Vitreous humor -
The space between the lens and the retina is filled with another liquid called the vitreous humor. The aqueous humor and the vitreous humor have almost same refractive index 1.336. The refractive index of the material of the lens is different in different portions but on the average, it is about 1.396. Then light enters the eye from air, most of the bending occurs at the cornea itself because there is a sharp change in the refractive index. Some additional bending is done by the lens which is surrounded by a fluid of somewhat lower refractive index. In normal conditions, the light should be focused on the retina.
Accommodation-
When the eye is focused on a distant object, the ciliary muscles are relaxed so that the focal length of
the eye lens has its maximum value which is equal to its distance from the retina. The parallel rays coming
into the eye are then focused on the retina and we see the object clearly. When the eye is focused on a closer object, the ciliary muscles are strained and the focal length of the eye-lens decreases. The ciliary muscles adjust the focal
length in such a way that the image is again formed on the retina and we see the object clearly. This process of adjusting the focal length is called accommodation.
Near point of the Eye - If the object is brought too close to the eye, the focal length cannot be adjusted to form the image on the retina. Thus, there is a minimum distance for the clear vision of an object The nearest point for which the image can be focused on the retina, is called the near point of the eye.
The distance of the near point from the eye is called the least distance for clear vision.
The average value of the least distance for the clear vision for a normal eye is generally taken to be 25 cm.
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