m,vnbm
Structure of the eye:
The eyeball has a diameter of about \(2.3 cm\) and is roughly spherical in shape.
The eyeball has a diameter of about \(2.3 cm\) and is roughly spherical in shape.
Important parts of the human eye:
Cornea:
It is the thin and transparent layer at the front surface of the eye. It allows light to enter the eye and performs most of the refraction (bending) of light rays.
Iris:
It is the coloured portion of the eye located behind the cornea. It controls the size of the pupil.
Pupil:
It is the small opening in the centre of the iris. It controls and regulates the amount of light that enters the eye.
Retina:
The retina is a delicate, light-sensitive membrane at the back of the eye. It forms a real and inverted image of the object and contains cells that detect light.
Eye Lens:
The eye lens is a transparent convex lens. It helps focus light and makes fine adjustments in focal length.
Ciliary muscles:
They are muscles attached to the eye lens. They help change the shape and thickness of the lens to adjust its focal length.
Power of accomodation:
The power of the eye's accommodation is the capability of the eye lens to focus nearby and distant objects. This is attained by modifying the focal length of the eye lens with the help of ciliary muscles.
The ciliary muscle relaxes when we see distant objects and causes the eye lens to thinner. This results in an increase in the focal length of the eye lens. Hence, the object in the distant location can be clearly seen. On the other hand, when we see a closer object, the focal length of the eye lens is decreased by the contraction of the ciliary muscle. Thus, the image of the closer object is formed on the retina.
The far point and near the point of the human eye:
The minimum distance needed to see the objects distinctly without strain is known as the least distance of distinct vision. It is called a near point of the eye. It is 25 cm for the normal human eye.
The eye's far point is the maximum distance at which the eye can clearly see objects. The normal eye sees infinity.
| Myopia | Hypermetropia | Presbyopia |
| Near-sightedness | Far-sightedness | Old age hypermetropia |
| The far point is moved nearer | The nearer point is moved farther | Power of accommodation is decreased due to aging |
| Lengthening of the eyeball | Shortening of the eyeball | Weakening of ciliary muscles |
| Focal length becomes smaller | Focal length becomes longer | Loss of flexibility of the eye lens |
| Image is formed in front of retina | Image is formed behind retina | Near point moves farther |
| Corrected by concave lens | Corrected by convex lens | Corrected by bifocal lens |
| Negative power lens | Positive power lens |
Concave on Upper for distant vision and convex on lower for near vision
|