Human Eye

Human eye is like a camera. The image is formed on light sensitive screen called retina.

Cornea: -

Light enters the eye through a thin membrane called the cornea. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.

Iris: -

It is behind the cornea. It is dark muscular diaphragm control the size of pupil.

Pupil: -

Pupil controls the amount of light that enter the eye.

Lens: -

The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. it forms an inverted real image on the retina.

Retina: -

Retina is a delicate membrane having enormous number of light-sensitive cells. Light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects.

Ciliary muscles: -

It controls the power of lens to see distant objects by changing lens’s curvature with ciliary muscles. When the muscles are relaxed, the lens becomes thin and its focal length increases, this enables us to see distant objects. When the ciliary muscles contract, increases the curvature of the eye lens. The eye lens then becomes thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly

We have 2 types of vision cell in our eyes, called rods and cones. Cones detect light color i.e. RGB and rods decides the intensity of light.

DEFECTS:

 Myopia: -

Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects clearly. The image of a distant object is formed in front of the retina, not at the retina. This defect can be corrected by using a concave lens of suitable power.

Hypermetropia: -

Hypermetropia is also known as far –sightedness .person can see distant objects clearly but cannot see nearby objects clearly. The near point, for the person, is farther away from the normal near point (25 cm).the image of an object is formed behind the retina. This defect can be corrected by using a convex lens of appropriate power.

Presbyopia: -

Presbyopia is also known as long-sightedness. The power of accommodation of the eye usually decreases with ageing. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.

Astigmatism: -

Astigmatism is the Inability to focus light on focus point. The Light coming from outside, bend on some particular angle. It is corrected by using cylindrical lens of particular angle. This arises due to difference in the radius of curvature of cornea.