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Eye is the most important sense organ. It is the sense organ through which we can see the world and we can watch the different colours.

Eye is similar to camera in the sense that the light enters through cornea and image is formed on a light sensitive screen called retina inside the eye.

There are following parts of eye –
(a) Cornea: This is thin membrane through which the light enters the eye. Maximum refraction of light rays takes place by cornea.

(b) Iris: Iris is behind cornea. It is dark muscular diaphragm.

(c) Pupil: It is small opening of variable diameter at the centre of iris. Pupil is useful to control and regulate the amount of light entering the eye. The pupil contracts if light intensity is more. The pupil expands if light intensity is less. This tendency of pupil to adjust the opening for light is called adaptation.

(d) Lens: Eyeball is spherical in shape with a diameter of about 2.3 cm. There is a transparent biconvex crystalline body behind pupil. It is a lens. The lens provides fine adjustment of focal length.

(e) Retina: Lens forms real inverted image on retina which is made of light sensitive cells. The image formed on retina reaches to brain by optic nerve and brain interprets information received through electric signals received from optic nerve.

(f) Ciliary muscles: The lens curvature can be modified by the ciliary muscles. The change in curvature changes its focal length. When the muscles are relaxed, the lens becomes thin. The focal length is increased. When focal length is more, we can see distant objects. When you are looking at an object, closer to the eye, the ciliary muscles contract. This decreases the focal length of lens. This enables us to see nearby objects.

Power of accommodation

The ability of the lens of adjusting focal length is known as power of accommodation.

Distance of distinct vision

The focal length of the eye lens cannot be decreased below a certain minimum limit. If we read a printed page by holding it very close to our eyes, we will find that the image being blurred or will feel strain in the eye. To see an object comfortably and distinctly, we must hold it at least 25cm from the eyes. The minimum distance at which objects can be seen distinctly is called the distance of distinct vision. It is also known as the near point of the eye.

For normal eyes the near point is about 25cm.

The farthest point up to which the eye can see objects is called far point.

For normal eyes far point is at infinity

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