HUMAN EYE

The human eye is one of the most valuable and sensitive sense organs in the human body. It enables us to see things around us.

HUMAN EYE: A SENSE ORGAN

The human eye is a sense organ which enables us to see.

PARTS AND FUNCTIONS:

IRIS:

The black part of our eye. Iris is a dark muscular diaphragm between the cornea and the lens. Iris controls the size of the pupil. It is the colour of the iris that we call as the colour of the eye.

PUPIL:

It is a black dot on iris. The pupil is a small hole between the iris through which light enters the eye.

In the dim light, it opens up completely due to the expansion of iris muscles,  but in bright light, it becomes very small due to the contraction of iris muscles.

CILIARY MUSCLES:

It is a soft muscle increase or reduces the size of eyelets to see the distant and nearby object. They hold the lens in position and help in modifying the curvature of the lens.

SCLERA:

The sclera is also known as the white part of the eye. The sclera is an opaque, fibrous, protective, outer layer of an eye containing collagen and elastic fibre.

CORNEA:

It allows light to enter. It is the transparent spherical membrane covering the front of the eye. Most of the refraction of light rays entering the eye occurs at the outer surface of the cornea.

CRYSTALLINE LENS:

It is a fibrous flexible tissue. And it is a convex lens made up of a transparent, soft and flexible material.

RETINA:

It is the main part of our eyes. Whenever light passes through eyelets it refracts and it image form on the retina. The retina is the light-sensitive surface of the eye on which the image is formed.

It contains light-sensitive cells rods and cones. Rod cells respond to the illumination. A number of rod cells are greater than the number of cone cells. These cells generate signals which are transmitted to the brain through optic nerves.

LIGHT-SENSITIVE CELL:

 Rods and cones are the light-sensitive cell which presents the retina.

BLINDSPOT:

Here no image formation occurs due to the absence of rod and cones. It is a point at which the optic nerve leaves the eye.

OPTIC NERVE:

Rod and cones make electrical signal which transmitted to the brain through this nerve.

AQUEOUS HUMOUR:

Between the cornea and eye lens, we have a space filled with a transparent liquid called the aqueous humour which helps the refracted light to be focused on the retina. It also maintains intraocular pressure.

VITREOUS HUMOUR:

The space between the eye lens and retina is filled with another liquid called vitreous humour.

FORMATION OF IMAGE:-

An image is formed on the retina by successive refractions at the cornea, the aqueous humour, lens and vitreous humour. It is real, diminished and inverted in nature.

The light-sensitive cells of retina get activated upon illumination and generate electricals signals.
These signals are then sent to the brain via the optic nerve. The brain interprets these signals and finally processes the information so that we perceive objects ad they are.

COLOUR OF OBJECTS:

The rod-shaped cells of retina respond to the intensity of light,i.e.the degree of brightness or darkness but do not respond to colours. The cone cells are sensitive to the different extent of primary colours such as red, blue and green.

Some terms related to the human eye are..............

(1).. Accommodation:

It is the ability or the property of the eye lens to focus both near and distant objects by adjusting its focal length.

However, the focal length cannot be decreased or increased beyond a certain limit, due to which a healthy person cannot view clearly if the objects are held too close or too far from the eye.

(2)..Power of Accommodation:

It is the maximum variation in the power of eye lens for focussing nearby or far objects, clearly at the retina. For a young adult with normal vision, the power of accommodation is about 4D. The eye loses its power of accommodation at old age.

(3)..The far point of the eye:

It is the farthest point up to which the eye can see clearly. It is infinity for the normal eye.

(4)..Near the point of the eye:

The minimum distance, at which an object can be seen most distinctly without any strain is called the least distance of distinct vision. For a normal eye of an adult, it is 25 cm. It is also called the near the point of the eye.

(5)..Persistence of vision:

The time for which impression or sensation of an object continues in the eye is called persistence of vision.

It is about 1/6th of a second which means that the minimum time for which we should view an object so that its clear image is formed on the retina is 1/16th of a second.

REFRACTION OF LIGHT THROUGH A PRISM:

You have learnt how the light gets refracted through a rectangular glass slab. For a parallel to the incident ray. However, as in a glass slab, the emergent ray is parallel to the incident ray. However, it is slightly displaced laterally. How would light get refracted through a transparent prism? Consider a triangular glass prism. It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between it's two lateral faces is called the angle of the prism.

ATMOSPHERIC REFRACTION:

You might have observed the apparent random wavering or flickering of objects seen through a trivalent stream of hot air rising above a fire or a radiator. The air just above the fire becomes hotter than the air further up. The hotter air is lighter (less dense) than the cooler air. Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the object as seen through the hot air fluctuates. This wavering is thus an effect of refraction ( refraction of light by the earth's atomphospere) on a small scale in our local environment. The twinkling of stars is a similar phenomenon on a much larger scale.

I hope this arrival had given you what you want or for what you came for.