HUMAN EYE by Jagdish Devnath

7/31/2019 HUMAN EYE by Jagdish Devnath

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The eye is that organ of the body that allows us to see the world around

us. It is one of the most precious gifts of God to mankind, most wonderful

and perfect optical instrument. Nature has provided the normal human eye

with the power of accommodation and it is because of this power that we

can see objects from about 25 cms. upto infinity. The eye is similar to

Camera in its functioning. The retina in eye corresponds to a film in acamera. The image formed in both is inverted.


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SIMILARITY :

LENS IS CONVEX

IMAGE FORMED IS REAL, INVERTED AND SMALLER IN SIZE.

DIFFERENCE

CAMERA

1. NON LIVING

2. CONVEX LENS IS MADE UP

OF GLASS HAS FIXED

THICKNESS AND FIXEDFOCAL LENGTH.

3. PHOTOGRAPHIC FILM OR

ROLE ACT AS SCREEN.

4. PHOTOGRAPHIC FILM IS

THEN WASHED TO GET

PHOTOS.

EYE

1. LIVING

2. CONVEX LENS IS MADE UP OF

LIVING CELLS.THICKNESS OF EYE

LENS CAN BE CHANGED BY ACTIONOF MUSCELS.

3. RETINA IS A THIN TISSUE WHICH

ACT AS A SCREEN.

4. RETINA SEND SIGNALS TO BRAIN

WHICH INTERPRET IT.


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VISUAL PARTS OF THE EYE :

1) CORNEA

2) PUPIL

3) IRIS

4) EYEBROWS

5) EYELASHES

6) EYELIDS

INTERNAL PARTS :

1) RETINA

2) LENS

3) OPTIC NERVE


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Virtual Image

Virtual image is that image which cannot be formed ona screen i.e. by mirror. In the mirror, the image seems to

be formed behind the mirror and is imaginary.

1) Real (Image can be formed on the screen)

2) Smaller than the object and

3) Upside down


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The eye sees by absorbing the light rays reflected by objects. These light

rays enter the eye through cornea and pupil. The lens focuses the imageon the retina which records it in an inverted position and transmits the

electronic signals to brains visual center by optic nerve. The braininterprets the image as erect and of the correct size.

In this model, the hole represents pupil and tracing paper represents

retina. When light enters the eye, it turns upside down. Accordingly, the

images on the tracing paper and retina are upside down.


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Light

The eye needs light rays in order to function. It can see in dim or

bright light but not in darkness.

Cornea

Cornea is the window of the eye and lies in front of the iris. It allows

light rays to enter and does 80% of the eyes focusing.

Iris

Iris is a coloured disk that controls pupil size by contracting or

expanding its muscles thereby modifying the amount of light enteringthe eye.


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Pupil

Pupil is a circular aperture in the center of the iris. It controls the amount

of light entering the eye.

Lens

The flexible lens lies behind the iris and constantly adjusts to provide asharp image all the time. It is a converging lens that forms images of

objects on the retina located at the back of the eye.

Retina

It is also called the screen of the eye ball where the images of object

are formed. It is made up of 137 million light sensitive cells knownas Rods and Cones. The most sensitive point on the retina is calledYellow Spot while the least sensitive point is known as the blindspot. Macula is the sensitive spot of the eye.


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Defects of Vision

Myopia

It is also called short sightedness or near sightedness.

Image is formed in front of retina.

Blurred image is formed.

Found in childhood.

Inherited disease.

Corrected by convex lens.


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Hyperopia (Hyper metropia)

It is also called long sightedness or far sightedness.

In ability to focus on near things.

Image is formed behind the retina.

Found in adults.

Corrected by concave lens.

Cataract

It is clouding of clear lens of eye compared to frosted window of the car.

It is ageing of the eye.

Mostly found in people above age of 40 years. Corrected by operation.


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1) The reading distance between book and eye should be minimum 30 cms.

2) Adequate light from over the right shoulder should fall on the book being read.

3) Use only prescribed medicines and not home made remedies to avoid irreparable loss.

4) Do not watch TV and work on computer for more than one hour at a stretch.

5) Wear sunglasses to avoid UV rays.

6) Secret of Good vision -- annual eye examination, well balanced diet and regular

exercise.


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HUMAN EYE by Jagdish Devnath

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Transcript of HUMAN EYE by Jagdish Devnath