EYE AS AN OPTICAL INSTRUMENT

EYE AS AN OPTICAL INSTRUMENT

WHAT IS AN OPTICAL INSTRUMENT?

OPTICAL INSTRUMENTIS A DEVICE THAT BASICALLY PROCESSES AND MANAGES LIGHT WAVES TO MAKE THE IMAGE CLEAR AND MAGNIFIED.OPTICAL SYSTEMS USE TRANSPARENT MATERIALS WITH REFRACTIVE INDEX SELECTED TO PROCESS AND BEND THE LIGHT WAVES TO CREATE THE IMAGES.OPTICAL INSTRUMENTS BASICALLY EITHER HAVE CONVERGING OR DIVERGING LENSES OR A COMBINATION OF BOTH SAME LENSES OR TWO DIFFERENT LENSES.CONVERGING LENS MAGNIFIES THE IMAGE AND GIVE A LARGER VIEW,WHILE DIVERGING LENS DIMINISH THE IMAGE AND GIVE A SMALLER VIEW.CAMERA IS A KNOWN OPTICAL INSTRUMENT.

HUMAN EYE:

A NORMAL HUMAN EYE FOCUSES LIGHT AND PRODUCES SHARP AND CLEAR IMAGE.THE ESSENTIAL PARTS OF THE EYES ARE BASICALLY RESPONSIBLE FOR THE CLEARENCE OF THE IMAGE

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CORNEA:IT IS A TRANSPARENT SPHERICAL STRUCTURE HAVING AN ANTERIOR RADIUS OF CURVATURE OF 7.8mm,POSTERIOR RADIUS OF CURVATURE OF 6.5mm AND A REFRACTIVE INDEX OF 1.3771.IT CONSTITUTES THE MAXIMUM REFRACTIVE POWER OF 43D AS THE LARGEST DIFFERENCE OF REFRACTIVE INDEX OCCURS BETWEEN CORNEA AND AIR FIRSTLY AND SECONDLY BECAUSE OF ITS CURVATURE.IT IS A BICONVEX LENS.THE CENTRE THICKNESS IS USUALLY BETWEEN 0.5mm TO 0.6mm.

AQUEOUS HUMOR:

IT IS A CLEA,WATERY FLUID BETWEEN THE CORNEA AND THE FRONT OF THR VITREOUS.IT PROVIDES NUTRITION TO THE LENS AND MAINTAINS THE PRESSURE OF THE EYE.SINCE IT IS A CLEAR FLUID,LIGHT EASILY PASSES THROUGH IT.

PUPIL:

PUPIL IS A CIRCULAR OPENING IN THE IRIS WHICH REGULATES THE AMOUNT OF LIGHT ENETERING THE LIGHT.THE INCREASE OR DECREASE OF ITS DIAMETER MAKES THE IMAGE BLURRED OR CLEAR.WHEN THE ILLUMINATION IS LOW,THE PUPIL INCREASES ITS DIAMETER TO ACCOMMODATE MORE LIGHT TO MAKE THE IMAGE CLEAR AND VICE VERSA IN HIGH ILLUMINATION.ABNORMALITIES IN PUPIL MAKE THE IMAGE BLUR OR LESS FOCUSSED.

CRYSTALLINE LENS:

THE LENS IS BICONVEX IN NATURE AND HAS APPROXIMATELY A DIAMETER OF 9mm.IT HAS A REFRACTIVE INDEX OF 1.42,AND HAS A TOTAL DIOPTRIC POWER OF 18D.IT BALANCES THE EYE’S REFRACTIVE POWER AND PROVIDES A MECHANISM FOR FOCUSSING AT DIFFERENT DISTANCES WHICH IS TERMED AS ACCOMMODATION.WHEN WE TRY TO VISUALIZE THE NEAR OBJECTS,THE LENS BULGES ITSELF AND HELPS US TO FOCUS AT NEAR.WHEN VIEWING DISTANT OBJECTS IT RELAXES OR  FLATTENS ITSELF TO FOCUS AT DISTANCE.IT IS COMPOSED OF LAYERS OF FIBRES WHICH IS ARRANGED IN RADIAL PATTERN TO ALLOW A SYMMETRICAL DIFFRACTION HALO TO BE FORMED.IT IS NOT A HOMOGENOUS STRUCTURE.ITS CENTRAL THICKNESS IS MORE THAN ITS PERIPHERAL THICKNESS AND ITS PARTS CAN BE IDENTIFIED TYPICALLY.

VITREOUS HUMOR:

THE POSTERIOR SURFACE OF THE LENS IS IN CONTACT WITH A TRANSPARENT GEL THAT FILLS THE POSTERIOR SEGMENT OF THE GLOBE.ITS REFRACTIVE INDEX CAN BE CONSIDERED SAME AS THE REFRACTIVE INDEX OF THE AQUEOUS HUMOR.

RETINA:

IT CAN BE CONSIDERED AS A SCREEN ON WHICH THE IMAGE IS FORMED FROM AN OPTICAL PERCEPTION.IT HAS A RADIUS OF CURVATURE OF APPROXIMATELY 12mm.IT HAS A CENTRAL AREA,THE FOVEAL AREA WHERE THE PHOTORECEPTORS ARE DENSELY PACKED TO PROVIDE HIGHEST RESOLUTION.THE RODS AND CONES HELPS TO GIVES EYE THE COLOUR PERCEPTION.THE CONES HELPS TO VISUALISE IN LIGHT AND THE RODS IN DARK. THE EYES ROTATES CONTINUOUSLY TO FIXATE THE DESIRED DETAILS INTO FOVEA.IT PLAYS A MAJOR ROLE IN THE OPTICAL SYSTEM OF THE EYE.

CAMERA:

A CAMERA HAS THE FOLLOWING PARTS:

APERTURE:IT ALLOWS THE LIGHT TO ENTER THE CAMERA BY OPENING AND CLOSING.IT IS OUR CHOICE HOW MUCH WE WANT TO OPEN THE LENS TO GET A DESIRED OPENING OF THE LENS.

LENS:THE LENS IS USED TO CAPTURE THE OBJECT AND TO STORE IT.THERE ARE VARIOUS TYPES OF LENSES WHICH CAN ZOOM TO GIVE A DETAILED PICTURE FEOM A DISTANCE AND THERE ARE LENSES ALSO WHICH CAN PROVIDE DISTORTED IMAGES.THE IMAGES DEPENDS ON THE QUALITY OF THE LENSES.

SHUTTER RELEASE BUTTON:THIS BUTTON HELPS TO DETERMINE THE APPROPRIATE SETTING OF THE LENS.IT CLOSES AND OPENS THE SHUTTER AT THE RIGHT TIME TO GIVE A CLEAR IMAGE BY CONTROLLING THE AMOUNE OF LIGHT ENTERING THE CAMERA WHILE TAKING A PICTURE.

MEMORY CARD:IT IS A PRIMARY STORAGE DEVICE OF A CAMERA USED TO STORE PHOTOS AND VIDEOS.TWO MOST POPULAR MEMORY CARDS ARE SD(SECURE DIGITAL) AND CF(COMPACT FLASH).

VIEWFINDER IN CAMERA:IT DISPLAYS THE IMAGE THAT HAS TO BE SHOT.IT IS USED TO SPOT THE FOCUS OF THE OBJECT WHEN THE DEVICE IS PLACED IN FRONT OF THE PHOTOGRAPHER’S EYE.

HOW DOES A HUMAN EYE FOCUS LIGHT?

FIRSTLY,THE LIGHT ENTERS THROUGH THE CORNEA ALSO KNOWN AS THE “WINDOW”OF THE EYE.ITS REFRACTIVE INDEX IS SUCH THAT IT BENDS THE LIGHT RAYS IN SUCH A WAY THAT IT TRAVELS THROUGH THE SMALL OPENING CALLED PUPIL AND REACH THE IRIS.THE IRIS CONTRACTS AND EXPANDS TO ALLOW SUFFICIENT AMOUNT OF LIGHT TO ENTER THE EYE.THEN IT CROSSES THE CRYSTALLINE LENS WHICH SHORTENS AND LENTHENS ITS WIDTH TO FOCUS LIGHT PROPERLY.THEN THE LIGHT PASSES THE DENSE VITREOUS.THEN THE RETINA RECEIVES THE IMAGES ANS THE RETINA CONVERTS IT INTO ELECTRICAL IMPULSES AND SENDS IT INTO OPTIC NERVE EHICH IN TURN SENDS IT TO THE BRAIN,WHERE ALL THE IMAGES AND THE DATA GET STORED.

COMPARISON BETWEEN A CAMERA AND A HUMAN EYE:

THE APERTURE OF THE CAMERA IS SIMILAR TO THE PUPIL OF THE EYE AS BOTH HELPS TO REGULATE THE AMOUNT OF LIGHT BY PARTIAL CLOSING OR OPENING TO PROVIDE THE PROPER CLARITY OF THE IMAGE.THE CRYSTALLINE LENS OF THE EYE IS SIMILAR TO THAT OF THE LENS OF THE CAMERA TO CAPTURE THE IMAGE.THE IRIS IS SAME AS THE SHUTTER RELEASE BUTTON TO ALLOW THE LIGHT TO ENTER AT THE RIGHT TIME.RETINA IS SIMILAR TO SCREEN OR FILM ON WHICH THE IMAGE IS FINALLY PROJECTED.PHOTORECEPTORS ARE PRESENT IN BOTH BUT IN A BIT DIFFERENT.BOTH THE RETINA AND IMAGING SENSOR CHIPS HAVE ONE THING IN COMMON THAT BOTH PRODUCES INVERTED IMAGE AND ELECTRICAL IMPULSES.THE BRAIN CORRECTS THE IMAGE IN HUMAN BODY WHILE THE DIGITAL CAMERAS ARE PROGRMMED TO MAKE CORRECTION ON THEIR OWN.JUST LIKE THE LENS ACCOMMODATE,WE CAN ALSO FOCUS THE OBJECT TO GET A BETTER VIEW.

Shreyosi Dasgupta

My job is my passion, proud to be an optometrist ❤

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  1. Kishor

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