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THE EYE

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sclera

pupil

iris

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cornea

sclera

choroid

retina

fovea

vitreous humour

aqueous humour

blind spot

optic nerve

pupil

iris

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The cornea's curvature accounts for 80% of the eye's focusing.

The lens alters its curvature to accommodate objects at varying distances.

The cornea does not have any blood vessels. Nutrients and oxygen are supplied directly by the tears and the aqueous humour.

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FAR POINTMuscles relaxedLens less sphericalFocus at infinity

NEAR POINTMuscles workingLens more sphericalFocus near point

FOR NORMAL EYE, ACCOMMODATION RANGE IS INFINITY DOWN TO 25 CM.

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PHOTODETECTION

CONES PREDOMINATE AROUND THE CENTRE OF THE OPTICAL AXIS

i.e.. AROUND THE FOVEA OR MACULA

THE RODS ARE FOUND MAINLY AROUNDS THE PERIPHERY

THE RETINA CONSISTS OF TWO TYPES OF LIGHT SENSITIVE CELLS, NAMED FROM THEIR SHAPE:

120 MILLION RODS

6 MILLION CONES

THESE ARE SURPRISINGLY SITUATED BEHIND A JUMBLE OF NERVE FIBRES THAT CARRY ELECTRICAL IMPULSES TO THE BRAIN.

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CONEROD

Rods and Cones - about 0.05 mm long and 1-3 μm in diameter.

They can react to a single photon.

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Vitreous humour

LIGHT

pigment cells

rods & cones

synapse

relay nerve cells

nerve fibres to

brainnerve cells

NOTE MORE THAN ONE ROD MAY BE ATTACHED TO THE SAME NERVE CELL

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THE RETINA

NERVE FIBRES

LIGHT

RODS

RETINA PIGMENT EPITHELIUM

Resynthesizes photosensitive

pigments RED CONE BLUE CONE GREEN CONE

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SPECTRAL RESPONSE OF THE EYE380 nm TO 700 nm

THERE ARE THREE TYPES OF CONES

RED CONES CONTAINS RED SENSITIVE PIGMENT – ERYTHROLABE

GREEN CONES CONTAINS GREEN SENSITIVE PIGMENT – CHLOROLABE

BLUE CONES CONTAINS BLUE SENSITIVE PIGMENT – CYANOLABE

CONES ARE ONLY EFFECTIVE IN BRIGHT LIGHT

WE CANNOT SEE COLOURS IN DIM LIGHT

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Relative Absorption

Wavelength / nm

400 500 600 700

Combined response

Peak sensitivity around 555 nm

In green - yellow

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Relative Absorption

Wavelength / nm

400 500 600 700

Cones have maximum sensitivity at 555 nm in YELLOW – photopic vision

Yellow safety clothing, tennis balls

Rods do not respond to colour but have peak sensitivity in the green at 510 nm – scotopic vision

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They are concentrated around the periphery of the retina – 108 cells

Rods allow us to see much lower light intensities because several of them are connected to a single nerve fibre.Objects look sharper at night when looking out of the side of the eye.The fibre is stimulated by the cumulative effect of photon arrival.

Because of this grouping, they give little perception of detail or precise position.

It is difficult to play ball games in poor light !!

RODS

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Concentrated around the fovea of human eyes, there are 160 000 cones per mm2, 106 cells in total, each 2 μm across

Responsible for visual acuity

Effective only in bright light

CONES

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DARK ADAPTATIONTHE PROCESS BY WHICH THE SENSITIVITY OF THE RETINA

INCREASES WHEN LIGHT INTENSITY DECREASES

PIGMENTS IN THE RODS AND CONES DECOMPOSE IN THE LIGHT, WHICH IN TURN STIMULATES NERVE CELLS.

ENZYMES REGENERATE THESE PIGMENTS.

THIS HAPPENS MORE QUICKLY IN THE CONES.

AS A RESULT, IN BRIGHT LIGHT, ONLY THE CONES REMAIN ACTIVE.

IF LIGHT INTENSITY SUDDENLY FALLS TO A LOW LEVEL, IT IS DIFFICULT TO SEE BECAUSE THE RODS ARE INACTIVE AND THE CONES NEED BRIGHT LIGHT TO FUNCTION.

BECAUSE THE RATE OF DECOMPOSITION OF THE PIGMENT IN THE RODS IS NOW MUCH LOWER, THE PIGMENT CONCENTRATION IN THEM GRADUALLY BUILDS UP AND SO DOES THE SENSITIVITY OF THE RETINA.

DARK ADAPTATION TAKES UP TO 30 MINUTES

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BRIGHT LIGHT

CIRCULAR MUSCLES CONTRACT

RADIAL MUSCLES RELAX

LESS LIGHT ENTERS THE EYE

DIM LIGHT

CIRCULAR MUSCLES RELAX

RADIAL MUSCLES CONTRACT

MORE LIGHT ENTERS THE EYE

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RED = GREEN MAKE YELLOW

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RED = BLUE MAKE MAGENTA

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GREEN = BLUE MAKE CYAN

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ALL 3 PRIMARY COLOURS

MAKE WHITE

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EYE FORMULAE

THE POWER OF A LENS in DIOPTRES (D)f

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where f is in METRES

Sign Convention “REAL IS POSITIVE”

The power of the unaccommodated eye is +59D

Convex lens is positiveConcave lens is negative

{ object at infinity }

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EYE FORMULAE

vuf

111 Where u = object distance

v = image distance

Magnification m

u

v

heightobject

heightimagem

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DEFECTS OF VISION – LONG SIGHT - HYPERMETROPIA

DISTANT OBJECTS ARE SEEN CLEARLY

CLOSE OBJECTS ARE FOCUSED BEHIND THE RETINA BECAUSE THE EYEBALL IS TOO SHORT

near point for normal eye at 25 cm

The patient’s near point is greater than the normal distance of 25 cm.

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LONG SIGHT - CORRECTION – CONVEX LENS

Object at near point for normal eye of 25 cm

The image is made to appear at the patient’s near point where he can see it clearly.

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LONG SIGHT - CORRECTION

Normal eye near pointuncorrected near pt.

Suppose the uncorrected near point is 150 cm, i.e. v = - 150 cm.

For normal vision u = 25 cm

DP 3.330.0

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N.B. The patient’s far point will now be the object distance that produces an image at infinity.

25 cm150 cm

150

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25

1111

vufSo f = 30 cm

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30

1

uThis gives u = 30 cm, so with glasses the patient’s range of vision is from 25 to 30 cm.

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DEFECTS OF VISION – SHORT SIGHT - MYOPIA

CLOSE OBJECTS ARE SEEN CLEARLY

DISTANT OBJECTS ARE FOCUSED IN FRONT OF THE RETINA BECAUSE THE EYEBALL IS TOO LONG

The patient’s far point is closer than infinity.

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SHORT SIGHT - CORRECTION - CONCAVE LENS

DISTANT OBJECTS ARE MADE TO APPEAR TO COME FROM THE UNCORRECTED FAR POINT

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SHORT SIGHT CORRECTION

UNCORRECTED FAR POINT

Example; A lady has a near point of 20 cm and a far point of 200 cm. What is the power of lens that she needs, and, for distance viewing, what is her range of vision with the spectacles?

Lens required to view objects at infinity must have a f = - 200 cm DP 5.02

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When using these spectacles the near point of the lady will be the object distance that produces a virtual image at her true near point of 20 cm.

Putting f = - 200, v = - 20 in the lens formula20

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200

1

u

This gives u = 22.9 cm and a range of vision from 22.9 cm to infinity.

200 cm

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Astigmatism is a condition in which the cornea of the eye is not spherical, causing out-of-focus vision in some planes

ASTIGMATISM

TEST

CORRECTION IS WITH CYLINDRICAL LENSES