JP© 1 THE EYE

JP© 2 sclera pupil iris

JP© 3 cornea sclera choroid retina fovea vitreous humour aqueous humour blind spot optic nerve pupil iris

JP© 4 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.

JP© 5 FAR POINT Muscles relaxed Lens less spherical Focus at infinity NEAR POINT Muscles working Lens more spherical Focus near point FOR NORMAL EYE, ACCOMMODATION RANGE IS INFINITY DOWN TO 25 CM.

JP© 6 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.

JP© 7 CONE ROD Rods and Cones - about 0.05 mm long and 1-3 μm in diameter. They can react to a single photon.

JP© 8 Vitreous humour LIGHT pigment cells rods & cones synapse relay nerve cells nerve fibres to brain nerve cells NOTE MORE THAN ONE ROD MAY BE ATTACHED TO THE SAME NERVE CELL

JP© 9 THE RETINA NERVE FIBRES LIGHT RODS RETINA PIGMENT EPITHELIUM Resynthesizes photosensitive pigments RED CONE BLUE CONE GREEN CONE

JP© 10 SPECTRAL RESPONSE OF THE EYE 380 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

JP© 11 Relative Absorption Wavelength / nm Combined response Peak sensitivity around 555 nm In green - yellow

JP© 12 Relative Absorption Wavelength / nm 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

JP© 13 They are concentrated around the periphery of the retina – 10 8 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

JP© 14 Concentrated around the fovea of human eyes, there are cones per mm 2, 10 6 cells in total, each 2 μm across Responsible for visual acuity Effective only in bright light CONES

JP© 15 DARK ADAPTATION THE 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

JP© 16 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

JP© 17 RED = GREEN MAKE YELLOW

JP© 18 RED = BLUE MAKE MAGENTA

JP© 19 GREEN = BLUE MAKE CYAN

JP© 20 ALL 3 PRIMARY COLOURS MAKE WHITE

JP© 21 EYE FORMULAE THE POWER OF A LENS in DIOPTRES (D) where f is in METRES Sign Convention “REAL IS POSITIVE” The power of the unaccommodated eye is +59D Convex lens is positive Concave lens is negative { object at infinity }

JP© 22 EYE FORMULAE Where u = object distance v = image distance Magnification m

JP© 23 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.

JP© 24 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.

JP© 25 LONG SIGHT - CORRECTION Normal eye near point uncorrected near pt. Suppose the uncorrected near point is 150 cm, i.e. v = cm. For normal vision u = 25 cm N.B. The patient’s far point will now be the object distance that produces an image at infinity. 25 cm 150 cm So f = 30 cm This gives u = 30 cm, so with glasses the patient’s range of vision is from 25 to 30 cm.

JP© 26 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.

JP© 27 SHORT SIGHT - CORRECTION - CONCAVE LENS DISTANT OBJECTS ARE MADE TO APPEAR TO COME FROM THE UNCORRECTED FAR POINT

JP© 28 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 = cm 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 formula This gives u = 22.9 cm and a range of vision from 22.9 cm to infinity. 200 cm

JP© 29 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