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Review about simple optic to the eye
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Optic, lens and IOL
Light
• Necessary to see things
• Can be made by the object eg lightbulb
• May come from other sources eg sun, light globe
– Bounce off the object = reflection
Light
• Why is it harder to see in the dark?
– Less light to be reflected
• In full light, what can be hard to see?
– Clear Glass, water or plastic (medium)
• Why are these hard to see?
– Light passes through them, no reflection
Light Rays
• Light travel in straight lines
• Optical diagrams show rays– An arrowhead shows
direction the light is travelling
– Useful for understanding what happens to light rays as they travel through or are reflected off surfaces
Parallel light rays: the rays are all travelling the same direction and stay the
same distance apart
Convergent light rays: the rays are
coming together
Divergent light rays: the rays are moving
apart
FIgure
1
Light Rays
• 3 groups of light rays
Parallel light rays: the rays are all travelling the same direction and stay the
same distance apart
Convergent light rays: the rays are
coming together
Divergent light rays: the rays are moving
apart
FIgure
1
Light Rays
• How does light change direction?
FIgure
1
• Reflection • Refractio
n
Angle of Reflection = Angle of Incidence
FIgure
2
Reflection
Refraction
• Change in direction of a light ray when it travels from a medium of one density to a medium of a different density is called refraction – Light travels through mediums (air, glass, water,
plastic)
– Light slows down through thicker (denser) mediums
Refraction
• Refracting surface– Meeting of mediums of
different densities
• Incident ray– Ray of light moving to
the refracting surface
• Refracted ray– Ray that has passed
through the refracting surface
Refractive Index
• A measure of how fast a medium will allow light to travel
• High refractive index– Thicker medium– Slower light travel
• Low refractive index– Thinner medium– Faster light travel
Refractive Index
Optical diagrams
• High Low RI– Dense medium to
less dense
– Light speeds up
– Bends away from the normal
Refractive Index
Optical diagrams
• Low High RI– Less Dense to
dense medium
– Light slows down
– Bends towards the normal
Prism
• Triangle shaped
• Transparent material
• Base
• Apex
apex Apical
angle
base
Prism
• Light bends towards the base
• Apparent deviation– Object appears to be moved when viewed
through the prism
Lenses
• Glass or plastic
• Focus light
• Examples
– Microscopes
– Magnifying glasses
– Spectacles
– Slide projector
What is a Lens?
• 2 prisms joined together
• Optical centre
• Focus
Lenses• Plus lens
– 2 prisms base to base
– Converging light
• Minus lens– 2 prisms apex to
apex
– Diverging light
Optical
centre
This ray
passes
through the
optical
centres
without
bending
Spectacle Lenses
• Lenses have two surfaces– One surface must be curved
Lenses
• Sphere– Perfectly round
object
• Convex– Outside of the sphere– Eg ball
• Concave– Inside of the sphere– Eg dish
Lenses
• Plus
• Minus
Lens Power
• Measurement of how much the lens bends the light to focus
• Measured in dioptres (D)
• Plus and minus
• ¼ steps
• Written in decimals eg 0.25
Lens Power– Light is parallel if coming from greater than 6m
– Focal point is where the light is focused
– Focal length is the distance between the lens and point of focus “f”
– “F” is power of lens
– F = 1/f f = 1/F
Plus Lenses
• Thicker in middle
• Thinner on the edge– Make images look
bigger
– Make images move in the opposite direction
• Also called positive, convex, converging lenses
Plus Lens
Minus Lens
Thicker in middle
Thinner in middle
Thinner on edge
Thicker on edge
Plus Lenses
Minus Lenses
• Thinner in the middle than on the edge– Make images look
smaller
– Make images move in the same direction
• Also called negative, concave, diverging lenses
Plus Lens
Minus Lens
Thicker in middle
Thinner in middle
Thinner on edge
Thicker on edge
Minus Lens Focus
Minus Lens Focus
Plus & Minus Lenses
WindowImage of window
Hand-held plus lensSheet of paper
Light from window
Plano Lenses
• Lenses with no power
• When may people need plano lenses?
– Sunglasses
– Safety spectacles
• May be flat or curved
Plano Lenses
Cylinder Lenses
• ‘Cyls’
• May be plus or minus
• Written as “DC”
Cylinder
Plus Cyl Minus Cyl
Sph vs Cyl
Sph vs Cyl
Sph vs Cyl
Focal Line
• Light is refracted to a focal line
• The focal line is parallel to the axis
Refractive state of the eye
• Focal point: – location of the image by an object ay optical infinity
through a nonaccommodating eye determine the eye’s refractive state.
• Far point– point in space thet is conjugate to the fovea of the
nonaccommodating eye
• Emmetropia• Ametropia:
– Myopia– Hyperopia– Astigmatism
Lens
• The lens is a biconvex structure located directly behind the posterior chamber and pupil.
• The lens measures:– At birth: 6.5 mm equatorially
• 3.5 mm antero-posterialy• 90 mg weigh
– Adult lens: 9-10 mm equatorially
• 5 mm antero-posteriorly• 255 mg weigh• The lens contributes 20D of the 60D focusing
power of the average adult eye.
lens
• The crystalline lens is a transparent, biconvex structure whose functions are- to maintain its own clarity- to refract light- to provide accommodation
• Has no blood supply or innervation after fetal development. It depends entirely on the humor to meet its metabolic requirements and to carry off its wastes.
• The lens is composed of:– Capsule– Epithelium– Fibers– Zonules
Accommodation and presbyopia
• mechanism eye changes refractive power by altering the shape of its crystalline
• Effort:– ciliary muscle contacts in parasympathetic stilumation
• Response– increase the lens convesxity
• Amplitude: (on Diopter)– decrease with age(up to 16D in children)– Highest in near point
• Presbyopia:– loses elasticity no accommodation response
IOL
• Classification:– Ìmplatation site– Optic profile– Optic material– Haptic style– Sphericity– Wavelength feature– Focality– Degree of accommodation– Edge finish– Power– Type of correction