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
Conve
rays:
comin
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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
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Light Rays
• How does light change direction?
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• Reflection • Refractio
n

Angle of Reflection = Angle of Incidence
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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