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