Human Vision and VisionCorrection

(PHR 177)CourseProf. Dr. Moustafa. M. Mohamed

Vice Dean Faculty of Allied Medical Science

Pharos UniversityAlexandria

Dr. Mervat MostafaDepartment of Medical Biophysics

Pharos University

The Physics of lightProperties of LightThe EyeImage FormationGlasses

LightThe Electromagnetic Spectrum

The Electromagnetic SpectrumRadio Waves - communicationMicrowaves - used to cookInfrared - “heat waves”Visible Light - detected by your eyesUltraviolet - causes sunburnsX-rays - penetrates tissueGamma Rays - most energetic

Properties of Light

Properties of LightWave model

– Classical sinusoidal wave

– Can travel through a vacuum

–Describes reflection, refraction, diffraction, interference, and Doppler Effect phenomena, etc.

Particle model

– “photon”

–Describes absorption and emission phenomena

The eyes mediate sightFunction

– Sensory organ for sight

–Detects light and converts it into neural responses that the brain interprets

Anatomy of the Human Eye

Eye AnatomyAnatomy – Light enters the eye through the pupil – Photoreceptors (light-sensing cells) are located in the retina – Retina acts like the film in a cameraHow are images formed?

Image Formation: AperturesApertures

– “openings”Basis of a pinhole camera

– Dark boxsmall “pinhole” to let in lightImage screen on opposite side of hole

– All light rays from a scene pass through single point (focusing)

The Pupil is an AperturePupil–Opening in the center of the eyeball

– Bounded by the IrisThe iris controls the size of the pupil

–Opening through which light enters the eye

Image Formation: AperturesTo achieve a clear image on an image screen, the

aperture must be very smallProblems:

–Smaller aperture:Fewer photons get through.Ratio of pinhole diameter to image distance should be

less than 1/100.

–Image screen must be large.

–Eye would have to be MASSIVESolution??

Lenses are the Solution to theAperture Problems

Lenses focus of the light waves past the apertureFocuses the image on the screenAllows for wider aperturesProduces smaller images

Lenses of the Eye

CorneaCrystalline LensPrimary function – To focus the image on the back of the retina

RefractionBending of the path of a light wave as it passes across

the boundary separating two media

– Cause:Change in the speed of the light waveNo change in speed = no refraction!

Optical DensityOptical density of a material determines the speed of

a wave passing through it↑ Op9cal density = ↓ Speed

Index of RefractionAbbreviated as “n”Indicator of optical density

Index of Refractionrefractive index or index of refraction of a

substance or medium is a measure of the speed of light in that medium.

It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium.

This can be written mathematically as:n = speed of light in a vacuum / speed of light in

medium.

Another common definition of the refractive index comes from the refraction of a light ray entering a medium.

The refractive index is the ratio of the sines of the angles of incidence θ1 and refraction θ2 as light passes into the medium or mathematically:

𝒏=𝒔𝒊𝒏𝜽𝟏

𝒔𝒊𝒏𝜽𝟐

Index of Refraction

Index of RefractionFor example, the refractive index of water is 1.33,

meaning that light travels 1.33 times as fast in vacuum as it does in water.

As light moves from a medium, such as air, water, or glass, into another it may change its propagation direction in proportion to the change in refractive index.

This refraction is governed by Snell's law,

Snell’s Law–Quantitative answer to the question of “By how much does the light ray refract?”

ni*sin(θi) = nr*sin(θr)

ni = index of refraction of incident media

nr = index of refraction of refractive medium

θi = angle of incidence

θr = angle of refraction

If ni = nr, then no refraction!!

Object-Image RelationshipImage location changes depending on object distance

for a given lens’ focal lengthThe Lens Equation

1/f = 1/d object + 1/d image

Image size is limited by short image distanceMost vision restricted to small region of the retina

Distance-Size Relationship =

ProblemRetina is a fixed distance from the cornea-lens system

(~22 mm or 2.2 cm)Lens Equation

– 1/f = 1/dobject + 1/dimage

– In the eye,dimage is fixed = distance between cornea lens system

and the retinadobject is fixed = distance between the eye and the

object being viewedSolution??

The Solution is AccommodationAccommodation

–The ability of the eye to change its focal length (f)

–Mediated by the lens and ciliary muscles

Nearby Objects Have a longer dimageShorten the focal length

Ciliary muscles contract

Squeeze the lens into a

more convex (fat) shape

Pushes cornea bulge out

further = greater

curvature

Distant Objects Nearby Objects

Have a shorter dimageLengthen the focal length

Ciliary muscles relax

Lens assumes a flatter

(skinnier) shape

Cornea is not pushed out

= less curvature

Near Point and Far PointNear PointClosest point at which an object can be brought into

focus by the eye

– Ideally ~25 cm

– Recedes with age (can lead to farsightedness)Far PointFarthest point at which an object can be brought into

focus by the eyeTypically is infinityDecreases with age

HyperopiaINABILITY of the eye to focus on NEARBY objects“Can see far” – no difficulty focusing on distant

objectsImages of nearby objects are formed at a location

BEHIND the retinaNear point is located farther away from the eye

Hyperopia: Causes

Shortened eyeball (retina is closer than normal to the cornea lens system)

Cornea is too flatLens can not assume a highly convex (fat) shape

Hyperopia: CorrectionNeed to refocus the image on the retina

– Decrease the focal length of the cornea-lens systemAdd a converging lens

MyopiaInability of the eye to focus on DISTANT objects“Can see near” – no difficulty focusing on nearby

objectsImages of distant objects are formed in front of the

retina

Causes of MyopiaNot usually caused by agingElongated eyeball (retina is farther away than normal

from the cornea-lens systemBulging cornea (greater curvature)

Correction of MyopiaNeed to refocus the image on the retina

– Increase the focal length of the cornea-lens systemAdd a diverging lens

Presbyopia“After – 40” visionProgressively diminished ability to focus on near

objects as one ages

– Similar to hyperopia, but different causeCause = diminished power of accommodation due to

natural process of aging

– Reduced elasticity of the lens

–Weakening of the ciliary muscles

– Changes in lens curvature due to continued growth

Astigmatism

Most common refractive errorBlurred or sometimes distorted vision at any distanceCause:

– Irregularly shaped cornea or lensMore oblong than sphericalRefractive power differs between regions

Correction

–GlassesLenses with different radii of curvature in different

planes