Download Refraction - Pharos University in Alexandria

Human Vision and Vision
Correction
(PHR 177)Course
Prof. Dr. Moustafa. M. Mohamed
Vice Dean
Faculty of Allied Medical Science
Pharos University
Alexandria
Dr. Mervat Mostafa
Department of Medical Biophysics
Pharos University
The Physics of light
 Properties of Light
 The Eye
 Image Formation
 Glasses
Light
 The Electromagnetic Spectrum
The Electromagnetic Spectrum
 Radio Waves - communication
 Microwaves - used to cook
 Infrared - “heat waves”
 Visible Light - detected by your eyes
 Ultraviolet - causes sunburns
 X-rays - penetrates tissue
 Gamma Rays - most energetic
Properties of Light
Properties of Light
 Wave 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 sight
 Function
– Sensory organ for sight
–Detects light and converts it into neural responses
that the brain interprets
Anatomy of the Human Eye
Eye Anatomy
 Anatomy
– Light enters the eye through the pupil
– Photoreceptors (light-sensing cells) are
located in the retina
– Retina acts like the film in a camera
 How are images formed?
Image Formation: Apertures
 Apertures
– “openings”
 Basis of a pinhole camera
– Dark box
 small “pinhole” to let in light
 Image screen on opposite side of hole
– All light rays from a scene pass through
single point (focusing)
The Pupil is an Aperture
 Pupil
–Opening in the center of the eyeball
– Bounded by the Iris
 The iris controls the size of the pupil
–Opening through which light enters the eye
Image Formation: Apertures
 To achieve a clear image on an image screen, the
aperture must be very small
 Problems:
–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 MASSIVE
 Solution??
Lenses are the Solution to the
Aperture Problems
 Lenses focus of the light waves past the aperture
 Focuses the image on the screen
 Allows for wider apertures
 Produces smaller images
Lenses of the Eye
 Cornea
 Crystalline Lens
 Primary function
– To focus the image on the back of the retina
Refraction
 Bending of the path of a light wave as it passes across
the boundary separating two media
– Cause:
 Change in the speed of the light wave
 No change in speed = no refraction!
Optical Density
 Optical density of a material determines the speed of a
wave passing through it
 ↑ Op9cal density = ↓ Speed
Index of Refraction
 Abbreviated as “n”
 Indicator of optical density
Index of Refraction
 refractive 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.
Index of Refraction
 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:
𝒔𝒊𝒏𝜽𝟏
𝒏=
𝒔𝒊𝒏𝜽𝟐
Index of Refraction
 For 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 Relationship
 Image location changes depending on object distance
for a given lens’ focal length
 The Lens Equation
1/f = 1/d object + 1/d image
Distance-Size Relationship
 Image Size=( Image distance )× Object Size
Object distance
 Image size is limited by short image distance
 Most vision restricted to small region of the retina
Problem
 Retina 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 retina
 dobject is fixed = distance between the eye and the
object being viewed
 Solution??
The Solution is Accommodation
 Accommodation
–The ability of the eye to change its focal length (f)
–Mediated by the lens and ciliary muscles
 Distant Objects Nearby Objects
 Nearby Objects
Have a shorter dimage
 Lengthen the focal length
Ciliary muscles relax
Lens assumes a flatter
(skinnier) shape
Cornea is not pushed out
= less curvature
Have a longer dimage
 Shorten the focal length
Ciliary muscles contract
Squeeze the lens into a
more convex (fat) shape
Pushes cornea bulge out
further = greater
curvature
Near Point and Far Point
 Near Point
 Closest point at which an object can be brought into
focus by the eye
– Ideally ~25 cm
– Recedes with age (can lead to farsightedness)
 Far Point
 Farthest point at which an object can be brought into
focus by the eye
 Typically is infinity
 Decreases with age
Hyperopia
 INABILITY of the eye to focus on NEARBY objects
 “Can see far” – no difficulty focusing on distant
objects
 Images of nearby objects are formed at a location
BEHIND the retina
 Near 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 flat
 Lens can not assume a highly convex (fat) shape
Hyperopia: Correction
 Need to refocus the image on the retina
– Decrease the focal length of the cornea-lens system
 Add a converging lens
Myopia
 Inability of the eye to focus on DISTANT objects
 “Can see near” – no difficulty focusing on nearby
objects
 Images of distant objects are formed in front of the
retina
Causes of Myopia
 Not usually caused by aging
 Elongated eyeball (retina is farther away than normal
from the cornea-lens system
 Bulging cornea (greater curvature)
Correction of Myopia
 Need to refocus the image on the retina
– Increase the focal length of the cornea-lens system
 Add a diverging lens
Presbyopia
 “After – 40” vision
 Progressively diminished ability to focus on near
objects as one ages
– Similar to hyperopia, but different cause
 Cause = 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 error
 Blurred or sometimes distorted vision at any distance
 Cause:
– Irregularly shaped cornea or lens
 More oblong than spherical
 Refractive power differs between regions
 Correction
–Glasses
 Lenses with different radii of curvature in different
planes