Transcript Properties of Light and Visual Function

Physical Optics and
Properties of Light
Resident Lecture
Amy C. Nau, O.D., F.A.A.O
The Uzumaki or 'rabbit ears' illusion: when in peripheral vision these spirals appear
to move in a clockwise direction.
Vision is the perception of light
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To understand vision, you must understand
the properties of light
Where does it come from?:
How does it interact with objects?
How does it move through the eye?
How can it be used to aid diagnosis of eye
disorders?
Photons
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When an atom is in the resting state, the
negative electron cloud is balanced with the
positive nucleus
Excited atoms have electrons that are in a
higher energy state, when they go back to the
relaxed state, the energy released is a
photon.
A photon is an energy packet that travels
through space as an EM wave
Wave Motion
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Wave propagation is
represented as a sine wave
that varies in time and
space
EM energy travels as a
transverse wave (vibrates
perpendicular to direction of
propogation)
Electromagnetic waves
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EM waves have both
electric and magnetic
fields.
Wave Motion
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Wavelength- (l m,nm) Distance from peak to peak, trough to trough or
any repeatable position. It is inversely proportional to the amount of
energy the atom gives up. So, short wavelengths have high energy.
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Cycle- (c) completion of a regular periodic event (one peak to the next)
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Frequency-(f, c/s, Hz) Number of cycles that pass a reference position in
a given time period. Constant for all media
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Period (T,P s/c) Inverse of Frequency
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Velocity (v, m/s) rate of travel. Light has same velocity in air as in a
vaccuum (3x108m/s)
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Amplitude (A) maximum displacement of wave
Velocity, frequency and wavelength are related by
v=f l
cycle
wavelength
peak
amplitude
trough
Electromagnetic waves
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Photons travel through a vacuum at a
constant speed (the speed of light, c)
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3x106 m/s
They will slow down outside a vacuum
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Index of refraction (n) of a media is the ratio of speed of
light in a vacuum to speed of light in the media
v=f l
Key Definitions
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Source- any object emitting EM radiation
Point source- so small or far that it acts as if
infinitely small
Extended source- source with measurable
area
Monochromatic- EM single wavelength or
frequency (laser, sodium gas, etc.)
Polychromatic- emits radiation of several
wavelengths (white light)
Wavefronts
Rays
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Used to represent the
propogation of light in
geometrical optics.
Rays are perpendicular
to wavefronts
A group of rays is a
pencil
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Convergent, divergent,
parallel
Beam- sum of pencils
Rays
Parallel Pencil
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Rays emitted from a source infinitely far
away. Since the wavefronts have such
large radii, they are functionally parallel
to each other. (20 feet)
Sign convention rules
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Assume light travels from
left to right
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Distances are + if they
travel in the same direction
of light. They are – if they
travel in the opposite
direction.
Radii of wavefronts are
measured from the
wavefront to the source or
image.
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-
Interference
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The addition of two waves to form a new
wave
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Constructive or Destructive
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http://www.colorado.edu/physics/2000/applets/fourier.html
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Used to measure the quality and shape of optical surfaces
Used to bypass the eye’s optics and project contrast sensitivity fringes on to
the retina directly
Seen with lasers (speckles)
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Coherence
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The ability of two waves to interfere
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Same wavelength, source and temporal
characteristics (lasers)
Incoherent light will not interfere
(incandescent light bulbs)
Coherence
Rectilinear Propogation
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Light rays travel in straight
lines
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Diffraction is when light hits
an object and becomes
distorted or bends. If that
distorted wave hits a surface,
there is a diffraction pattern
Double-slit diffraction
Double-slit diffraction
(red laser light)
2-slit and 5-slit diffraction
http://en.wikipedia.org/wiki/Diffraction
Scatter
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Another diffraction effect in which light
interacts with a series of small particles
The size and spacing of the particles
determines the degree of scatter
Materials scatter the light they don’t absorb
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Smoke, fog, edema, cataract produce glare,
reduced contrast
IR less prone to scatter, can penetrate to the
retina/choroid
http://medinfo.ufl.edu/cme/hmoa2/light_scatter_t.jpg
http://www.3dvf.com/DATA/PUBLISH/404/images/scattervolumelight.jpg
Fluorescence
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Molecules absorb photons
and become excited. If they
return to the original state, a
photon is emitted.
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Fluorescein Dye- illuminated
with a blue wavelength of light
and emits in the green.
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Used for cls, ocular surface
inspection and FA
ICG- illuminated with IR- note IR
also does not scatter!
Electromagentic Spectrum
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The amount of energy the atom releases
determines the wavelength.
- Small wavelength= high energy
- Long wavelength = lower energy
LIGHT is EM waves that are within the visible
spectrum 380nm-780nm
The term “visible light” is redundant…………
EM Spectrum
Visible light (for humans) is 380nm to 780 nm
Vision and EMR
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An object’s color is determined by the
characteristics of the wavelengths that are
reflected off of it’s surface
We have 3 populations of cones that are
sensitive to different wavelengths
Short (S)- blue
Middle (M)- green
Long (L)- red
Spectral Sensitivity
Color is a property of the way the
Visual system detects light
http://www.photo.net/photo/edscott/vis00010.htm
Normal
Protanopia
Unable to percieve red
Deuteranopia
Unable to percieve green
Ways that light changes
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Wavelength (color)
changes depending on
where light is travelling
The speed of light
(velocity) also changes
depending on media
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Wavelength decreases
in any media other than
a vacuum.
It is directly related to
velocity, so if velocity
decreases, so will the
wavelength.
Example Problem
Red light, with a wavelength of 700nm in a vacuum, enters a lens so that
the wavelength reduces to 450nm. What is the velocity of the light in the
block of glass?
Solve for frequency 3x108m/s /700x10-9 = 4.28x1014
Use this to solve for the second veocity
V= 4.28x1014 x 450 x 10-9
So, you see as the wavelength decreased, so did the speed at which
light travels.
v=fl
Vergence
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The reciprocal of the radius of the
wavefront
When measured in meters, is referred to
as a DIOPTER
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Divergent pencils have negative vergence
Convergent pencils have positive vergence
VD=1/rm
Problem
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What is the power of the following lens?
VD=1/rm
l=-40cm
l’=90cm
Object vergence (L) = 1/l = 1/-.4 = -2.5D
Image vergence (L’) = 1/l’= 1/.9 = +1.11D
Total change by the lens (power, or F) is +3.61D
Problem
Determine the vergence of a wavefront with a
radius of -.04meters.
VD=1/rm
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So, V=1/-.04 = -25D
Pinhole camera
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h
h’
a
a’
h/h’=a/a’
No matter where the object is placed, a clear
image will form, but the size of the image will change.
A pinhole takes the
place of the lens.
If the pinhole is
small enough to
only let one ray
from each object
point to pass, each
point of the image
will be formed by a
single ray, forming
an inverted image
that is in focus.
::
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Small aperture concepts
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Depth of field- distance over which an object
can be moved without affecting the image
(ph, small pupils)
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Range of vision at near increases
Prosthetic cls
Depth of focus- distance over which an image
screen can be moved while maintaining
sharpness
Field of view- maximum angular size of object
imaged by system (or eye)
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FOV of the 20D v 28D
Problem
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A PH camera is used to photograph an
object. Where must the object be placed so
the image formed on film 5cm behind the PH
is .1 times the size of the object?
h
.1(h)
?
Just set up a ratio.
5cm
h/? = .1(h)/.5 solve for ? And you get 50 cm.
Hermann Grid Illusion
In the above illustration, black dots appear to form and vanish at the intersections of the gray
horizontal and vertical lines. When focusing attention on a single white dot, some gray dots
nearby and some black dots a little further away also seem to appear. More black dots seem
to appear as the eye is scanned across the image (as opposed to focusing on a single point).
Strangely, the effect seems to be reduced, but not eliminated, when the head is cocked at a
45° angle. The effect seems to exist only at intermediate distances; if the eye is moved very
close to or very far away from the figure, the phantom black dots do not appear.
The illusion is known as the scintillating grid, and was discovered by E. Lingelbach in 1994. It
is a modification of the Hermann grid illusion.
Begin!