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Outline Of Today’s Lecture
1. “The Facts of Light”
2. “The Birds & The Bees”
3. Demo on “Smoked” Filters
4. The Human Eye: Physiological Optics
Part 1
“The Facts Of Light”
Part 1: “The Facts Of Light”
• Light is the region of electromagnetic radiation
spectrum that humans can typically see (~400 to ~700
nanometers).
• Light has both wave-like and particle-like properties.
• Let’s consider some different types of waves….
Part 1: “The Facts Of Light”
• Let’s “Do the Wave”!
• Stadium-Style = Transverse Wave
• Transverse Wave - A wave in which oscillations
are perpendicular to the direction the wave
travels.
• Electromagnetic energy is a transverse wave.
Part 1: “The Facts Of Light”
• Let’s “Do the (other) Wave”!
• Geek-Style = Longitudinal Wave
• Longitudinal Wave - A wave in which
oscillations are parallel to the direction the
wave travels.
• Acoustic energy is a longitudinal wave.
Part 1: “The Facts Of Light”
• Now, let’s consider the particle-like properties of light.
• The smallest (particle or) unit of light is the photon.
• Not all photons are equal!
Part 1: “The Facts Of Light”
• Some photons oscillate at relatively high rates. These
are said to be of high energy.
• High energy photons travel a short distance over the
course of each oscillation, so they have short
wavelengths.
• Short-wave (i.e., high energy) light typically appears
blue-ish to most humans.
• On the other hand….
Part 1: “The Facts Of Light”
• Some photons oscillate at relatively low rates. These
are said to be of low energy.
• Low energy photons travel a long distance over the
course of each oscillation, so they have long
wavelengths.
• Long-wave (i.e., low energy) light typically appears
red-ish to most humans.
Part 1: “The Facts Of Light”
“The rays are NOT colored.”
Sir Isaac Newton
Part 1: “The Facts Of Light”
“The red’s in the head.”
Billy Wooten
Part 1: “The Facts Of Light”
• The four F’s of evolutionary survival
(according to Terry Sejnowski)
• Fighting
• Fleeing
• Feeding
• Reproduction
Part 1: “The Facts Of Light”
• What are the evolutionary advantages of being sensitive to
electromagnetic energy (versus some other energy form)?
• Electromagnetic energy travels quickly, thereby conveying
(almost) immediate information about distant objects, food,
predators, potential mates, etc..
• Electromagnetic energy tends to move in straight lines, thereby
preserving information about the shape of objects, food,
predators, potential mates, etc..
Part 1: “The Facts Of Light”
• What are the evolutionary advantages of being sensitive to
light, per se (versus some other part of the E.M. spectrum)?
• Light is “bouncy”. Unlike longer-wave energy, which passes
through many opaque objects, light can be reflected
(‘bounced’) off of objects, making them visible. Light’s a better
messenger.
• On our planet, light is plentiful. By contrast, shorter-wave
energy tends to be absorbed by our atmosphere (i.e., by oxygen
and nitrogen), and is less plentiful.
Part 1: “The Facts Of Light”
• Summary of evolutionary advantages to using light as the
“messenger”.
• Plenty fast.
• Plenty straight.
• Plenty bouncy.
• And just-plain-old, PLENTY!
Part 1: “The Facts Of Light”
• Regarding the evolutionary advantage to using
light…
• “The Eyes Have It! ”
• Some eyes even more so than others….
Part 2
“The Birds & the Bees”
(Demo on Polarized Filters)
Part 2: Polarized Filter Demo
• Some eyes are sensitive to the manner in which light
is polarized --that is-- to the plane of polarization
(plane of oscillation). Bow-&-Arrow Demo
• Some birds and insects have eyes that can pick-up
polarization information.
• Humans, other mammals, and other so-called
“higher vertebrates” are blind to polarization
information.
Part 2: Polarized Filter Demo
• Put on your polarized filters, and close your
left eye.
• Take one of the filters from a second pair, and
rotate it in front of your right eye.
• Repeat this with the other filter.
Part 2: Polarized Filter Demo
• What’s happening with the polarized filters?
• On a given pair of filters, one filter blocks certain
planes of polarization, and “passes” others.
• The other filter does the same for a complimentary
set of planes.
• By properly orienting two polarized filters, you can
block all the light.
Profile of Light Reaching Retina
Number of Photons
No Filter
100
0
Orientation A
Orientation A
+ 90 degrees
Plane of Polarization
Number of Photons
Profile of Light Reaching Retina
100
A filter that passes Plane A,
but blocks the perpendicular plane.
0
Plane A
Plane A
+ 90 degrees
Plane of Polarization
Number of Photons
Profile of Light Reaching Retina
100
A filter that blocks Plane A,
but passes the perpendicular plane.
0
Plane A
Plane A
+ 90 degrees
Plane of Polarization
Number of Photons
Profile of Light Reaching Retina
100
Two properly aligned filters
will block (nearly) all the light
0
Plane A
Plane A
+ 90 degrees
Plane of Polarization
Part 3
Demo on “Smoked” Filters
Part 3: “Smoked” Filter Demo
• Put a “smoked” filter in front of your right
eye.
• Take a smoked filter from a second pair, and
rotate it in front of your right eye.
• Does orientation matter in the same way that it
did for polarized filters?
Part 3: “Smoked” Filter Demo
• What’s happening with the “smoked” filters?
• On the first pair, your filter partially attenuates all planes of
polarization equally.
• Your second filter does the same. It attenuates whatever is
remaining from the first filter. Again, all planes of
polarization are equally attenuated.
• Unlike polarized filters, orientation makes no difference for
smoked filters.
Profile of Light Reaching Retina
Number of Photons
No Filter
100
0
Orientation A
Orientation A
+ 90 degrees
Plane of Polarization
Number of Photons
Profile of Light Reaching Retina
100
A single smoked filter
attenuates all planes equally
0
Orientation A
Orientation A
+ 90 degrees
Plane of Polarization
Number of Photons
Profile of Light Reaching Retina
100
A second smoked filter
further attenuates what little
light passes through the first filter.
Again, all planes are reduced equally.
0
Orientation A
Orientation A
+ 90 degrees
Plane of Polarization
Part 3: “Smoked” Filter Demo
• The point is ….
• If I placed just one of the filters before your eye, you could
NOT determine whether (A) the light was reduced equally
across all planes of polarization, or (B) the reduction was
polarization-plane specific.
• This is because, as a mammal, you are “polarization blind”.
• Not all animals are polarization blind.
• In principle, sensitivity to the plane of polarization could guide
migration patterns in birds.
Learning Check
• Be able to draw a luminance-bypolarization profile for polarized filters.
• Be able to draw a luminance-bypolarization profile for “smoked” filters.
Part 4
The Human Eye:
Physiological Optics
The Eye & Physiological Optics
The eye is said
to have two
optical components:
The Cornea
&
The Lens
The Eye & Physiological Optics
•
Image formation on the retina depends on the cornea.
•
The cornea refracts (or “bends”) light. That is, it
changes the direction of light entering the eye.
•
Because the cornea is convex, it makes incoming light
rays converge to a focal point.
•
The refractive power of the cornea, or any other optical
device, is measured in diopters (40 for the cornea).
The Eye & Physiological Optics
Diopter = 1 / focal length in meters
Focal length is the distance from the
optical device, to the focal point.
Focal Point
Focal Length
The Eye & Physiological Optics
•
Image formation on the retina also depends on the
crystalline lens.
•
Unlike the cornea, the crystalline lens can change shape,
providing additional optical power of 10 to 30 diopters.
This variability in the lens’ shape is called
accommodation.
•
Together, the cornea and the lens provide refraction
ranging from 50 and 70 diopters --at least in healthy,
young eyes.
The Eye & Physiological Optics
Myopic
Near-sighted
The eye ball is “too long”, given the refraction.
Retina
Can be corrected with a concave “-” lens.
The Eye & Physiological Optics
Hyperopic
Far-sighted
The eye ball is “too short”, given the refraction.
Retina
Can be corrected with a convex “+” lens.
The Eye & Physiological Optics
Presbyopic
“Old Sight”
Caused by sclerosis (hardening) of crystalline lens,
i.e., reduced refraction.
Corrected with bifocals
Reading is corrected with a strong convex lens.
Distance can be corrected with a weaker convex lens.
The Eye & Physiological Optics
1. Demo on lenses.
2. Two lenses will be passed around the classroom.
3. One is a +3 diopter (convex) lens. The positive
value indicates an increase in refractive power.
4. One is a -3 diopter (concave) lens. The negative
value indicates an decrease in refractive power.
5. Put one on top of the other, and they “cancel”.
The Eye & Physiological Optics
1.
How is image size measured on the retina?
2.
Size (or retinal subtense) is expressed in degrees of
visual angle.
3.
If a 1-centimeter long stimulus is viewed from 57.29
centimeters away, the stimulus subtends 1 degree of
visual angle on the retina.
4.
One degree of visual angle corresponds ~300
micrometers (millionths of a meter) on the retina.
The Eye & Physiological Optics
Visual Angle = Arctan (stimulus size / viewing distance)
The arctangent is the “inverse” of the tangent function.
We use arctan when we know the lengths,
but need to find the angle.
The Eye & Physiological Optics
1.
“Rule of thumb” - your thumb nail subtends
approximately 1 degree of visual angle, when your arm
is fully extended.
2.
The full moon subtends approximately 0.5 degrees of
visual angle.
3.
What about the “blind spot”?
The Eye & Physiological Optics
1.
The “blind spot” is a perceptual phenomenon that arises
from the optic disc (an anatomical structure on the nasal
side of each retina).
2.
It is vulgar to call the optic disk ‘the blind spot’.
3.
The blind spot is approximately 6 degrees of visual angle
in diameter, so the optic disk subtends about 1,800
micrometers on the retina.
4.
The blind spot is “many, many moons” big!
(approximately 12).
The Eye & Physiological Optics
Blind Spot Demo
Hand-out