Physics 1230: Light and Color
Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Email:
ivan.smalyukh@colorado.edu
Phone: 303-492-7277
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
jhihan.yang@colorado.edu
Class # 15
Chapter #5: The
human eye & vision
Review: For photography of objects
at different distances from us we use
• A. Camera with CCD that
has at least 5 megapixels;
• B. Small aperture (large fnumber);
• C. Large aperture (small fnumber);
• D. Short exposure;
• E. A, B, C, & D
Review: For photography of
moving objects we use
• A. Camera with CCD that
has at least 5 megapixels;
• B. Small aperture (large fnumber);
• C. Large aperture (small fnumber);
• D. Short exposure and
larger aperture;
• E. A, B, C, & D
Ch. 5 The Eye
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Announcements
• New HW assigned today;
• Will discuss the outcome of the midterm on
Tuesday;
• Overall everybody did a very good job!
• Reading – Textbook Chapter 5
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We will learn
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Anatomy: The parts of the eye
How it “works”
Night vision
Response time
Eye problems and fixes
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Eye parts that you see
Pupil
Opening to the inside of
the eye
Iris
changes its opening to
adjust light
the pupil is the opening
Sclera the white outer wall
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Helpful concept map of vision:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/visioncon.html#c1
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Lens + Cornea
Sclera
Retina
Lens
Film/CCD
Light tight box
Iris
Diaphragm
pupil
aperture
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The Eye: Analogy
to the Camera
Lens and cornea
Iris (diaphram)
Ciliary muscle (focus)
Retina (film/CCD??)
We will see that the
retina does FAR more
than film or CCD
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Image is upside down and right-left
reversed (like a simple camera)
Do we see things upside down? Why?
Comparison of eye to camera
The camera
Lens
Diaphragm (f-stop)
Focusing knob
Film/CCD
The eye
Lens and cornea
Iris
Ciliary muscle
Retina
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Evolution
of the eye
All these
visual
system
still exist
in various
creatures.
Wikipedia
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Iris
Wide open at night less depth of field
aberrations
Closed down in daytime or for close
work,
fewer aberrations
more depth of field
You can check depth of field:
Try it: Close one eye, hold up
thumb, stuff behind thumb is out
of focus.
Try it: Pinhole pupil
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The Lens system: Imaging
We have two lenses: Cornea and lens
Cornea accounts for most of optical power (so,
does cornea or lens have greater focal length?)
Cornea has fixed focal length. Lens = fine-tuning.
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The Lens system: Focusing
mostly by cornea
assisted to varying degrees by eyelens
ciliary muscles puff up to relax the lens for
close focusing
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How thin lenses add
Ftot = final focal length
F1 = focal length lens 1
F2 = focal length lens 2
Diverging lenses (concave) have
negative focal lengths
This is the same as adding powers:
Dtot = D1 + D2
Demo: put together some lenses
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Question
You have two converging lenses, used
close together. The effective lens has a
focal length that is :
Dtot = D1 + D2
A) Larger than either alone
B) Shorter...
C) The same as one of them..
D = 1/F
So the cornea + eyelens gives a shorter focal
length (thus more power) than either alone
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Which has the shorter focal length: Cornea or eyelens?
How and where is the image
produced in your eye?
• Dual (compound) lens system
– Cornea (on outside) does
most of the ray bending
Cornea does most of ray bending
Retina
– Eyelens does fine tuning:
F1
– The image is focused on
your retina
– We'll use an equivalent
single lens in examples
F1'
Final
image
is on
retina
Eyelens behind cornea does fine-tuning;
focuses by changing its (long) focal length
How does eye focus?
• A camera moves the lens relative to screen
• In the eye the “screen” (ie., retina) is at a fixed
distance
• So, the eye changes the focal length of the
lens! (eyelens only, not cornea) =
“accomodation”
• The “focusing knob” is the ciliary muscle
Camera:
Eye:
1/f = 1/xo + 1/xi
fixed number
changes as xo changes
varies
1/f = 1/xo + 1/xi
varies
fixed number
Accommodation is achieved by the eyelens' ability
to change its focal length by changing its bulge
• Less bulge means less bending
power and longer focal length
• More bulge means more
bending power and shorter
focal length
large f
smaller f
What does accommodation of the eye have to do with looking
at me or your thumb? How does it work?
(Lens represents combined cornea-eyelens system)
Thumb is out of focus
Focusing your eye on a nearby
thumb requires shorter focal length
(more bulgy) eyelens than focusing
on me far away, since rays must be
bent more for image to fall on retina.
large f
Prof is in focus
Thumb is in focus
smaller f
Prof is out of focus
A near-sighted or MYOPIC eye produces an
image that is not far enough behind the lens, so
is blurry on the retina. Therefore, the eye lens
focal length is:
A)
B)
C)
D)
Too long for a focused image.
Too short for a focused image.
Actually, the iris is closed too much
None of these.
So, we want to
lengthen the effective
focal length of their
eye
So what kind of corrective lens do they
need?
A. Convex (f>0)
B. Concave (f<0)
Remember, Dtot = D1 + D2 and D = 1/f 24
Nearsighted (myopic; can’t see far) = concave lenses
Diverging Lens:
f<0
Farsighted (hyperopic; can’t see near) = convex lenses
f>0
What about the stuff inside?
The Retina (the “film” of the eye)
• Light-sensitive cells = “rods”
and “cones”
– Cones = color, precise
– Rods = low light, B&W
• Fovea = high density of cones
– That’s where you “look at”
someone.
– Unlike camera, we “scan” to get
image of room
• Signal gets out through optic
nerve
– No rods and cones here = blind
spot.
But the retina needs blood supply
• Blood vessels
are in front of
the cones and
rods….
Will learn much more about retina later
Dark adaptation
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Rods = dark vision
When you are in the light, you use your cones
In the dark, you use your rods
Detection threshold:
– When you go into the dark, you take about 7 minutes
for your eyes to switch to using all rods, no cones (25
min for full adaptation)
Retina
has 108 nerve endings to detect image
rods, for night vision
cones, for color and detail, 7 million
optic nerve = 106 transmission lines
fovea, region of best vision (cones)
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Rods and cones
• Rhodopsin, a photochemical, responds to light
It is destroyed and reformed.
• There are 3 kinds of cones for 3 colors
red, green, blue
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Retina details
Photopic vision, in bright light, cones are used
Scotopic, in low light, rods are used
more rods per nerve combines signals
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Retina Sensitivity
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More about the eye
Night vision
Response time
Eye problems
hyperopia, myopia,
presbyopia, astigmatism
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Dark adaptation (night vision)
• Time to adapt to dark: ~20-30 minutes
• Night adaptation: shifts from rods to cones
• You see blue better in the dark
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Persistence of vision
• Images remain on receptors for
– 1/25 second in low light
– 1/50 second in bright light
• Movies do 24 frames per second in a darkened
room.
• TVs do 60 frames per second, ok in lighted
room.
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Eye problems
• Loss of accomodation:
ability to focus from 10 inches to infinity
• Cataracts = cloudy eyelens,
replacement lens does not accommodate
• Floaters = dead cells floating in vitreous
humor (seen against a clear sky)
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Eye problems continued
• Myopia, see close objects clearly, only
fixed by a negative lens
• Hyperopia, see things far, only
fixed by a positive lens
• Presbyopia, stiff lens, no accommodation
Bifocal glasses have near and far focal
lengths
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The Eye as a cool instrument:
The eyelens
We know that lenses suffer from various
aberrations. What happens in the eyelens?
• Spherical aberration is mostly corrected
Cornea is not spherical surface (aspherical)
Iris cuts out rays through the edge of the lens
Index of refraction is not uniform.
• Curvature of field
retina is curved to correct for this
• Chromatic aberration:
Bluest light is absorbed
Many of these tricks are now used in technology.
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The Retina: Detecting the light and
processing the images
Has 108 nerve endings to detect image
rods, for high sensitivity (night vision)
cones, for color and detail, 7 million
optic nerve = 106 transmission lines
fovea, region of best vision (cones)
More nerves in your retina than some creatures
have in their entire brains.
Processing Power.
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Very common eye problems:
Issues in the lens focusing affect many
of us:
• Myopia, see close objects clearly, only
fixed by a negative lens
• Hyperopia, see things far, only
fixed by a positive lens
• Presbyopia, stiff lens, no accommodation
Bifocal glasses have near and far foci.
How do we fix these problems?
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Eye problems
• Loss of accomodation:
ability to focus from 10 inches to infinity
• Cataracts = cloudy eyelens,
replacement lens does not accommodate
• Floaters = dead cells floating in vitreous
humor (seen against a clear sky)
• Diseases of the eye components.
• Nerve damage
• Etc. …
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Review: Parts of the human eye
Specialized optical instrument
and image analysis computer.
References
http://www.costaricacoffeeart.com/alternative_photograph
y_make_your_own_negative_film_or_plates.php
http://unblinkingeye.com/Articles/Emulsion/emulsion.html
http://en.wikipedia.org/wiki/Charge-coupled_device
http://www.camerapedia.org/wiki/CCD
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