Chuck Rogers, [email protected] Ryan Henley ... · Eyeglass prescription is in diopters...
Transcript of Chuck Rogers, [email protected] Ryan Henley ... · Eyeglass prescription is in diopters...
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Physics 1230: Light and Color
Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried
physicscourses.colorado.edu/phys1230
Lecture 25: More about eyes!
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Project Progress Reports
Due Friday (in dropbox)
15 points towards the 65 point project total
• Turn in an outline of your project paper
• Try to include one picture or figure for your
project.
• Let us know how your observations are
going. Are there questions you have? Are
there some confusing things that you’d like
feedback on? This point in the project is a
great time to ask us if you need special
feedback.
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Physics 1230: Light and Color
Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried
physicscourses.colorado.edu/phys1230
AND:
Online_HW11 and Written_HW11
are due Friday at 8PM.
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Physics 1230: Light and Color
Chuck Rogers, [email protected] Rightley, [email protected]
www.colorado.edu/physics/phys1230
Lecture 25:
The Eye and correcting and
improving vision
Last Time: Language for inside parts of the
human eye
Multi-component lens made of
cornea, humor, and inner lens.
Structure of the eye: cornea and lens
• There are t wo lenses in your eye, t he cornea and t he
eyelens
• The cornea, t he f ront sur f ace of t he eye, does most of
t he f ocusing in your eye, most opt ical power, short er
fixed f ocal lengt h
• The eyelens provides adj ust able fine- t uning of t he f ocus
Structure of the eye: Cornea and Lens
The eye lens: Accomodation
The muscles inside the eye feel more relaxed when you look at something far away
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The Eye: Analogy
to the Camera
You want to produce a focused image at the
back of the eye. Therefore, you:
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A) Adjust the focal length of the lens.
B) Adjust the diaphragm to change DoF.
C) Adjust the object distance for sharp image.
D) Adjust the retina to avoid blurred exposure.
E) None of these.
Quite different than for a camera
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Last Time: Eye and:
Analogy to the
Camera
Lens and cornea
Iris (diaphram)
Ciliary muscle
(focus)
Retina (FILM??)
The retina: Light sensitive back surface of your eyeDetects light through specialized cells called
rods and cones
Rods
• Rod-shaped
• Black and white
• Throughout entire retina
• Respond in low light too
Cones
• Cone-shaped
• Respond to color
• Concentrated in “fovea”
• Require a lot of light to respond
“antennae” that resonate to incoming light
Light receptors: Rods and cones
Much variation between creatures
Light comes in
from here
http://webvision.med.utah.edu/anatomy.html
Lots of specialization here for detection and
processing. More in the next couple of lectures…
Human retina Cat retina
Cones are concentrated in the fovea
So you see color most sharply for something in the center of your vision.
Look at the x in this sentence
• Notice how, when looking at the x, it is sharp. Or, look at the y, then it is sharp. However, the other things that you are NOT looking at are a little blurry?
Then look at the y in this sentence
• You only have precise vision in one area of the retina, called the “fovea”
• You are moving your eye so that the image of the X or Y falls on your fovea
• When you look at something, its image is on your fovea
Notice in this image, there are blood vessels in the back of the retina.
“Red eye” is the reflection of a flash off these blood vessels To eliminate red eye, flash a light so pupil closes down, OR use
a flash that is off-center, so red-eye bounces away from lens
• Light passes through this network of veins to get to your retina
• It’s always there, so it fades from your vision
• But the light casts a shadow of the blood vessels on the retina.
Why don’t we always see this network of blood vessels?
Your eye ignores images that don’t change = adaptation
Lots of information!
Questions??
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You now know a fair amount about the retina. We will cover more in a
week or so.
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Very common eye problems:
• Myopia, see close objects clearly, onlyfixed by a negative lens
• Hyperopia, see things far, onlyfixed by a positive lens
• Presbyopia, stiff lens, no accommodationBifocal glasses have near and far foci.
Issues in the lens focusing affect many
of us:
How do we fix these problems?
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:
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A) Too long for a focused image.
B) Too short for a focused image.
C) Actually, the iris is closed too much
D) None of these.
You have a lens with a short focal length, f,
and you wish f was longer. You can make it
longer by using a second lens. The correct
choice for this case is:
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A) A focusing lens of negative power
B) A diverging lens of positive power
C) A focusing lens of positive power
D) A diverging lens of negative power
Recall:1 2
1 2
1 1 1
TOTAL
TOTAL
f f f
OR
D D D
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Eyeglasses: Our most common optical instrument
For nearsighted people (can’t focus far away)
Eyeglasses are diverging (thinner in middle)
For farsighted people (can’t focus up close)
Eyeglasses are converging (thicker in middle)
Demo: eyeglasses
Normal vision: you can focus from 25 cm to infinity ()
18th century HUGE
improvement in quality of life
Ben Franklin
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Printer, publisher, author, scientist, signer of the US Declaration
of Independence, discoverer of the two signs of electric charge.
As on the US
$100 bill.
Man of fashion!
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Eyeglass prescription is in diopters
Optometrists use diopters to measure the powerof a lens
Diopters [or D] = 1 / (focal length in meters)
Example: f = 50 cm or f = 0.5 m
D = 1/f = 2 diopters (units are 1/meters)
The example above would be:
(A) reading glasses (B) distance glasses
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Astigmatism
Vertical and horizontal lines focus differently
This problem is fixed by a cylinder lens
Sharply focused
Out of focus
Focuses in one
direction, but not the
other!
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Action of a cylinder lens
Focuses in one
direction, but not the
other!
If a cylinder lens is needed for your eyeglasses,
your cornea and eyelens is curved more in one
direction than in the other!
I encourage: Play with your (or with a friend’s) eyeglasses to see
what they can do!
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The Magnifying glass (again): New insight!
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Ray optics lets us
determine the ray paths.
A model of the observer lets us predict
an image where rays converge.
Our first effort
to explain:
The Magnifying glass (again): Another view
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Typical closest focus is 25 cm from the eye.
A magnifying glass is like READING
GLASSES: It lets you focus on closer
things.
The eye perceives via focused images:
25 cm
The Magnifying glass (again): Another view
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Best focused image for eye alone.
The eye perceives via focused images:
25 cm
If the object is 1 cm high and the eye is 2.5 cm
from lens to retina, how high is the image?
A) 1 cm B) 0.1 cm C) 1 mm D) B and C
E) Could be either A or B
The Magnifying glass (again): Another view
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Best focused image for eye alone.
The eye perceives via focused images:
25 cm
Focal
length of
magnif.
Use a Magnifying glass!!
Bigger image
at back of eye
Or: