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Physics 1230: Light and Color

Chuck Rogers, Charles.Rogers@colorado.eduRyan 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, Charles.Rogers@colorado.eduRyan 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, Charles.Rogers@colorado.eduShane Rightley, Shane.Rightley@colorado.edu

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: