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Physics 1230: Light and Color

Lecture 13: The Eye, eyeglasses and other optical

instruments, start retina signal processing.

Reading: Chap. 9,10 Color perception.

Exam 4 cancelled: Exam extra credit

assignment will be due Wed. at 5PM

Extra credit to improve exam scores!

HW9: Due today, Monday, 5PM

FCQ at end of lecture.

Review: Parts of the human eye

Specialized optical instrument

and image analysis computer.

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The Eye: Analogy

to the Camera

Lens and cornea

Iris (diaphram)

Ciliary muscle

(focus)

Retina (FILM??)

We will see that the

retina does FAR more

than film or CCD

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The Lens system: Imaging &Focusing

Most converging of light is by the cornea.

Assisted to varying degrees by eyelens focus.Ciliary muscles puff up to relax the lens for close focusing

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)

large f

smaller f

Focusing your eye on a nearby thumb requires shorter focal length (more bulgy) eyelens than focusing on the Prof far away, since rays must be bent more for image to fall on retina.

Thumb is out of focus

Prof is in focus

Thumb is in focus

Prof is out of focus

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The Eye as a cool instrument: The eyelens

• Spherical aberration is mostly correctedCornea is not spherical surface (aspherical)Iris cuts out rays through the edge of the lensIndex of refraction is not uniform.

• Curvature of fieldretina is curved to correct for this

• Chromatic aberration:Bluest light is absorbed

We know that lenses suffer from various

aberrations. What happens in the eyelens?

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)

The retina and optic nerves are recognized as actually

parts of the brain (like your olifactory bulb in the nose).

They start development IN the brain and migrate…

More nerves in your retina than some creatures

have in their entire brains. Processing Power.

Human versus cat retinas

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…

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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?

The remaining lectures:

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• Ch. 5 (the eye),

• Ch. 6 (optical instruments),

• Ch. 7 (Retina and visual perception),

• Ch. 9 & 10 (color & color perception).

We

are

here

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Optical instruments we’ll cover:

• Single lens instruments– Eyeglasses

– Magnifying glass

• Two lens– Telescope & binoculars

– Microscope

We

are

here

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

and you wish it 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

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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!

Play with your 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

22Demo with thumb and eyepiece from telescope kits.

Best focused image alone.

The eye perceives via focused images:

25 cm

Focal

length

Magnifying glass

Bigger

image

25 cm

cmM

f

By similar triangles:

Or:

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Magnification of a one-lens magnifier

Example:

5 cm focal length has a magnification of

cmin length focal

cm 25

Interesting thought:

2 mm focal length has 125 magnification

A) 5 B) 4 C) 25 D) 1/5 E) None of these

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van Leeuwenhoek’s microscope

25 cm, reading distance,

approximately

Focal length, approximately

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van Leeuwenhoek’s microscope

Years developed: 1660s

Tiny lens with 2 mm focal length

(a lens cannot be much bigger than the focal length)

Magnification = 125cm 0.2

cm 25

Problem: image was still small, and very dim.

Technology edge: Outstanding single lenses

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Robert Hooke’s

microscope, also

circa 1660

Discovered: Blood cells,

Microbes, etc.

van Leeuwenhoek

No existing pictures

of Hooke…

A TWO LENS system.

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Robert Hooke’s two-lens microscope

object

lens 1

Nosepiece

or

Objective

lens 2

eyepiece

lens 1 image

lens 2 image

A magnifying glass (the eyepiece) magnifies

the first image further.

The first lens, the

nosepiece, is used as

a projection lens.

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Hooke’s discoveriesThe cell

Detailed structure of creatures.

Example: The flea (plague).

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Modernbinocular

microscope is very much the same as

Hooke’s.

A beamsplitter, a half-

silvered mirror, sends

half the light to each

eyepiece.

Many optical instruments can be understood step by step, as we

did for Hooke’s microscope:

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The first lens collects light and

produces an image.

The second lens produces a

new image of the first image.

The third lens produces a new

image of the second image…

And so on.

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Galileo’s telescope (~1600)

Negative lens for eyepiece gives right-

side-up image.

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Kepler’s telescope (~1600)

Positive lens for eyepiece gives upside down image.

It’s upside down, but brighter and is easier to see.

lens 1

Objective

lens 2

eyepiece

lens 1 image

lens 2 image

Your telescope kit makes a:

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A)Galilean telescope

B)Keplerian telescope

C)Another type that we have not seen.

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Galileo’s telescope

30 x magnification

Tiny lens means not

much light entered,

so image is dim.

Discoveries:

Sunspots

Craters on the Moon

Phases of Venus

Moons of Jupiter

Magnification of a telescope(refractor or reflector)

Magnification =

Example:

Objective focal length = 1 m = 100 cm

Eyepiece focal length = 1 cm

Magnification = 100

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eyepiece oflength focal

mirroror lens objective oflength focal

Limits to magnification

A bigger fuzzy image is not useful (no additional information).

A high resolution image requires large lenses and mirrors.

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8 inch reflector

telescope Hubble telescope, 96 inch

(2 meter) mirror

Smallest visible feature 1/(diameter of lens or mirror)

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Yerkes observatory,

Largest refractor40 inch lens

1897

Larger lenses would

sag under their own

weight.

The lens was

achromatic (two kinds

of glass).

Yerkes Today

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Still available for

observations in the

original observatory.

Lake Geneva Wisc.

COLD winters yield

great seeing.

Constant pressure

from developers for

the lakeshore.

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Newton’s reflector telescope

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Newton’s telescope

Advantage: no chromatic aberration

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Palomar reflector - 5 meter mirror

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South Africa Large Telescope (10 m)

This shows 7 of the 91 mirrors. Diameter is about 10 m.

Each mirror is a hexagon so that they pack closely.

Former CU student

Amanda Gulbis

uses this telescope.

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Largest single mirrors are 8 m now

Prof. Roger Angel’s group. University of Arizona

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Finished mirror(don’t sneeze)

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KeckTelescope36 mirrors10 m dia.

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Magellan Telescope (yr. 2018)24.5 m dia. segmented mirror

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European Extremely Large Telescope (42 m dia.) would use many smaller mirrors

The remaining lectures:

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• Ch. 5 (the eye),

• Ch. 6 (optical instruments),

• Ch. 7 (Retina and visual perception),

• Ch. 9 & 10 (color & color perception).

We

are

here

Stop here for the FCQs

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The retina is where the image falls at the

back of your eyeball

• Inverted image falls on retinainstead of film.

– Can demonstrate this inversion

•Open left eye only.

•Press gently with left finger on eyeball just above tear duct.

•Observe dark spot in lower left corner of your field of view.

• Rods & cones packed into retina.

– Sensitive to light like camera film

• Optic nerve

– Nerve fibers connect rods & cones to brain. (transform light into electrical signals)

– Blind spot is where optic nerve leaves eyeball. Demo.

• How are rods and cones distributed in the retina? Fig. 5.12– The fovea is the small region near the

center of the retina

• Used for sharp, detailed viewing.

• Has the most cones (precise, color vision)

• Has no rods (used for low light, less precise viewing).

– Looking at someone means their image is on your fovea

• If their image is not on your fovea you see them "out of the corner of your eye."

• Eyeball moves to see a sharp image

– It scans to make all parts of an image eventually fall on your fovea

• Like TV image scanning

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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)

Human and cat retinas

Light comes in

from here

http://webvision.med.utah.edu/anatomy.html

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Rods and cones

• Rhodopsin, a photochemical, responds to lightIt is destroyed and reformed.Signal goes to a synapse, a gap between nerve cells

• There are 3 kinds of cones for 3 colors

red, green, blue

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Retina details

Choroid, outside layer with blood supply

Photoreceptors: rods and cones

Plexiform layer, inside layer with nerves

Photopic vision, in bright light, cones are used

Scotopic, in low light, rods are usedmore rods per nerve combines signals

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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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Robert Hooke’s two-lens microscope

Magnification = M1 x M2

Example: Eyepiece M2 = 25

Nosepiece M1 = 40

Final magnification = 40 x 25 = 1000

M1, like for any projection lens, is Xi / Xo

M2, like for any magnifier, is 25 cm / focal length

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Telescope drives

• Telescopes must rotate once every 24 hrs (approximately) to follow the stars, or the pictures will have streaks.

Northstar

Student telescopes

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View of galaxyNGC1300

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Catadioptric telescope

Also called Schmidt-Cassegrain Front glass lens corrects aberrationsWhy buy this? It’s shorter.

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Hubble Space Telescope

image

Compare to student

telescope image.

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Binoculars

Simply folded telescopes for each eye.

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What does 7 x 50 binoculars mean?

• 7x is the magnification

• 50 is the diameter of the front lens (the objective lens) in millimeters

• 6 x 30 binoculars are easy to carry

• 7 x 50 binoculars are heavy, butthese give a brighter image

• 15 x 80 binoculars need a tripod

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35 mm slide projector

Field lens is used to put the most light on the slide.

Field lensProjection

lensColor

slide

Mirror

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Viewgraph projector

Curved mirror

Fresnel lens (condenser)

and viewgraph location

Projection

lens

Mirror

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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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A dash past The Iris

Higher light levels (say daytime): Closed down, f/8We sense colors!Fewer aberrationsMore depth of field

Low light levels (say at night!): Wide open, f/2 or f/3Get more of the available light!We lose our color vision.More aberrations from the larger lens openingLess depth of field

You can check depth of field:

Try it: Close one eye, hold up thumb, stuff behind

thumb is out of focus.