and the tricks we can do - university of edinburghwjh/scifun/talks/visual.pdf · 1998-09-01 ·...

SCI-FUN Schools Talks TH

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How do we See ?

and the tricks we can do

Will HossackDepartment of Physics and Astronomy

The University of Edinburgh

The Visual System I -1- 1998


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The Human Eye

Basic anatomy of the eye is:

Pupil

CrystallineLens

Vitreous Humor

Macula

Aqueous Humor

Iris

Cornea

Optic Nerve

Retina

where the Cornea and Crystalline Lens form an image of

the outside world on the Retina.

Information from the Retina sent to the Brain via the Optic

Nerve.



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Look at Object

View an object, image formed in Retina:

Object

Pupil

Lens

EyeInvertedImage

but the image is Inverted, (Upside down).

Information then sent to the Brain which then interprets

what you are looking at.



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The Blind Spot

Look the Optic Nerve a bit closer,

Cornea

Pupil Iris

CrystallineLens

Retina

Nerve FibresLight Detector Cells

Optic Nerve

‘‘Hole’’ in Retina

Nerve Fibres run on surface of Retina and leave through“a Hole”. The eye is not sensitive to light there so has aBlind Spot

To “see” Blind Spot.

1. Hold Card “1” at arms length.

2. Stare at the letter “A” with your RIGHT eye, and close your LEFTeye.

3. Move card towards you SLOWLY, while staring at letter “A”.

4. Letter “B” should disappear.



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Focus on Objects

Look at distant object,

Object

Radial Muscle

CorneaLens

Eye Sharp Focus

Cornea and Lens form a sharp image in Retina

Look at close object,

Eye

Radial

Sharp Focus

Muscle

Lens is BentObject

Radial Muscle “bends” the Lens to give higher power and focus largerimage into the Retina.

In older people the Lens gets “stiff”, so not able to read close. Needspectacles “for reading”.



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Why Spectacles

If the distance from the Lens to the Retina is too big then light fromdistant object will be focused Short.

Distant Object

Radial

CorneaLens

Eye

Light Focused‘‘Short’’ of Retina

MuscleRelaxed

Known as Short Sighted (myopic).

ObjectDistant Retina

Sharp Image on

EyeNegative Lens

Radial MuscleRelaxed

This can be corrected by putting a Negative lens in-front of the eye.Light then correctly focused on Retina.

Opposite problem if lens to retina distance too small, need positivelens.



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Stereo Vision

The human visual system has two eyes, separated by 60! 70mm,

so you get two overlapping views of objects each from a slightly dif-ferent direction.

Two images passed to different sides of the Brain.



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Depth Information

Look at close object. Eyes move to place object in centre of view,

Three DimensionalObject

Eye Rotated Anti-clockwise

Right Eye

Left Eye

Eye Rotated Clockwise

So for Three Dimensional objects we see differences in the two views.

The Brain interprets these differences as Depth, so

Two-Eyes) See Three-D Objects

Used to judge distance:

1. Stand a pencil upright on the desk.

2. Close ONE EYE.

3. Try and put your finger on TOP of the pencil point.

4. Repeat with TWO EYES open.



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Seeing Three-D Objects

Use Red and Green printing and Coloured filters to give Stereo effect,

��

��

��

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Left View Right View

Print as GreenPrint as Red

Red Filter

Green Filter

Left Eye

Right Eye

Red ImageGreen Image

Use the effect to record and see three dimensional objects.

Use Red-Green filter



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Stereo Vision in AnimalsAnimals that need to judge distance have stereo vision:

With Stereo Vision:

Active hunters: (cats, birds of prey, stoats)Tree dwellers: (monkeys, apes)

Helps hunters, allowing them to judge distance,

Not able to see behind



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Without Stereo Vision:

Passive grazers: (cows, zebra, bison etc)Foragers: (mice, rats, most birds)

all have eyes on sides of head.

Have wide field of view (able to see almost behind themselves), soable to see hunters coming.



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Playing Tricks

If you have Two-dimensional images of Three-dimensional objects,you can start to play Tricks with the visual system,

So without the full Three-dimensional information we can make “im-ages” of Impossible objects.



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Escher DrawingsThis effect was used by the Dutch artist, M.C. Escher in many famouslithographs,

“Waterfall”, lithograph, 1958.



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Escher Drawings II

and most famously in,

“Ascending and Descending”, lithograph, 1960.



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Stick Illusion

By matching up incorrect lines we can also get the three-stick illusion.(Schuster 1964),

where the “middle” stick is not connected to the correct place at thebase.



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Impossible Stone Arch

which can become very confusing is drawn as a real building,

∆φηβσα



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We see what we expect

Our brain tries to Make Sense of the world, and we see “what weexpect”,

Picture A

Picture B



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We see what we expect II

“How old is the woman in this picture ?”

Answer will mainly depend on how old you are.



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Confusing Objects

We can use the model fitting ability of the visual system to produceConfusing objects,

What shape is this:

Are these real stairs:

The more you look at them the more confusing they become.



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Seeing Lines

Since the visual system uses Lines and Edges to interprets images,we can make you see edges that are not really there.

What Shape is in the Centre



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Seeing Lines II

which can be very substantially changed by very small changes in theobject,

(1) (2) (3)

What shape is in the Centre

Note the very small change in whole object gives major change isapparent “object” in the Centre.



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Difficult Pictures

What do you see?


and the tricks we can do - university of edinburghwjh/scifun/talks/visual.pdf · 1998-09-01 ·...

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