Early Observations

• And God said, Let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness. - Genesis 1:3-4

• And God said, This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations: I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth. - Genesis 9:12-13

Early Observations

• Early lens dicovered in ruins of Nineveh believed to be 4000 years old

• Empedocles (500 BC)– Light has motion and travels at a fixed speed

• Aristotle (332 BC)– Pure light changed into colors by contamination

• Egyptians (200 AD)– Light travels from eye to illuminate object

• Alhazen (965 AD)– Light comes from sun or other objects– Made curved mirrors and pinhole camera obscura

Early Observations• Leonardo da Vinci

– Studied the ''Artificial Rainbow'' obtained by passing pure light through a prism

• Galileo (1600)– Tried to measure speed of light– Used lanterns separated by increasing distances– No valuable estimate

• Olaus Roemer (1675)– Used eclipses of Jupiter's satellites– Obtained a speed of 140,000 miles per second– Ridiculed by the French Academy of Sciences:

''It is absolutely impossible for anything, unless by the spirit of God, to attain such a speed''

Early Observations

• Newton– Studied chromatic aberration in lenses– Studied light passing through prisms

• Selected the brightest seven: Red, Orange, Yellow, Green, Blue, Indigo, and Violet– These are arbitrary

• Thought light was comprised of corpuscles– Unable to completely explain color

• Nature and Nature's Law lay hid at night. God said, 'Let Newton be,' and all was light.

Early Observations

• Christian Huygens– Published the wave theory of light in 1690– Could easily explain refraction– Hard to explain light traveling in a vacuum– Light carried on ''ether''

• Thomas Young (1801)– Double slit experiment– Determined wavelength of red light of 750 nm

• Augustin Jean Fresnel (1816)– Created diffraction gratings

Early Observations

• Sir William Herschel (1799)– Using a prism and a thermometer discovered

infrared radiation– Showed it behaves like visible light

• Johann Wilhelm Ritter– Used silver chloride to detect ultraviolet light

• James Clerk Maxwell (1864)– Theorized radiation beyond infrared– Produced and identified in 1887 by Heirich Hertz

Early Observations

• Jean Bernard Leon Focault (1850)– Used a rotaing mirror system– Obtained a speed of 187,000 miles/sec

• Albert Abraham Michelson (1879)– Obtained a speed of 186,284 miles/sec

• Current Speed is 186,284 miles/sec

Röntgen discovers x-rays

• Röntgen– Discovery

November 8, 1895– Public announcement

December 28, 1895– Public demonstration January

13, 1896• Worldwide acclaim in early

1896• Awarded first Nobel Prize in

Physics, 1901

Dual Nature of Light

• Max Planck (1901)– Radiation has a wave form– Made of small units of energy, called a quanta– Unified earlier theories

• On Monday, Wednesday, and FridayWaves boasted, ''It's my day,'‘On Tuesdays, Thursdays, and SaturdayParticles sang, ''Due it my way.'‘Sundays, though, were the very bestThe good Lord said, ''It's a day of rest.''

Electromagnetic Radiation

Albion College

Eye

• Every organism is light sensitive– Plants– Insects

• Complex eye• Simple brain

– Vertebrates• Simple eye• Complex brain

Eyes

• Sensory without contact• Optical Instrument– Described by Descartes

in 1637

• Some parts isolated from blood flow

• Image is inverted, just like a camera!

Eye Anatomy

• Eyelid• Eye Muscles• Cornea• Iris• Pupil• Lens• Retina• Optic Nerve

Eyelid

• Provides protection from objects

• Keeps eyes moist– Covers eye when sleeping– Blinking

• keeps eyes moist during activity

• occurs without stimulus• Increases with stress• Decreases with

concentration

• Semi-transparent

Muscles

• 3 pairs move and stabilize eyes– Always in tension

• Provide stimulus to brain– Search or saccades

• Blind during saccade motion

– Follow

• You cannot see your eyes move in a mirror!

Saccade Motion

• Hard to detect subtle differences in pictures

Saccade Motion

• Look at the picture on the right• Do you know the person?• What are your eyes looking at?

Saccade Motion

• The path shows the point of gaze of an observer

• Did your eyes follow a similar path?

Saccade Motion• Gaze depends on task• 1: Free examination• 2: estimate the material

circumstances of the family• 3: give the ages of the people• 4: surmise what the family had been

doing before the arrival of the "unexpected visitor"

• 5: remember the clothes worn by the people

• 6: remember the position of the people and objects in the room

• 7: estimate how long the "unexpected visitor" had been away from the family

Schlera

• The white of the eye• Provides clues to where

we are gazing– Used by infants– Less developed in other

animals, even primates– Compare human and

chimpanzee– It is difficult to talk to

someone when we can't see their eyes

Cornea

• Provides most of the refraction and focusing of light

• No blood flow• Shape can be changed

with surgery• Effective shape can be

changed with eyeglasses

• Outgrowth of the skin

Iris

• Colored part of the eye• Greek for rainbow• Restricts incoming light• Color not related to

function• Albinos have no

pigment and cannot block light

Pupil

• Window into eye• We can't see into our

own eyes• Closes for better acuity• 16:1 Change in size• Can oscillate if a laser or

bright light is shown in the edge

Lens

• Changes to allow us to focus objects near and far

• Normally transparent– Cataracts are cloudy

regions of the lens

• Looses elasticity over time

Retina

• Contains the photoreceptors

• Nerves form an extension of the brain

• Kepler– 1684: Saw it as a light

sensitive screen– 1625: Verified by

examining ox eye

Retina

• Fovea– Central region– high accuity– Color vision– About 1 degree of the field

of view– Dime at 15 inches

• Outside fovea– Monochrome– Periphery– Sensitive only to movement– Stimulates eyes to move

Retinal Structure and Function

Rods

• Found throughout retina

• Few in the fovea• Smallest is about the

size of two wavelengths of red light

• Take over in low lighting conditions– No color– Limited acuity

Rods

• Only one rod type• Contain purple dye

Rhodopsin• Most sensitive at 498

nm• Can detect about 1

photon• Scotopic vision– Most sensitive at 510 nm– Insensitive to red

Rods

• Connected in parallel to increase sensitivity

• 100 million rods• Light sensitivity of 100

photons through cornea (15/500 rods)

• Can see single candle at 17 miles

Cones

• Photopic vision• Three kinds– Red– Green– blue

• About 6 million in each eye

Distribution of Photoreceptors

Dark Adaptation

• Takes about 30 minutes• Rhodopsin ½ time

regeneration in 7 minutes• Peripheral vision becomes

dominant• Everything in the

moonlight is monochrome

• Can't read newspaper in moonlight because of second blindspot

• Purkinje shift– Color sensitivity changes

with dark adaptation– Color sensitivity of

cameras do not change

• Mesopic– Between scotopic and

photopic– Viewing of computer

displays

Color Vision

• Varies among species– Most mammals don't– Birds do– Insects do– Fishes do– Reptiles do

• Why this narrow EMR band?

Optic Nerve

• Sends signals from retina to brain

• Results in a blind spot in the visual field

• Brain fills in missing image!

• About 800,000 nerves per eye

Nerves

• Brain contains billions of neurons• Send signals at a finite rate• Electrical signals sent via Na-K conduction

Chiasm

• Optic nerve sections cross at chiasm

• Eye is split into L/R sides, which go to R/L side of brain