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 • DiscoveryNovember 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.''
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
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
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