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Lecture 6 1. Give two examples of how the development of a new pigment has had an impact on painting and painting style. 2. Describe the chemical process responsible for leaves turning: Color Perception + Mixtures Additively Color perception Rods & c

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Natural Science
NATS 1870
Robin Kingsburgh

SC NATS 1870 (lecture) Monday, October 19, 2009 1. Give two examples of how the development of a new pigment has had an impact on painting and painting style.  Ultramarine used for religious figures  hard to make, expensive  Household & car paint  arsenic based green paints & less choice of paint depended on mass market 2. Describe the chemical process responsible for leaves turning: a) green to yellow; b) green to red  Keratin  green to yellow  Anthocyanin  green to red  Not all leaves go red, needs high sugars  anythocyanin  Usually outside facing south turns red first st  Red outside of tree 1 , green more on inside Color Perception + Mixtures Additively  Background color affects way color perceived  Cones affect/help perceive color, doesn’t see fixed color  Process called simultaneous, adaptive process Color perception  Is an active process  eye + brain are continually comparing objects, surrounding, illumination conditions  Give rise to certain “adaptive processes” like simultaneous contrast  A color will appear different depending on its surroundings  Visual sense responds to visible light  Color vision: process of decoding 1) wavelength (λ) 2) intensity (I)  Response to visible light is based on chemical process in cells  cones & rods  Photons of visible light are absorbed by specific molecules  photo pigment molecules  very large molecules that resemble caratine, Vitamin A  Millions of them are stacked in each rod & cone Cones Rods  Large cells  Small cells  Responsible for black & white vision  Responsible for color vision  located: tend to be centrally concentrated  Located: tends to be concentrated towards edge of reat back of eye in foeva  Less precise vision  About 100 million rods  give sharp detailed vision  approximately 6 million cones  One type  3 types: red, green, blue  Contains “rhodopin”  Function in low levels of light  R  sensitive cones  G  sensitive cones  B  sensitive cones  Each contain slightly different photo pigment molecules called “opsins”  Need bright light sources to trigger cones The Color Theory  Postulated in 1801 by Thomas Young  3 types of “resonators” in eye, that responded to red, green, blue light  Combination could produce all other colors  Theory was expanded by James Clark Maxwell? Who devised a spinning disk & light box that allowed color combination & he demonstrated that nearly all colors could be produced by red & green & blue in different amounts  When color top spins, eyes are additively mixing colors o Eyes can’t pick up separate color components o Where colors are overlapping each other while spinning = another color o Colors are less saturated, higher value = paler o When all 3 painter primaries colors spin (red, yellow, blue) = white o No unique set of primary  Proved in 1964  actual absorbance of 3 types of cones in macaque monkeys measures  experimented confirmation of Young’s idea Spectral Sensitivity/Absorption in Eye l l l Mechanics of Vision  Photon of light travels with eye  get absorbed by rod or cone o Photo pigment molecule, e- jumps from low  high orbit as it absorbs photon o Structural change of opsin molecule from kinked to straight o Initiates a nerve impulse/message relay thru different cells type o Cones  bipolar cells  ganglion cells  optic nerve  visual cortega  An enzyme re=converts the straight to kinked version for the photo pigment molecule  capable of absorbing
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