VIEWS: 4 PAGES: 38 POSTED ON: 8/12/2012
VISION Physics 6B Fall 2004 Elizabeth Ko, Rana Hojatmehr, Francis Baking, Steven Vision: An Introduction The visual field is mapped as an image on a surface made up of light-sensitive cells Absorption of light by pigments Electrical effect ensues— leads to an impulse in a fiber of the optic nerve, which travels to the brain The Human Eye: Basic Anatomy Rods and Cones Rods: Vision in low light Rhodopsin Cones: Color vision and detail Color pigments Perceiving Light Rhodopsin: a mixture of scotopsin and 11-cis- retinal Eventually forms activated rhodopsin Activated rhodopsin causes electrical impulses that are transmitted to the brain and interpreted as light Rhodopsin reformed Activated Rhodopsin The cell membrane of a rod cell has an electrical charge—activated rhodopsin causes this electrical charge to increase. This produces an electrical current along the cell. Path of electric impulse: ganglion cell → optic nerve → optic chiasm → optic tract → occipital lobe → primary visual cortex The Vision Process Monochrome vision (occurs in the rod cells) Color vision ( occurs in the cone cells) Monochrome Vision (Rod Cells) Isomerization of retinal Protein conformational changes Signal transduction cascade to generate a nerve impulse Isomerization of Retinal Chromophore 11-cis-retinal isomerizes to all- trans-retinal. This event is best understood in terms of molecular orbitals, orbital energy, and electron excitation. Isomerization of Retinal Absorption of a photon by 11-cis-retina promotes a p electron to a higher-energy orbital (a p-p* excitation). This excitation "breaks" the p component of the double bond, thus allowing free rotation about the bond between carbon atom 11 and carbon atom 12. The double bond then reforms and locks the molecule back into position in a trans configuration of the all-trans-retinal. 11-cis-retinal has a maximum absorbance in the ultraviolet part of the spectrum, but the maximum absorbance for rhodopsin is 500 nm (in the visible green part of the spectrum). The observed color of a substance is actually the complementary color to the color that is absorbed. Thus, the name "visual purple" describes the complementary color for rhodopsin. Conformational Changes The all-trans chromophore adopts a twisted conformation, which is energetically unfavorable. Therefore, a series of changes occurs to expel the chromophore from the protein. Phototransduction Pathway Signal Transduction Cascade to Generate a Nerve Impulse Activation of the enzymes transducin and phosphodiesterase Hydrolysis of cyclic GMP Closing of Na+ channels (hyperpolarization) Propagation of an electrical impulse to the brain The Threshold of Sensitivity Rods are most sensitive near 500 nm Threshold: 90 photons/s entering the eye in .1 sec ! E=90 hf =100 hc/lamda =4x10^-17 watts (10% efficiency) Refraction Light rays bend when they pass from one transparent medium to another When a ray strikes the cornea, because the speed of light differs in the two media, it will bend. How does the brain interpret images? Rays of light reflected off of an image are focused through the lens onto the back of the eye, forming an upside-down image on the retina. We can think of the image as a pixellate map of activated and non-activated photocells on the retina. A nerve from each photocell connects to a particular location in the visual cortex of the brain. The photocells that are activated send a nerve impulse to the brain, while the photocells that are not activated do not send any impulse to the brain. The brain, when it receives a collection of nerve signals from the eye, interprets where each signal comes from, and reconstructs the pixellate map. The brain judges the image location to be the location where light rays appear to originate from. Why we see a world full of color Electromagnetic Spectrum ~400nm ~750nm The major importance of light intensity Colored vision depends heavily on intensity of light – Cones will only work when there is a high intensity of light – Low intensity light triggers vision by the rods Purkinje effect – Because rods can see more toward the blue spectrum colors which appear brightest in the dark, will not appear so in the light – Triggered by differences in rods and cones Color Sensation Any color can be defined by a basic equation: – C=aA+bB+cC A, B, and C represent the primary colors Red, Blue, and Green a, b, and c represent relative amounts of each primary color Two different colors can be added together and be plotted as vectors (e.g. C=aA+bB+cC and C’=a’A+b’B+c’C) – To simplify the drawing of vectors, we assume that everything will be set to the same light intensity a+b+c=n, where N is a constant number We can then colors along the XY plane using XY coordinates Mechanism of colored vision The three primary colors are detected by the cones located in the retina The highest concentration of cones is in the fovea Three types of cones General Test of dependence on stimulated cones Some Problems that occur with colored vision Colorblindness – 8 percent of males – 0.5 percent of females Types of Colorblindness – Red/Green Colorblindness – Dichromats Missing one of the cone pigments – Deuteranopes See colors not as points but as lines – Protanopes Focus is near the red end of the visible spectrum Deuteranope Protanope WHY IS SKY BLUE? Speculations Is it because air molecules are by themselves blue? Is it because there is only blue light in the sky? Or is there any other reason? Light in the sky From the sun Visible white light that contains all wavelengths = all colors Thus, there is more than blue light in the sky Sunlight Electromagnetic wave Oscillate charged particles (protons and electrons) in air molecules Oscillation produces electromagnetic wave of same frequencies. Result, scattering of sun light Also, air molecules absorb all frequencies. Physics of Blue Higher frequency than other colors (except violet) Molecules oscillate more -> blue light scattered more strongly Acceleration œ square of frequency Intensity of light œ square of frequency Thus, light intensity œ frequency ^4 And blue light is scattered ten times more than red Sky is Blue The color we see in the sky is the redirected sun light We see blue because it has higher frequency and is scattered much more. Why don’t we see violet sky? Somehow, our eyes are more sensitive to the frequency of blue light. Works Cited Casiday, Rachel and Frey Regina. Vision and Light-Induced Molecular Changes. 2000. Clayton, Roderick K. Light and Living Matter, Vol. 1: The Physical Part. McGraw Hill, Inc. 1970. Clayton, Roderick K. Light and Living Matter, Vol. 1: The Biological Part. McGraw Hill, Inc. 1970.
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