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Chapter 8

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McGill University
PSYC 212
Remy Allard

Chapter 8The Visual System Light Optics and the EyeA Nature of LightParticle theory of lightstraight line motion of lightParticle theory of light then questioned and wave theory emergedParticlelight reflects off smooth surface mirror oBends going from one medium to the nextoReflects in geometrically predictable waysoLight beam can produce an ejection of electrons from an object under appropriate conditionsWavediffraction of lightoAll waves must occur as deformation of a medium therefore there must be something that the light goes throughMust be rigid to allow light to travel through it quicklyLight travels in small packets photons that have wavelike quality to themoEnergy in light actually concentrated in particlesoPhotons themselves traveled in wavelike manneroDual Theory of LightAccounts for number of wave phenomena diffraction but also particle phenomena electron emission when light strikes an objectoPropagation of light best described by wave theoryoInteraction of light with matter best explained by particle theoryGeneral Properties of lightoAn oscillating changed particle produces electromagnetic radiation type of moving energy field that is found everywhere in natureCharacterized by flow of photonsMovement of photons through space produces oscillating electricmagnetic field that together move in the same direction as the photonMutually linked to each othervary in intensity over time2 fields lie perpendicular to eachotherif intensity of electric field is oscillating along vertical axis magnetic field will oscillate along horizontal axisrepresents waves moving through spacedont have to show bothshows fluctuation in electric magnetic force field as a function of psace and distancesingle cycle represents segment of wave before it repeats itselfdistance covered by full cycle separating two identical points in waveformwavelengthwavelength can vary considerably ovisible light is an example of electromagnetic radiationfull range or spectrum of electromagnetic radiation produced by naturalartificial sourcesnumber of different units often used to express wavelength values across the range meters for long wavelength ration to nanometers for short wavelength but usually use nanometersshortestgamma rayscosmic radiation that travels distances from universe to EarthlongestELF wavessmall segment of electromagnetic spectrum actually used for sensory function visible light400700 nmovisual system responsive to energy contained in light different wavelengths produce different perceptual impressions we label in terms of coloursowhy do humans only see visible spectrumCause major source of electromagnetic radiation to the Earth is the SunEmission spectrum to sun restricted to narrow band of radiation spanning from 1004000 nm ie UV VS IRReaches surface of Earth transmission properties in atmosphere are factorTransmission spectrum of atmosphere has similar profile to Suns emission spectrumoverlaps significantlyVS is comfortable in this regionlife evolved to take advantage of light as sensory stimulusmuch of suns emissionoSpeed of lightLight transmitted along straight linesat awesome speedVisioninstantaneous senseallows us to perceive events with little delayRapid transmission of flight in eyes allows visual system to assume role as most distant senseSpeed of light in a vacuum is 299792 kms aka 186282 milessLight can travel in absence of mediumemptiness of outer spaceoLight propagationbasic features of light propagation through spaceimplication for visual processingoften convenient to collapse light source to an imaginary pointlight emanates from point source and spreads in all directionswave fronts very small in immediate vicinity of point sourceexpand progressively as they radiateconcentric series of wave fronts increasing in diametereventually will flatten out optical infinityoptical infinitythe distance of 6 meters or more at which wave fronts flatten out light rays become parallelmore convenient to represent light propagation in terms of rays rather than waveswe consider light ray as particle analog of traveling wave
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