What is light a valuable thing to sense? It travels essentially instantaneously through air. EM radiation propagates rectilinearly. Light is pervasive on earth. Light provides differential information about many terrestrial objects.
Layers of the eye Fibrous tunic (sclera) –1mm thick –Fibrous to contain internal pressure of eye Vascular tunic –Dark choroid tissue 0.2 mm thick –Dark color prevents light scatter Retina
Anterior chamber Contains aqueous humor –Nourishes the cornea and lens –Under pressure –Glaucoma is excessively high pressure in eye
Iris –Two layers Pigmented Vascular Pupil –Two sets of muscles: circular & radial –Varies in size: 2-8 mm in young adults (16-fold variation in light) 5-2 mm in elderly adults –Variations in size influence depth of field (cf. p. 44)
Crystalline lens –Capsule Modulates flow of aqueous humor Modulates shape of lens –Epithelial layer –Lens proper Grows constantly, quadrupling in size by 90 years of age. Subject to hardening and opacities (cataracts) Brunescence (yellowing)
Vitreous chamber –Contains vitreous gel-like substance –Not renewed, so can contain floaters.
Macular degeneration Leading cause of impaired vision in industrialized nations. Can sometimes be arrested by laser surgery. One of the few health risks where African Americans have reduced rates over other racial groups.
Diabetes Causes cataracts Growth of blood vessels in eyes.
Light structure is usable only if Light must reach the retina (~50% passes through cornea) The image cast on the retina must be focused and not blurred. The structural relations among points of light must be preserved.
Image formation in the eye Optical power of cornea and crystalline lens –Variable due to change in shape of cornea and crystalline lens Shape of eye
Both rods and cones contain photopigments Each photopigment has two parts –Opsin Comes in three forms –Retinal (vitamin A derivative) Isomerizes when it absorbs light Isomerization slows spontaneous dark current
Phototransduction (1 msec) 1.Retinal isomerizes when it absorbs light 2.Isomerization releases all-trans retinal which eventually decreases cGMP concentration. 3.Lower cGMP concentration lets ion channels at the synapse close.
Rods have one type of opsin Cones have three different types –440 nanometers (aka S-cone, blue cone) Wavelength of light looks violet –530 nanometers (aka M-cone, green cone) Wavelength of light looks green –560 nanometers (aka L-cone, red cone) Wavelength of light looks yellow
Mammalian color processing Blue cones are found in the retinas of most species, hence appear to be oldest in evolutionary terms. Two cone retinas generally have blue and green, indicating that green is next oldest in evolutionary terms. Primates have three cone types, indicating that the red cones are the most recent in evolutionary terms.