Chapter 2 Flashcards
electromagnetic spectrum
spectrum where energy is described as its wavelength, ranging from short wave gamma rays (10^-12m) to long length radio waves (10^4m). Visible light is a band in this spectrum and is identifiable by colour (short - blue, mid - green, long - yellow, orange, red)
photon
small packet of light energy
cornea
transparent, fixed covering of the front of the eye that accounts for 80% of the eye’s focusing power
lens
transparent, ciliary muscles adjust curvature of lens, accounts for 20% of focusing power of the eye,
retina
a layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed; layered with 5 types of neurone - signals generated in receptors travel to bipolar cells and then to ganglion cells with long axons to transmit messages out retina; horizontal and amacrine cells connect neurons across retina
accommodation
change in lens shape that occurs when ciliary muscles in front of eye tighten, increasing curvature of the lens so that it gets thicker (increased curvature = increased light bending)
near point
distance at which you can no longer accommodate
myopia
inability to see distant objects clearly; refractive myopia - cornea/lens bend light too much, axial myopia - eyeball is too long
far point
distance at which light becomes focused on retina (a myopic can see an object clearly at far point)
laser assisted in situ keratomileusis
LASIK - cut a flap, sculpt cornea with laser, flap replaced
hyperopia
far sighted - focus is on a point behind the retina, the eyeball is too short
visual pigments
present in outer segments of rods and cones; opsin - long protein, retinal - smaller, light sensitive component; when it absorbs one photon of light, retinal component changes shape (bent to straight) in a process called isomerization. Triggers chemical reaction that activates thousands of charged molecules to create electrical signals in receptors, amplifying the effect of isomerization
isomerization
process by which one molecule is changed into another molecule with exactly the same components in a different organization
dark adaptation
eye adapts to darkness by increasing sensitivity to light
rods and cones
visual receptors with inner and outer segments interspersed throughout peripheral retina (+ rods [120 million] than cones [6 million-cones in fovea]); rods most sensitive to light at 500 nm, cones most sensitive to light at 560 nm
fovea
small area on retinal that has only cone receptors (50,000), looking directly at an image it falls here
macular degeneration
destroys cone rich fovea and area around it producing a blind region in central vision so that an object can not be seen if looked at directly
retinitis pigmentosa
hereditary degeneration of retina attacks peripheral rod receptors and results in poor peripheral vision
blind spot
has no receptors (brain “fills it in”, other eye compensates, located on edge of visual field) - location where optic nerve leaves eye
dark adaptation curve
function relating sensitivity, light and time in the dark and beginning when light is extinguished
light adapted sensitivity
sensitivity in light
method to measure dark adaptation
- observer looks at small fixation point centred on fovea while paying attention to flashing test light on peripheral retina
- in light, observer adjusts strength of flash with a knob so that it can barely be seen (threshold - converted to sensitivity high threshold = low sensitivity)
- lights are turned off and experiment is repeated, observer must continually reduce strength of flash as dark adaptation occurs
- adaptation occurs rapidly for 3-4 mins (cones) and levels out, at 7-10 minutes further adaptation occurs (rods) until observer has been in darkness for 20-30 mins
dark adapted sensitivity
sensitivity at the end of dark adaptation (100000 x greater than light adapted sensitivity)
measuring cone adaptation
must measure response of fovea
