Lecture 5: Light meets the eye Flashcards
What is light
Narrow band electromagnetic radiation that can be conceptualized as a wave or stream of photons
What is a photon
Quantum of visible light (or other form of em radiation) demonstrating both particle and wave properties
How do we consider light
Light as waveform when travels in space
Light as photon when hits retina and is transduced in neuronal activity
What is light refracted by
Cornea and lens
Describe light as electromagnetic wave
Em spectrum = characterized by frequency/speed of oscillations
Up rays, visible light, infrared radiation
Short 400nm = blue
Green/yellow = 500-600nm
700nm = longer, red/purple
Describe em wave
Electrical and magnetic part
When is light vertically polarized
Electric fields oscillation restricted to vertical plane = light is vertically polarized
What type of light do we usually experience
Mostly unpolarized
Bc every potential angel of polarity is equally present in most light sources
Describe polarizing glasses
Only let light pass with a certain polarity
Can be used to reduce total amount of light that reaches retina = sunglasses, or to present slightly diff visual info to right and left eye = modern 23 glasses (light of diff polarities to diff eyes)
Single wave = certain polarities
Describe a single ray of light
Electrical radiation = goes in one direction and oscillates
Perpendicular oscillation in magnetic field
= polarity/angle perpendicular to electromagnetic wave
Describe light scattering
Light hit small molecules
Redirect of Light as it interacts with molecules in medium = causes light to deviate from og path
Depends on size, shape and composition of particles relative to wavelength of light
Describe Rayleigh scattering
Scattering of light by particles much smaller than wavelength of light
Typically occurs in gasses, like sun light ray hits Moelecule oxygen = scatters
Causes shorter wavelengths (blue) to scatter more than longer wavelengths = why sky blue
Light scattered in all directions
Describe mie scattering
Scattering of light by particles that are comparable in size to wavelength of light
Like water drop, dust
Affects all wavelengths equally
Results in white/grayish appreance = clouds or haze
Maintains direction
Describe non selective scattering
Scattering of light by particles much larger than wavelength of light
Like water droplets in fog
Affects all wavelengths equally
Result in uniform white appearance
Larger molecules and direction bit less affected than mie
What types of scattering Can happen when light hits larger molecule
Mie or non selective
Describe light absorption
Process by which ligth energy taken up by material - converts it into other forms of energy
Like heat,
Rather than it being trasnmitted or reflected, like when light hits skin = disappears bc turned into heat
Ligth interacts with surface
Usually light hits object = some absorbed and reflected
Describe light reflection
Light bounces off surface of a material
Reflection can be specular or diffuse
Describe specular reflection
Light reflects off smooth surface in a single predictable direction
All ligth redirected in same direction
Like mirror or still water
Describe diffuse reflection
Ligth reflects off rough surface
Scatter in many directions
Define light transmission
Passage of ligth through material
Light continues to propagate without being absorbed or reflected
Define light refraction
Bending of light as it passes from one medium to another with a different refractive index
Changes speed and direction of light
Name layers that light goes through eye in order
Cornea
Anterior chamber
Pupil
Lens
Vitreous humor
Retina
Choroid
Sclera
Define cornea
Transparent
Dome shaped
Outer layer at front of eye
Helps focus ligth on retina
Define anterior chamber
Fluid filled space betwene cornea and iris
Contains aqueous humour = nourishes cornea and lens
Define pupil
Circular opening in Center of iris
Regulates amount of light entering eye
Acts as diaphragm
Attached to ciliary muscle via zonular fibres
Behind pupil = lens, refract and redirect light on retina (fovea)
Define vitreous humour
Clear gel like substance
Fills space between lens and retina
Helps maintain eye shape and transmit light to retina
Define choroid
Behind retina
Vascular layer betwene retina and sclera
Provides oxygen and nutrients to outer layers of retina
Define sclera
Tough, outer layer of eye
Provides structural support and protection
Whites of eye
Define retina
Thin
Light sensitive tissue lining back of eye
Where photoreceptor cells - rods and cones
Convert light into neural signals for visual processing (nerve fibres = process light)
What is optical infinity
Objects at a distance of 20 feet (6 meters) or more from eye = at optical infinity
Do not require accommodation to be seen distinctly by a healthy eye
Emmetropia
Ligth rays coming from objects at optical infinity are naturally focused on retain without any anomalies
Lens in relaxed state, focuses light
Accommodation
Process by which eyes lens changes shape
Controlled by ciliary muscles - to focus light on retina for clear vision at diff distances
Refract light more, divergent angle, light ray more divergent - not parallel
Focal length
Distance between optical Center of a lens and its focal point
Where parallel rays of light converge or appear to diverge
Determines lens magnifying power and ability to focus on objects at various distances
What is focal point
Goal = refract light, converge them on focal point = Center of retina - light get focused here
What are eye problems
Bc capacity of lens to focus light on retina not good enough
Describe presbyopia
Age related
Eye gradually loses ability to focus on nearby objects
Caused by reduction of elasticity of lens - and weakening fo eye ciliary muscles
= limit ability of eye to accommodate for near vision
~40 y/o
Lens stiffer, usually need to accommodate for close objects but lens cannot shrink enough, light focused behind retina,cannot perceive properly
Describe hyperopia
Farsightedness
Refractive error where distance objects appear clearer than near ones
Bc light focuses behind retina
Eyeball too short
Lens fine
Describe myopia
Nearsightedness
Refractive error
Near objects appear clearer than distant ones bc ligth focuses in front of retina
Often due to longer eyeball, problem with objects at optical infinity
Describe astigmatism
Refractive error causes by irregularly shaped cornea or lens
Leads to distorted or blurred vision at all distances because light focused unevenly on retina
Lens cannot focus light on one focal point, so looks kinda blurred
What does blue light do
Stimulate receptors in retina that control sleep/wake cycles
Photopigemnt = melanopsin
Describe Hermann con Helmholtz
1821-1894
Physicist and pshyiologust
Amazed by how poor an eye is = bad design, lose lots of info
“PERCEPTION IS UNCONSCIOUS INFERENCE”
BRAIN makes up rest, agreed with Kant = neo kantian = more scientific interpretation of Kant theory’s
Describe blind spot - gen
Area in visual field = close to centre, bit to right of visual field - do not see anything, brain makes up for it
Dot will dissapear, brain can fill in yellow in line
Brain makes you percieve it = brain guesses
Describe blind spot - procedure
Close left eye
Look at cross with right eye
Bring closer to eyes while looking at cross
When 30-40cm from screen = YELLOW DOT DISSAppears
If move right eye = will reappear
Move back to cross = dissapear
That spot of visual field wehre see nothing= blind spot
Name parts of retina
Fundus
Optic disc
Macula
Fovea
Descrube fundus of retina
Back of eye
Interior surface of eye
Visible through Ophthalmascope
Includes retina, optic disc, macula, blood vessels
Describe optic disc of retina
Region wehre optic nerve exits eye
Contains no photoreceptor cells
Creates blind spot
Area where blood vessels that feed the retina enter eye and where axons fo retinal ganglion cells merge into optic nerve (origin of optic nerve)
Describe macula of retina
Centra area of retina responsible for detailed central vision, critical for tasks like reading and recognizing faces
Macula = area that contains high density of photoreceptors, almost no blood vessels
Darker spot
Describe fovea of retina
Small, central depression within macula that contains high concentration of CONE CELLS
Provides sharpest visual acuity and colour vision
Inside macula
Depression
Describe image of fovea
Capillary - bv in front
Bipolar cells, then photoreceptors then pigment epithelium (Need to be close to for regeneration of piegmtns)
So fovea dips down to here, all other cells pushed aside = most ligth enters, v high density cones = accurate vision
Describe photo receptors and angle from fovea graph
Nose side and temple side = mostly rods, so like evryehwere
Except = fovea, has most cones
Optic disk = no rods or cones=blind spot
What is fovea used for
See things with high precision in tiny fraction visual field
Describe central vision
Tasks that require high visual acuity
Like reading
Limited to visual angle of 2-3 degrees
No more than width of thumbnail when viewed at arms length - 57cm in front of us
Cannot see things very well, seems like we can bc keep moving eyes
Define visual acuity
Clarity or sharpness of vision
Measured by ability to discern fine details, typically evaluated using standardized eye charts
Define visual angle
Angle forms by an object at the eye
Determined by objects size and distance form observer
Used to quantify how large an object appears in the field of view
Describe ganglion cells
Ligth hits this first
Neurons located in retina that receive visual info from bipolar cells and transmit it to brain via their axons = form optic nerve
Convey info to brain
Describe bipolar cells
Light hit second
2 polarities
Intermediate neurons in retina = connect photoreceptors (rods and cones) to ganglion cells
Transmit visual signals from outer to inner retina
Synapse with photoreceptors and ganglion cells
Describe photoreceptors
Light hit third
Rods and cones
Specialized cells in retina that detect light and convert it into electrical signals
Define rods
Sensitive to low light levels
Responsible for vision in dim lighting (scotopic vision) = v sensitive but not accurate, night time
Define cones
Active in bright light
Responsible for colour vision and fine detail (photopic vision)
