light and the eye Flashcards
visible light
- Visible spectrum for humans ranges from 400 to 700 nanometers (tiny portion)
- Most perceived light is reflected light
- Is a sensible choice for a visual system - X rays and UV are harmful for DNA and IR may cause damage
what is camera obscura
- A small hole in a wall (i.e. an aperture) will produced an inverted “scaled version” of the original object.
- Adopted by many artists to capture true perspectives and likeness.
The images that are formed were very faint and had to be viewed in the dark - the amount of light admitted is proportional to the area of the hole
- Adopted by many artists to capture true perspectives and likeness.
cornea
The transparent tissue at the very front of the eye.
sclera
tough white tissue that coats the rest of the eye
iris
Muscle tissue that controls the size of the Pupil and, thereby, the amount of light that enters the eye
optical components of the eye
the cornea and the lens
what does the lens do
- Adjusts its shape according to the viewed object distance (the remaining 20%)
- Accommodation happens when the ciliary muscles contract which causes the lens to thicken and bend the light more.
If an object is moved closer without the focus adapting - lens will change shape through muscle contraction to bring it into focus again
- Accommodation happens when the ciliary muscles contract which causes the lens to thicken and bend the light more.
what is the cornea responsible for
is fixed accounts for about 80% of focusing
what is myopia
(near-sightedness) - inability to see distant objects clearly
how does myopia work
○ Image is focused in front of the retina when the lens is relaxed
○ Caused by: Refractive myopia: cornea or lens bends too much light. Axial myopia: eyeball is too long
Fixed using corrective lens
presbyopia
old eye- The near point is the distance where the lens can no longer adjust for close objects.
what is presbyopia caused by
Due to hardening of lens and weakening of ciliary muscles
what is hyperopia
(far-sightedness) - inability to see nearby objects clearly
explain hyperopia
○ Focus point is behind the retina.
○ Usually caused by an eyeball that is too short
Constant accommodation for nearby objects can lead to eyestrain and headaches.
describe photoreceptors
- The outer segments of photoreceptors contain the light-sensitive visual pigment.
- It is here that the light is transduced into electricity.
- The electrical signal is then propagated from synaptic terminal via a set of neurons to the brain.
what are the two components of pigment molecules
►Opsin - a large protein ►Retinal - a light sensitive molecule
explain what dies retinal do
Visual transduction occurs when the retinal absorbs light.
Retinal changes it shape, in a process called isomerization.
what is the fovea
highest visual resolution (adjacent to the blind spot)
what is the distribution of photoreceptors in the fovea
Around 120 million rods and 5 million cones
what is the blind spot
the place where the optic nerve leaves the eye
○ One eye cover the blind spot of another
○ Located away from the centre of the visual field
The brain “fills in” the spot
differences in shape of rods and cones
►Rods - large and cylindrical ►Cones - small and tapered
difference in distribution on the retina
►Fovea consists solely of cones.
►Peripheral retina has both rods and cones.
►But more rods than cones in periphery.
Cones, rods and dark adaptation
explain method used for measuring dark adaptation
- Observer is light adapted.
- Light is turned off.
- Observer adjusts the intensity of a test light until they can just see it – the detection threshold.
Threshold values are recorded at different times after the light was turned off.
results for rods vs cones
cones -sensitivity increases for three to four minutes and then levels off
rods- sensitivity increases for about 25 minutes and then levels off
what are the roles of the rods and cones
keeping the fovea for cones gives us high resolution during day time while, using our rods, we can still get a chance to find your way in the dark
describe the retinal flow of rods and cones
1) Photoreceptor 2) Bipolar cell 3) Ganglion cell 4) Optic nerve 5) To the brain
explain neural convergence
►Higher convergence of rods than cones
- Average of 120 rods to one ganglion cell
- Average of six cones to one ganglion cell
explain the arrangement of cones in the fovea and its advantage
one to one connections to ganglion cells - gives maximum spatial resolution
what is spatial summation
Many (rods or cones)-to-one bipolar gives more sensitivity to light
what is more sensitive to light
rods - take less light to respond
why do cones have high visual acuity
- One-to-one wiring leads to ability to discriminate details and accurately encode spatial positions.
Trade-off is that cones need more light to respond than rods
