light and the eye

Question 1

visible light

Answer 1

• 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

Question 2

what is camera obscura

Answer 2

• 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

Question 3

cornea

Answer 3

The transparent tissue at the very front of the eye.

Question 4

sclera

Answer 4

tough white tissue that coats the rest of the eye

Question 5

iris

Answer 5

Muscle tissue that controls the size of the Pupil and, thereby, the amount of light that enters the eye

Question 6

optical components of the eye

Answer 6

the cornea and the lens

Question 7

what does the lens do

Answer 7

• 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

Question 8

what is the cornea responsible for

Answer 8

is fixed accounts for about 80% of focusing

Question 9

what is myopia

Answer 9

(near-sightedness) - inability to see distant objects clearly

Question 10

how does myopia work

Answer 10

○ 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

Question 11

presbyopia

Answer 11

old eye- The near point is the distance where the lens can no longer adjust for close objects.

Question 12

what is presbyopia caused by

Answer 12

Due to hardening of lens and weakening of ciliary muscles

Question 13

what is hyperopia

Answer 13

(far-sightedness) - inability to see nearby objects clearly

Question 14

explain hyperopia

Answer 14

○ 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.

Question 15

describe photoreceptors

Answer 15

1. The outer segments of photoreceptors contain the light-sensitive visual pigment.
2. It is here that the light is transduced into electricity.
3. The electrical signal is then propagated from synaptic terminal via a set of neurons to the brain.

Question 16

what are the two components of pigment molecules

Answer 16

►Opsin - a large protein ►Retinal - a light sensitive molecule

Question 17

explain what dies retinal do

Answer 17

Visual transduction occurs when the retinal absorbs light.
Retinal changes it shape, in a process called isomerization.

Question 18

what is the fovea

Answer 18

highest visual resolution (adjacent to the blind spot)

Question 19

what is the distribution of photoreceptors in the fovea

Answer 19

Around 120 million rods and 5 million cones

Question 20

what is the blind spot

Answer 20

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

Question 21

differences in shape of rods and cones

Answer 21

►Rods - large and cylindrical ►Cones - small and tapered

Question 22

difference in distribution on the retina

Answer 22

►Fovea consists solely of cones.
►Peripheral retina has both rods and cones.
►But more rods than cones in periphery.

Cones, rods and dark adaptation

Question 23

explain method used for measuring dark adaptation

Answer 23

• 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.

Question 24

results for rods vs cones

Answer 24

cones -sensitivity increases for three to four minutes and then levels off
rods- sensitivity increases for about 25 minutes and then levels off

Question 25

what are the roles of the rods and cones

Answer 25

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

Question 26

describe the retinal flow of rods and cones

Answer 26

1) Photoreceptor 2) Bipolar cell 3) Ganglion cell 4) Optic nerve 5) To the brain

Question 27

explain neural convergence

Answer 27

►Higher convergence of rods than cones
- Average of 120 rods to one ganglion cell
- Average of six cones to one ganglion cell

Question 28

explain the arrangement of cones in the fovea and its advantage

Answer 28

one to one connections to ganglion cells - gives maximum spatial resolution

Question 29

what is spatial summation

Answer 29

Many (rods or cones)-to-one bipolar gives more sensitivity to light

Question 30

what is more sensitive to light

Answer 30

rods - take less light to respond

Question 31

why do cones have high visual acuity

Answer 31

• 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