9.2 Human Performance And Limitations - Vision

Question 1

what is the basic structure of the eye? (3)

Answer 1

aperture (iris)
lens
light-sensitive surface (retina)

Question 2

what is the cornea?

Answer 2

clear window at the front of the eye which allows light to enter

Question 3

what is the cornea responsible for?

Answer 3

70-80% of the total focusing ability of the eye

Question 4

what is the iris?

Answer 4

the coloured part of your eye,

controls the amount of light that enters the eye

Question 5

what is the pupil?

Answer 5

the black section of your eye in the centre of the iris.

rapidly changes its size to changing light levels

Question 6

how many times can the amount of light be adjusted by?

Answer 6

factor of 5:1

Question 7

what does light pass through after the pupil?

Answer 7

the lens

Question 8

how is the shape of the lens changed? what is this called?

Answer 8

by ciliary muscles by thickening or flattening the lens according to the distance of focus

accommodation

Question 9

how does the lens adjust for close and far distances?

Answer 9

close = thickening
far = flattening

Question 10

how is accommodation affected?

Answer 10

ageing
fatigue
tiredness

Question 11

what is the retina?

Answer 11

complex layer of nerve cells connected to the optic nerve

Question 12

what two cells are found in the retina?

Answer 12

cones and rods

Question 13

what are cones?

Answer 13

function in good light
capable of detecting fine details
colour sensitive (1000 different shades of colour)

Question 14

what are rods?

Answer 14

cannot detect colour
poor at distinguishing fine detail
good at detecting movement in the edge of the visual field (peripheral vision)
much more sensitive in lower light levels

Question 15

why do we see only black and white and shades of grey in poor light?

Answer 15

due to the sensing task being passed from the cones to the rods

Question 16

what is the central area of the retina known as? what receptors does this have?

Answer 16

fovea

all cones

Question 17

what is typically focused at the fovea?

Answer 17

the visual image

Question 18

what happens when light stimulates the cells on the retina?

Answer 18

small electrical impulses by the way of the optic nerve to the visual cortex of the brain, where an image is perceived

Question 19

what about blind spots?

Answer 19

a blind spot occurs at the back of the eye where the optic nerve join,

but rarely affects viewing as it is not possible for the image of an object to fall on the blind spot of both eyes at the same time

Question 20

how many times are the eyes more sensitive than the ears?

Answer 20

24 times

Question 21

what is visual acuity? which chart is used?

Answer 21

the ability of the eye to discriminate sharp detail at varying distances

Snellen chart

Question 22

what is the meaning of 20/40 vision?

Answer 22

the observer can read at 20 feet what a non-impaired person can read at 40 feet

Question 23

how does angular disatnce affect the resolving power of the fovea?

Answer 23

drops as angular distance increases

5 degrees = drop of 50%

Question 24

what can affect the visual acuity of the eye? (13)

Answer 24

age
physical imperfections in one or both eyes (myopia and hypermetropia)
drugs
medication
alcohol
cigarettes
available light
clarity of the air
size and contours of the object
the contrast of the object with its surroundings
motion of the object
distance from the object
angle from the object

Question 25

what is hypermetropia?

Answer 25

known as long sightedness

Question 26

how is hypermetropia caused? (long sightedness)

Answer 26

shorter than normal eyeball causing the image to be formed behind the retina

blurred vision looking at close object

Question 27

what is myopia?

Answer 27

known as short sightedness

Question 28

how is myopia caused? (short sightedness)

Answer 28

a longer than normal eyeball causing an image to be formed in front of the retina

distant object blurred

Question 29

what visual problems may happen? (6)

Answer 29

-cataracts (clouding of the lens with ageing)
-astigmatism (misshapen cornea causing objects to be irregularly shaped)
-glaucoma (build-up of pressure of the fluid within the eye causing damage to optic nerve)
-migraine
-presbyopia (lens less flexible, unable to accommodate sufficiently)
-fatigue

Question 30

how does carbon monoxide affect the eye?

Answer 30

builds up in the bloodstream allowing less oxygen to the eye.

rapidly impairs sensitivity of the rods

Question 31

how may environment affect vision? (4)

Answer 31

lighting level
dust
glare
spectacles

Question 32

who does colour vision deficiency affect most?

Answer 32

men

men 8%, women 0.5%

Question 33

what is the most common type of colour vision deficiency?

Answer 33

distinguishing between red and green

Question 34

how can ageing affect colour vision deficiency?

Answer 34

progressive yellowing of the lens results in a reduction of colour discrimination in the blue-yellow range

Question 35

what consideration must be taken with eyesight for an engineer?

Answer 35

that a reasonable standard of eyesight is needed to perform their duties
frequency of checks to be maintained