Eye Refraction and Accommodation

Question 1

What is refraction?

Answer 1

Light rays bend to from a sharp image on the retina
Can be biconcave and biconvex

Question 2

What is accommodation?

Answer 2

Changing how much light rays are bending so can focus

Question 3

What is the refractive media in the eyeball?

Answer 3

Cornea, AH, lens and VH are transparent to allow light to fall on retina

Question 4

How does a sharp image fall on the retina?

Answer 4

Light waves from an object bend at cornea, bend more at lens to form a clear image on retina
Cornea is the most powerful bender but the lens has the power to change its bending power

Question 5

What happens when objects are at a distance (>6m) compared to close up?

Answer 5

Only parallel rays from object reach the eye
When close up divergent rays from object reach the eye but need to be bent more to focus on retina

Question 6

How does the eye focus on an object?

Answer 6

Lens becomes thicker so more powerful and clear image formed on retina again

Question 7

What 3 things happen simultaneously and compromise accommodation?

Answer 7

Lens changes shape - thicker and more spherical
Pupil constricts
Eyes converge

Question 8

How does the lens become thicker?

Answer 8

Ciliary muscle contracts making the body bulge which means space in middle decreases
So suspensory ligaments become lax - lens no longer under stretch

Question 9

Explain pupil construction and increased focus

Answer 9

Pupil constricts to allow only a few rays to pass through
Pupillary constrictor - sphincter pupillae is around border of the pupil - parasympathetic innervation

Question 10

Describe eye convergence

Answer 10

Eyes have to turn in to look at the object
Medial rectus muscle is in use - this muscle then thicker than lateral rectus

Question 11

What is myopia?

Answer 11

Short-sightedness

Question 12

What is hyperopia?

Answer 12

Long-sightedness

Question 13

What is astigmatism?

Answer 13

Non-spherical curvature of cornea or lens

Question 14

What is presbyopia?

Answer 14

Long sightedness of old age

Question 15

What is emmetropia?

Answer 15

Perfect vision - 20/20

Question 16

What is the most common cause of myopia?

Answer 16

Eyeball is too long

Question 17

Describe myopia

Answer 17

When cornea and lens bend rays they make an image in front of retina so far off objects not seen clearly
Bending power is too much for the eyeball

Question 18

Describe myopia when an object is brought close up

Answer 18

Divergent rays from object reach cornea and then use the bending power
Image is formed on retina without needing to increase curvature of the lens - no accommodative power

Question 19

What are the symptoms of myopia?

Answer 19

Headaches, cant see blackboard or distant objects, can have divergent squint, toddlers can have more image in books and pictures and can lose interest in class

Question 20

Describe myopia correction

Answer 20

Bending power needs to be decreased
Biconcave lenses - (-sign)
Spectacles, contact lenses and laser eye surgery

Question 21

Describe hyperopia

Answer 21

Eyeball is too short or cornea and lens too flat
Image formed behind the retina
Person uses accommodation power and makes lens thicker causing image to form on retina - so using this power when usually not needed

Question 22

Describe hyperopia and seeing up close

Answer 22

Uses more accommodation power until all used up as been used on focusing on long sight
Cant see nearby objects

Question 23

What are the symptoms of hyperopia?

Answer 23

Eyestrain after reading, convergent squint which needs immediate correction to prevent lazy eye

Question 24

What is the correction for hyperopia?

Answer 24

Biconvex glasses alleviates use of glasses for focussing on distant objects and rests accommodation power
Contact lenses
Laser eye surgery

Question 25

How do images appear in astigmatism?

Answer 25

Close and distant images appear hazy as more than 1 image formed

Question 26

Describe astigmatism

Answer 26

Surface has different curvatures in different meridians
Bending of light rays along one axis will never be the same as that of other axis so image hazy whatever distance

Question 27

What is the correction for astigmatism?

Answer 27

Cylindrical glasses - curved in only one axis ( in degrees)
Laser eye surgery
Toric lenses - special contact lenses

Question 28

Describe presbyopia

Answer 28

With age lens gets less mobile and elastic
When ciliary muscle contracts it is not as capable of changing shape so seeing near images becomes more difficult

Question 29

What is the correction of presbyopia?

Answer 29

Biconvex reading glasses

Question 30

What does the energy in light waves need to stimulate?

Answer 30

Photoreceptor cells on retina - photo transduction (conversion of light energy to electrochemical response by photoreceptors)

Question 31

What do phototransduced rods and cons activate?

Answer 31

Optic nerve to generate an action potential

Question 32

What are the main parts of the rods and cones?

Answer 32

Synaptic terminal, axon, nucleus, inner segment containing organelles and outer segment with lamellae (have photoreceptors)

Question 33

Describe visual pigment in photoreceptor outer segments

Answer 33

Each lamellae is made up of cell membrane
In this cell membrane is visual pigment rhodopsin in rods and cone opsins S, M and L in cones
Molecules differ in their spectral sensitivity

Question 34

What happens when light falls on 11-cis retinal?

Answer 34

Isomerises to all-trans retinal
Can not fit into opsin so rhodopsin splits resulting in bleaching of visual purple

Question 35

What is opsin formed by?

Answer 35

Vit A

Question 36

How does bleaching of visual pigment result in phototransduction?

Answer 36

Phototransduction cascade
Photoreceptor cells at rest kept at depolarised state - depolarised by open Na/Ca channels

Question 37

What role does Vit A play on visual pigment?

Answer 37

Visual pigment regeneration
Cis-retinol transported back to rod/cone to reform opsin

Question 38

Describe the cascade of reaction in rods and cones

Answer 38

Hyperpolarises - impulse is transmitted - becomes AP in optic nerve cells

Question 39

Describe the effects of vitamin A deficiency

Answer 39

Vision affected - night blindness
Essential for healthy epithelium so conjunctiva and corneal epithelium are abnormal