Lecture 2

Question 1

What connects the eye to the brain?

Answer 1

The optic nerve

Question 2

Where is light focused onto in the eye?

Answer 2

The retina

Question 3

What is the focal distance in the eye?

Answer 3

The distance between the lens and the retina

Question 4

Describe a lens

Answer 4

An optical device that transmits and refracts light to either converge or diverge the light beam

Question 5

What happens to a collimated beam of light travelling parallel to the lens axis in a biconvex lens?

Answer 5

It is converged (focused) to a spot on the axis at a certain distance behind the lens (focal length)

Question 6

Define optical power

Answer 6

The degree to which a lens, mirror or optical system converges and diverges light

Question 7

What is a unit of measurement of optical power of a lens

Answer 7

A diopter

Question 8

What is the relationship between diopters and focal length (in metres)?

Answer 8

Reciprocals

Question 9

What happens to a lens immersed in a refractive medium?

Answer 9

Its optical power/focal length change

Question 10

What does spherical aberration cause?

Answer 10

Beams parallel to but distant from the central lens optical axis are focused in a slightly different place than beams close to the axis

Question 11

Name 3 types of lens aberattion

Answer 11

Spherical, coma and chromatic aberration

Question 12

Properties of the cornea (5)

Answer 12

• Most of the focusing power of the eye
• Power of about +40 diopters and thus acts like positive power lens even though it doesn’t look like one
• It has a central thickness of about 0.5mm
• Most of the power comes from front surface (back surface isn’t curved)
• Focusing power arises from curvature and refractive index difference between air (n=1) and tear layer at corneal surface (n=1.376)

Question 13

Properties of the Iris and Pupil (4)

Answer 13

• The pupil controls amount of light entering eye, and hence retinal illuminance
• Pupil diameter varies between 2-8 mm
• However does not ensure maintenance at constant level of retinal illuminance
• This is because pupil area varies over a ratio of 1:16 while light levels can vary over a range of 1:10^10

Question 14

What is the role of the pupil?

Answer 14

To set the depth of field and control aberrations

Question 15

How do the iris and pupil respond in dark and bright conditions

Answer 15

At low light levels the iris pulls back, and increases the size of the pupil, allowing more light in

Question 16

What is a side effect of an increased pupil size?

Answer 16

More aberrations as more marginal light rays come through and hit the lens, giving spherical aberration

Question 17

Describe the pupillary light reflex?

Answer 17

Light is sensed by the retina, signals are sent to the brain which cause the iris to pull back and allow more light in or close in and allow less as required.
Concussions can affect this reflex

Question 18

Properties of the Lens (4)

Answer 18

• The Lens has a power in relaxed state of about +20 diopters (not as much as cornea)
• It can adjust its power (by increasing curvatures) to allow eye to focus at different distances (accommodation)
• Looking near is accommodated
• Lens has a gradient refractive index (n=1.35 at edge, 1.41 centrally) that correct for spherical aberration

Question 19

What does accommodation do? How?

Answer 19

• Accommodation provides ability to change point of focus of eye from distant to near objects
• Changes dioptic power of eye, this is achieved by change in shape (curvature of lens)

Question 20

List the accommodative apparatus anatomy (5 items)

Answer 20

• Ciliary body
• Ciliary muscle
• Choroid (connective tissue of eye)
• Zonular fibers
• Lens Capsule and lens

Question 21

What do ciliary processes of ciliary body secrete?

Answer 21

Aqueous humour

Question 22

Where is the ciliary body?

Answer 22

Between the sclera spur and the retina

Question 23

Where are ciliary processes?

Answer 23

In the pars plicata

Question 24

Where do zonules attach?

Answer 24

To the Lens capsule and arise from the pars plana of the ciliary epithelium

Question 25

Where is the ciliary muscle attached?

Answer 25

• The sclera spur, which serves as a fixed anchor against which it contracts
• The inner surface of the anterior sclera
• The stroma of the choroid

Question 26

Mechanism by which accommodation occurs?

Answer 26

Ciliary muscle contracts and pulls the stroma of the choroid, which zonules are attached to, closer to the lens. Ciliary muscle consists of radial, circular and longitudinal fibres, and contraction of all 3 groups pulls choroid forward, releasing resting zonule tension at lens equator, allowing accomodation to occur, as the inherent elasticity of lens substance and capsule means it wants to round up.

Question 27

What is another name for the mechanism of accomodation?

Answer 27

The Helmholtz theory of accommodation

Question 28

What happens when accommodation ceases?

Answer 28

• The elastic choroid and posterior zonular fibers return the ciliary muscle to the unaccommodated state.
• The reconfiguration of the ciliary muscle increases tension on the zonular fibers at the equatorial edge of the lens.
• This pulls edges of lens towards sclera to flatten and unaccommodate the lens

Question 29

What is the accommodation range?

Answer 29

Distance in Diopters betweed far point and near point

Question 30

Define presbyopia

Answer 30

The decrease in accommodation range that occurs with age. It is usually gone by age 50

Question 31

List and describe other refractive errors (3)

Answer 31

Emmetropia- Normal
Myopia - Nearsighted (can see close) focal point in front of retina. Partly hereditary e.g. in Singapore 80% of population is myopic
Hyperopia - Farsighted (can see far) focal point behind retina. Occurs when combined curvature of lens and cornea is insufficient.

Question 32

How do you correct myopia?

Answer 32

Lenses with negative optical power (i.e. concave) compensating for excessive positive dioptres of myopic eye

Question 33

How do you correct hyperopia?

Answer 33

Convex lenses to increase positive dioptric value, causing light to focus closer.

Question 34

Describe astigmatism?

Answer 34

An optical defect in which vision is blurred due to inability of optics of eye to focus point obect into a sharp focused image on retina. Maybe due to irregular or toric curvature of cornea and/or lens

Question 35

What shape is an unaccommodated lens?

Answer 35

Flat compared to a round accommodated lens