Lecture 2 Flashcards
Optical Properties of the Eye
What connects the eye to the brain?
The optic nerve
Where is light focused onto in the eye?
The retina
What is the focal distance in the eye?
The distance between the lens and the retina
Describe a lens
An optical device that transmits and refracts light to either converge or diverge the light beam
What happens to a collimated beam of light travelling parallel to the lens axis in a biconvex lens?
It is converged (focused) to a spot on the axis at a certain distance behind the lens (focal length)
Define optical power
The degree to which a lens, mirror or optical system converges and diverges light
What is a unit of measurement of optical power of a lens
A diopter
What is the relationship between diopters and focal length (in metres)?
Reciprocals
What happens to a lens immersed in a refractive medium?
Its optical power/focal length change
What does spherical aberration cause?
Beams parallel to but distant from the central lens optical axis are focused in a slightly different place than beams close to the axis
Name 3 types of lens aberattion
Spherical, coma and chromatic aberration
Properties of the cornea (5)
- 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)
Properties of the Iris and Pupil (4)
- 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
What is the role of the pupil?
To set the depth of field and control aberrations
How do the iris and pupil respond in dark and bright conditions
At low light levels the iris pulls back, and increases the size of the pupil, allowing more light in
What is a side effect of an increased pupil size?
More aberrations as more marginal light rays come through and hit the lens, giving spherical aberration
Describe the pupillary light reflex?
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
Properties of the Lens (4)
- 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
What does accommodation do? How?
- 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)
List the accommodative apparatus anatomy (5 items)
- Ciliary body
- Ciliary muscle
- Choroid (connective tissue of eye)
- Zonular fibers
- Lens Capsule and lens
What do ciliary processes of ciliary body secrete?
Aqueous humour
Where is the ciliary body?
Between the sclera spur and the retina
Where are ciliary processes?
In the pars plicata
Where do zonules attach?
To the Lens capsule and arise from the pars plana of the ciliary epithelium
Where is the ciliary muscle attached?
- 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
Mechanism by which accommodation occurs?
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.
What is another name for the mechanism of accomodation?
The Helmholtz theory of accommodation
What happens when accommodation ceases?
- 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
What is the accommodation range?
Distance in Diopters betweed far point and near point
Define presbyopia
The decrease in accommodation range that occurs with age. It is usually gone by age 50
List and describe other refractive errors (3)
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.
How do you correct myopia?
Lenses with negative optical power (i.e. concave) compensating for excessive positive dioptres of myopic eye
How do you correct hyperopia?
Convex lenses to increase positive dioptric value, causing light to focus closer.
Describe astigmatism?
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
What shape is an unaccommodated lens?
Flat compared to a round accommodated lens
