Optical properties of the eye Lecture Two

Question 1

What refracts light rays onto the retina?

Answer 1

The cornea and the lens

Question 2

Describe the basic structure of the eye from anterior to posterior:

Answer 2

Tear Layer - Cornea - Aqueous Humor - Lens + Cillary body set up - Vitreous Humor - Retina

Question 3

Describe the lens set up - with regards to accomodation:

Answer 3

The Lens is surrounded by a lens capsule which the Zonular fibres attach to. The Zonular fibres attach to the ciliary body/ muscle.

Question 4

What types of muscle are in the ciliary body?

Answer 4

Longitudinal
Circular
Radial fibres.

Question 5

Define the Aqueous Humor:

Answer 5

Clear watery fluid - anterior compartment : Maintains pressure and nourishes the cornea and lens.

Question 6

Define the Vitreous humour:

Answer 6

Clear jelly like fluid - Posterior compartment : Maintains the shape of the eye and attaches to the retina.

Question 7

What are the ciliary muscles?

Answer 7

Involuntary muscles that change the lens shape and thus optic power (accomodation) to allow focusing of objects at different locations.

Question 8

Whats the cornea?

Answer 8

The cornea is a transparent tissue. It is avascular and anueral. It has a 40 diopter value and therefore refracts light onto the retina

Question 9

What is the iris?

Answer 9

Circular band of muscle that controls the size of the pupil and therefore changes the degree of light entering the eye.

Blue eyes have the least pigment and brown the most.

Question 10

What is the lens?

Answer 10

The lens is transparent tissue that refracts light, focusing it onto the retina of the eye. The lens can change shape (accomodation)

Question 11

What is the optic nerve?

Answer 11

A bundle of over one million axons from ganglion cells that carry visual signals from the eye to the brain.

Question 12

What is the pupil?

Answer 12

Hole in the centre of the eye where light passes through.

Question 13

What is the choroid?

Answer 13

Thin layer of tissue containing blood vessels that sits between the retina and the sclera of the eye.

Question 14

What is the retina?

Answer 14

Layer of tissue at the back of the eye containing photoreceptors

Question 15

What is the sclera?

Answer 15

Tough white outer covering of the eyeball. Extra ocular muscles attach here to move the eye.

Question 16

Despite being a optic tool, it must also be remembered that the eye is what?

Answer 16

A biological tissue and acts so accordingly.

Question 17

What is optical power?

Answer 17

The degree to which light either converges of diverges.

Question 18

What is a dioptre?

Answer 18

A unit of measurement for optical power.

Question 19

What is the reciprocal of a diopter?

Answer 19

the reciprocal of a diopter is 1/meters. I.e 3 diopters brings light into focus at 1/3 meters.

Question 20

What must also be considered with light entering the eye other than the solid structures?

Answer 20

The liquids of the eye aqueous and vitreous humour will both have different refractive indices from air and therefore will cause the light to refract.

Question 21

When the lens doesn’t form a perfect image, what is this flaw called?

Answer 21

Abberations

Question 22

Whats an example of an aberration?

Answer 22

Spherical abberation. This is when light hits the edge of the lens which is distant enough from the central axis that the light refracts more than those closer to the central axis and forms a blurred image.

Question 23

Can spherical aberration be solved?

Answer 23

Yes the body has biological processes to counter this. i.e the iris constricts reducing the diameter of the pupil and prevents rays from striking the edge of the lens.

Question 24

What are some other aberrations?

Answer 24

Comm and chroma abberations.

Question 25

The eye can change its point of focus, what does this mean?

Answer 25

The eye is a dynamic system - changing its point of focus.

Question 26

How does the lens correct for spherical aberration without pupil constriction?

Answer 26

The lens has a refractive gradient so the edge of the lens has a lower refractive index than the centre.

Question 27

Describe how accomodation occurs?

Answer 27

To focus on something closer, the lens must change its shape to accomodate this. The ciliary body muscle contract and RELAXES the zonular fibres that attach the muscle to the lens capsule. The lens becomes more spherical.

The muscle relaxes, zonular tension increases and the lens becomes flat.

This is hemholt theory.

Question 28

What is the accomodation triad?

Answer 28

The accomodation triad is a series of events that occur to put near things into focus. It includes:

1) Accomodation (Change in optical power(increase))
2) Constriction of pupils (Increase depth of field, decreased light)
3) Convergence of the eyes. (the brain forms one image)

Question 29

Where does the most optical power come from in the cornea?

Answer 29

The front. 40dioptres

Question 30

What does the pupil diameter vary between?

Answer 30

2-8mm

Question 31

Despite changing pupil size, does luminance remain constant?

Answer 31

No, this is not guaranteed.

Question 32

What are the pupils main job?

Answer 32

1) Setting the depth of field

2) Controlling abberations

Question 33

What is myopia?

Answer 33

Nearsightedness. Distant objects appear blurry as focal point is infant of the retina.

Question 34

What is hyperopia?

Answer 34

Farsightedness. Close objects appear blurry. Young people can accomodate for this and cope.

Question 35

What is astigmatism?

Answer 35

Refractive power is not uniform throughout the lens. Therefore multiple focal points can occur on the retina. Thus vision is completely blurry- with multiple variants.

Question 36

What is presbyopia?

Answer 36

Age related blindness. Caused by the inability to accomodate (ciliary muscle tire) therefore close objects appear blurry.

Question 37

What is emmetropia?

Answer 37

Normal vision.

Question 38

What does the zonular fibres attach to specifically?

Answer 38

The zonular fibres attach to the lens capsule and the ciliary processes in the pars plana.

Question 39

What is the ciliary muscle attached to?

Answer 39

The sclera spur and the storm of the choroid