Vision - Refraction and Accommodation

Question 1

When happens to light rays when they pass through a different medium?

Answer 1

They bend

Question 2

How do biconvex lenses affect light rays?

Answer 2

Bends light which focuses at a singular point

Question 3

How do biconcave lenses affect light rays?

Answer 3

Causes light to diverge

Question 4

What is the function of refractive media in the eyeball?

Answer 4

Transparent refractive media in front of the retina, i.e. the cornea, AH and VH, is what allows light to fall on the retina and create an image

Question 5

What is refraction?

Answer 5

Light waves from an object bend at the cornea, and then bend some more at the lens to form a clear image on the retina

Question 6

What occurs to allow the eye focus on close up objects?

Answer 6

The eye needs more bending powering to focus on an object (as eye receives more divergent rays) -> the lens becomes thicker and hence more powerful, and a clear image is formed on the retina

Question 7

Define accommodation

Answer 7

The changes occurring in both eyes as it changes focus from a distant to a close object

Question 8

What three changes occur with accommodation?

Answer 8

• Lens changes shape (becomes thicker & more spherical)
• Pupil constricts
• Eyes converge

Question 9

Describe the steps in lens thickening during accommodation

Answer 9

1. Ciliaris contracts making the ciliary muscle body bulge
2. Space in the middle decreases, as bulging brings the ciliary body ends closer
3. Suspensory ligaments becomes lax which released the stretch on the lens
4. Len thickens

(Parasympathetic)

Question 10

Describes what happens in pupil constriction during accommodation

Answer 10

To sharpen focus to see objects up close, the pupil constricts to allow only a few rays (those from the object) to pass through

Question 11

What muscle in involved in pupil constriction during accommodation?

Answer 11

Pupillary constrictor (sphincter pupillae) which is a concentric muscle around the border of the pupil which gets parasympathetic innervation (CN III)

Question 12

Describe what when the eyes converge during accommodation

Answer 12

When focussing on an object up close, our eyes have to turn in to look at the object convergence

Medial rectus muscles of both eyes are uses - oculomotor nerve (CN III) innervated

Question 13

What is the defect in myopia (shortsightedness)?

Answer 13

Eyeball is slightly longer in length that normal, so when light rays converge it forms an image in front of the retina so they are not seen clearly. Close objects look clear, distant objects appear hazy.

‘Bending power’ of cornea and lens is too much for the eyeball

Question 14

What are the symptoms of myopia?

Answer 14

• Headaches
• Infants & preverbal children divergent squint
• Toddlers loss of interest in sports/people
• Teachers notice child is losing interest in class

Question 15

How is myopia corrected?

Answer 15

• Biconcave lenses - diverges light so light is focused on the retina
• Glasses
• Laser eye surgery – removes flap of cornea and exposes stroma and burns a bit away to flatten lens

Question 16

What is the defect is hyperopia (longsightedness)?

Answer 16

Eyeball is shorter in length, so the image of a distant object is formed behind the retina. The person then automatically starts to use the accommodative power and makes the lens thicker

However, since the lens power is use to see distant objects (that should normally be seen without use of any power), when closer objects are viewed, there’s no power to enable a clear image

Question 17

How is hyperopia corrected?

Answer 17

• Biconvex glasses alleviate use of glasses for focussing distant objects and ‘rests’ the accommodative power
• Contact lenses
• Laser eye surgery

Question 18

What is astigmatism?

Answer 18

Close and distant objects appear hazy, they are formed out of sink and so some images may form and some do not.

The bending of light rays along on axis will never be the same as that of the other axis, there the image is always hazy, whatever the distance of the object.

Question 19

How are astigmatisms corrected?

Answer 19

• Needs special glasses, cylindrical glasses which are curved in only one axis.
• Laser eye surgery can also be used to correct defect
• Needs special contact lenses called toric lenses

Question 20

What is prebyopia?

Answer 20

Longsightedness of old age

Question 21

What is the defect in presbyopia?

Answer 21

With age the lens gets less mobile/elastic, so when the ciliary muscle contracts, it is not as capable as before to change shape.

Therefore, seeing near objects/reading the newspaper starts to become difficult -> needs glasses to read