Correcting Refractive errors

Question 1

What are the optical components of the eye?

Answer 1

cornea
lens
length of the eye

Question 2

What happens during accomodation?

Answer 2

The lens becomes shorter and fatter and becomes more convex to focus on near targets which bend the light to focus on the retina

Question 3

What nerve controls accomodation?

Answer 3

it is stimulated by the parasympathetic nerves of the oculomotor nerves (think when a person is in love, they are relaxed so they can see their person up close)

Question 4

Explain hypermetropia

Answer 4

Hypermetropia is long-sightedness where the patient can initially focus on objects far away but struggles with near targets. This is because the eye is too short or the lens or cornea is not refracting well. Objects focus behind the eye and so corrective convex lens is required to focus the object at the retina.

Question 5

Explain myopia

Answer 5

This is short-sightedness where the patient cannot see far away since the eye is too long and the object focuses in front of the retina. This is corrected by a concave lens

Question 6

Explain astigmatism

Answer 6

The cornea in astigmatism is shaped more like a rugby ball than a soccer ball and so the focusing elements is different in different medians. For example the light coming from the top and bottom of an object may be focused in one plane, but light coming from the side of the object in another plane. So a correction lens shaped like that rugby ball rotated 90 degrees is needed to correct this. This can occur in myopia and hypermetropia.

Question 7

Explain presbyopia

Answer 7

With increasing age (especially after 40) the deformability and flexibility of the lens decreases meaning that accommodation starts to fail. This is overcome by a convex lens (reading glasses).

Question 8

Explain cataract

Answer 8

Increasing density of the lens nucleus as the lens accumulates all its cells over life time, means that vision becomes obscured and hazy as we age. This increase in density can also cause an increase in refractive power making the light cross over in front of the retina and often the elderly become myopic. This means distant objects may become obscured but then reading close unaided is great.