Optics of the eye

Question 1

What are the 6 different refracting surfaces of the eye

Answer 1

Front and back of cornea

Front and back of anterior lens

Front and back of posterior lens

Question 2

What is the optical axis of the eye

Answer 2

The straight line passing through the geometrical cetner of a lens, joining the two centres of curvature of its surfaces

Question 3

What is the visual axis of the eye

Answer 3

A line passing from the fovea through the nodal point of the eye

Question 4

What is Gullstrand’s simplified Schematic eye

Answer 4

Consists of a single refrating surface and thin lens

Equivalent power of +59.7 D in the relaxed state

Question 5

What is the Reduced schematic eye

Answer 5

Assumes the eye as one refractive surface with a refractive index of n= 1.333

Question 6

What are the characteristics of the reduce schematic eye

Answer 6

2 principal points that conincide with each other and with the vertex of the surface

2 nodal points coincide with each other and with the centre of curvature of the surface

Optical axis becoem the visual axis

Hypothetical pupil lies at the refracating surface

Enterance and exit pupils coincide with hypothetical pupil

Question 7

What are the parameters of the standard reduced emmetropic eye

Answer 7

Reduced power = +60D

Axial length of eye k’ = +22.22 mm

Refractive index of eye= 1.333 (n’ when calculating power)

Question 8

What are the 4 types of refractive erros

Answer 8

Emmetropia
Myopia
Hyperopia
Astigmatism

Question 9

What is far point

Answer 9

The point farthest from the eye at which an object is accurately focused on the retina when the accomdation is completely relaxed

Question 10

What is near point

Answer 10

The point nearest the eye at which an object is accrately focused on the retina at full accomodation is called near point

Question 11

What is emmetropia

Answer 11

When is an unaccomodated eye, te parallel pencils of rays from a distsant obj to a sharp focus on the retina

The far point is at infinity and secondar focal point coincides with fovea

Question 12

How is near point (accomodation determined)

Answer 12

By age:

Esstimated accommodation = 15 - (age/4)

Question 13

What is ametropia

Answer 13

Having a refractive error which results in imbalance between power of the eye and its axial length

Question 14

What is axial amertropia

Answer 14

Assummued to have standard eye power = +60D

Refractive error attributd to error in axial length

Question 15

What is refractive ametropia

Answer 15

Axial legnth of reduce eye is assumed its standard value = 22.22mm

Defect attributed to an eror in power

Question 16

What is myopia

Answer 16

rays from a distant object point focus in front of the macula in the unaccommodated eye

Optical system too powerful for its axial length/ axial length too long

Question 17

What is the sign of the far point in myopia

Answer 17

Negative as it is in front

Question 18

How is myopia corrected

Answer 18

Negative, concanve lense

Question 19

What is hyperopia

Answer 19

rays from a distant object point focuses behind the macula in unaccommodated eye

Eye not powerful enough, axial length too short

Question 20

What is the sign of the far point in hyperopia

Answer 20

Posistive, as far point is behind the eye

Question 21

How is hyperopia corrected

Answer 21

Posistive, convex lenses

Question 22

What is astigmatism

Answer 22

Two focal lines image formed at different ditance generally perpendicular to each other for a point object

Eyeball is not round thus the horizontal and vertical axis have different focal points

Question 23

How is astigmatism corrected

Answer 23

Sphero-cylindrical lenses

Question 24

What isthe circle of least confusion (astigmatism)

Answer 24

Dioptric midway point betwen the 2 focal planes, cross sction of which is circular

Question 25

What is the interval of sturm

Answer 25

dioptric or physical distance between the 2 focal lines

Question 26

What is Presbyopia

Answer 26

Associated with the againg of eye that results in progressively worsenening ability to focus clearly on close objects

Crystallne lenses losing its flexibilty as we get older

Question 27

How is presbyopia corrected

Answer 27

Posistive, convex lenses