emmetropia and myopia 3.2

Question 1

what is emmetropia ?

Answer 1

-Second focal point Fe’ falls on the retina for the unaccommodated eye OR the far point (MR) lies at infinity. A sharp image is formed on the retina and vision cannot be improved with corrective lenses but it doesn’t have to be 6/6

Question 2

what is far point Mr ?

Answer 2

• Object point conjugate with the retina in the unaccommodated eye.
• Also known as punctum remotum hence the subscript R

Question 3

what does conjugate mean ?

Answer 3

points which are conjugate are pairs
e.g. object point will form image so object and image are known as conjugate pair
. object point at infinity will be conjugate with 2nd focal point

Question 4

what is the power and refractive index of the standard emmetropic reduced eye?

Answer 4

n=1
n’=4/3
Fe= 60D

Question 5

what do symbols K K’ k k’ mean ?

Answer 5

k - far point distance (measured from the surface of the reduced eye)
K - ocular refraction. It is the vergence of light striking a reduced eye from the far point. K=n/k
k’ - axial length of the reduced eye - distance from single surface of eye to retina
K’ – Dioptric length of the eye,It is the vergence of light
leaving the surface of the reduced eye that forms an image at the retina.
K’=n’/k’
n’=4/3

Question 6

what is the vergence of light when travelling from infinitely distant object ?

Answer 6

0

Question 7

what is formula for F (e)?

Answer 7

K’=K+F (e)

Question 8

what is myopia ?

Answer 8

Second focal point falls in front of the retina in the
unaccommodated eye OR the far point (MR) lies in front of the eye
retinal image blurred - vision reduced

Question 9

where is far point in myopia ?

Answer 9

.Mr is in front of the eye - real point
anything before far point produces sharp image and anything beyond far point will produce blurry image
. objects closer to the unaccommodated eye are seen clearly

Question 10

what is ocular refraction ?

Answer 10

Refractive error measured at the eye. It is given the symbol K for the reduced eye

Question 11

why is K ( ocular refraction ) equal to the power of a thin lens placed in contact with the eye to fully correct ametropia ?

Answer 11

this is because incident vergence required for a sharp optical image to be formed on the retina, the object will be at the far point location , at distance k
therefore ocular refraction is the thin lens power of a contact lens needed to correct ametropia

Question 12

when is vergence L and L’ equal to k and K’ ?

Answer 12

if the image is formed on the retina and the object is at the far point

NB ; K and k’ are parameters of an eye and not always vergence from ray tracing
linked by K’= K+Fe