prisms

Question 1

how is the rotation of the eye controlled ?

Answer 1

rotation of the eye is brought about by the 6 extrinsic ocular muscles attached to the globe of the eye.

Question 2

what does defects in the innovation of the muscle or in the muscle them selves cause ?

Answer 2

can bring about an imbalanced rotation in the eyes this will cause a failure in the fusion of the two images and result in diplopia for the patient and the appearance of strabismus

Question 3

what is strabismus ?

Answer 3

eye not looking straight ahead so it might appear that the eyes are turning in or out or up or down and can affect both eyes or one eye

Question 4

what do we do when a patient suffers from diplopia or strabismus ?

Answer 4

we must place a lens which is going to deviate the light displacing the image and allowing binocular fusion

Question 5

what is property of a prism ?

Answer 5

. may be with or without focal power
. it deviates the light from its original path it doesn’t refract
. prisms may be incorporated into an ophthalmic lens by surfacing or induced by decentration

Question 6

what is the base of the prism ?

Answer 6

the base is the thickest part and thats the place we use to tell people the prism direction

Question 7

what do we refer to prisms when prescribing them ?

Answer 7

we refer to them in the direction of the base

Question 8

what is the structure of prism ?

Answer 8

. the point where the surfaces meet is the apex , the thinnest edge of the prism
. opposite the apex is the base , the thickest edge of the prism

Question 9

where is the image displaced ?

Answer 9

in the direction of the apex

Question 10

where do we measure prism ?

Answer 10

by direction of the base

Question 11

what happens when surfacing prism ?

Answer 11

when surfacing prism, tools are tilted during manufacture to produce specified amount of prismatic effect

Question 12

how would you write prescription for

+2.00DS + 5 △ down plano prism

Answer 12

+2.00DS 5 △ DN lens

Question 13

what is the function of prism ?

Answer 13

. deviate light from its originial path

Question 14

how can we measure prisms ?

Answer 14

by the amount of deviation they produce

d=(n-1) a
a=angle of apex
d= angle of deviation
n= refractive index of prism material

Question 15

why is it difficult to measure apical angles

Answer 15

due to ophthalmic prims having very small apical angles therefore we can’t calculate deviation

Question 16

what do we use instead of apical angles ?

Answer 16

unit of the prism diopter

Question 17

how is the unit of prism diopter represented ?

Answer 17

.1 prism diopter displaces an image 1 unit when viewed at a distance of 100 units

. 1 prism displaces an image 1cm when viewed at 100 cm

. 2 prism displaces an image 2cm when viewed at 100 cm

Question 18

when prescribing what do we need to give ?

Answer 18

. we need to give the magnitude and direction of the prism

. we state if base direction is pointing up , down , in or out

Question 19

what are the prism direction ?

Answer 19

base up
base down
base out - temporal towards the ears
base in - towards the nose

Question 20

what is other way you can describe prism direction ?

Answer 20

using 360 notation 
up - 90
down -270
right eye nasal - 360
right eye temporal - 180 
left eye nasal - 180 
left eye temporal - 360

Question 21

what happens if prism base directions are the same or parallel in one lens if expressed in vertical and horizontal component ?

Answer 21

if the prism base direction are the same or parallel in one lens they can be added together

Question 22

what happens if prism or base direction are not parallel if oblique prism is expressed in vertical and horizontal component ?

Answer 22

then it is necessary to find the single resultant prismatic power and oblique base direction

Question 23

when is compounding prisms required ?

Answer 23

. compounding prisms is required if prism is given in horizontal and vertical form into a resultant prism in 360 degree notation
-apply Pythagorus theorem.

Question 24

what is to do if resultant prism is between 0 and 90 ?

Answer 24

answer would be θ

Question 25

what if resultant prism is between 90 and 180 ?

Answer 25

180- θ

-in and up

Question 26

what if resultant prism is between 180 and 270 ?

Answer 26

180 + θ

-down and in

Question 27

what if resultant prism is between 270 and 360 ?

Answer 27

360 - θ

-down and out to left eye

Question 28

when is resolving prism required ?

Answer 28

required if prism is given as a single oblique resultant prism in 360 deg notation and it is requires in horizontal and vertical component form