Prism

no

does prism affect vergence?

base

light rays are deviated towards the ______ of a prism

apex

the virtual image is displaced toward the _____ of a prism

normal

the angle of incidence & refraction are defined with respect to the _________

direction of the incident ray

the angle of deviation defined with respect to _____

d=a(n-1)

thin prism approximation equation

how far light was displaced from the point it would have struck at a given distance from prism had it not been bent by the prism

how is prism power defined?

P/100

tan(d)=

1.75

1degree of deviation produces ______ prism diopters

0.57

1 prism diopter produces ______ degrees of deviation

same

the eye turns in the _____ direction the prism points

base out

base out prism over one eye is the same as ______ prism over the other eye

base in

base in prism over one eye is the same as _______ prism over the other eye

base down

base up prism over one eye is the same as _______ prism over the other eye

base up

base down prism over one eye is the same as ______ prism over the other eye

actual prism power / (1-(distance from prism to center of rotation/distance from prism to near object))

effective prism power =

F

T/F: when equally powered rotary prisms are placed base-to-base, the resultant amount is 0

0pd

if 2 rotary prisms of 4pd each are placed on top of one another base-to-apex, what would the total prismatic effect be?

10

for prism with an apical angle of less than _____ degrees, you can use the dan formula to find the angle of deviation

base in

what base direction does base 180 correspond to on the left eye?

base out

what base direction does base 180 correspond to on the right eye?

decrease

the effective power of a prism, when measured as the angle that light enters the eye will ______ as an object moves closer to the prism being worn

F

T/F: base up prism in front of the right eye has the opposite effect as base down prism over the left eye

does not change

considering Risley prism, if 2 prisms begin base right on top of one another, what happens to the overall vertical prismatic effect when one is rotated clockwise & the other is rotated counterclockwise?

increases

prismatic power of a lens increases as the dioptric power of the lens _________

increases

prismatic power of a lens increases as the distance from the optical center _______

same

for plus lenses, base direction corresponds to the ______ direction of decentration

opposite

for minus lenses, base direction corresponds to the ______ direction of decentration

power of the cyl, orientation of the cyl axis

what 2 things does prismatic effect depend on for a cylinder lens?

0

if the axis of a plano cylinder is oriented in the direction of the decentration, there is ______ prismatic effect

prentice’s rule

if cylinder axis is at right angles to the direction of decentration, amount of prism induced varies with _______

F=Fcyl(sin²theta)

sin squared equation for approximating prism

1/2g

it is assumed that most of the time a lens will be thicker by _______ when prism is present regardless of the prism orientation

increases by ½ prism thickness difference

how does center thickness change for a plus lens w/ base in prism?

decreases by up to full thickness difference

how does center thickness change for a plus lens w/ base out prism?

does not change

for plus lenses, how does center thickness change with vertical prism & a small frame B

increases by up to ½ thickness difference

for plus lenses, how does center thickness change w/ vertical prism & a large frame B

increases slightly

how does center thickness change for a minus lens w/ base in prism?

increases slightly

how does center thickness change for a minus lens w/ base out prism?

base, apex

when light travels through a prism, it bends towards the _____ & the image is displaced towards the ______

T

T/F: if we know lens decentration in cm & lens focal length, the prismatic effect caused by the decentration may be calculated

out

moving a plus lens temporally in front of the eye produces base ______ prismatic effect

0

what prismatic effect is created when a cylinder lens is decentered along its axis?

T

T/F: there is no optical difference between the prism created by decentration & prism that has been ground in, but it may be possible to create a thinner prismatic lens by surfacing instead of decentering a finished lens

F

T/F: a plus lens is normally decentered outward because of the wearer’s PD

temporal

for minus lenses BO prism increases the thickness of the ________ edge by an amount equal to the base thickness of the prism

identical, equal but opposite

initially rotary/risley prisms are ______ but as the prisms are rotated, their bases move by ______ directions

decrease

as risley prisms are rotated in opposite directions, vertical components _______

increase equally to cancel each other out

as risley prisms are rotated in opposite directions, horizontal components ______

0

when the risley prisms are fully horizontal with one base left and one base right, there is _____ prism

increases, up

if the risley prisms are rotated past the horizontal, base up prism ______ until both prisms are fully base ______

left, clockwise, counterclockwise

to produce only horizontal prism on risley prisms, begin with both prisms base ____ & rotate one _____ and the other _____ equally

phoropter, lensometer

what are the 2 common forms of Risley prism in ophthalmic practice?

decreases

effective power of a prism _______ as the object moves closer toward the spectacle plane

less

the eye turns ____ when looking through prism at an object at near than when looking through the same prism at a distant object

less

the power of the prism is ____ the closer the object is to the prism

same

when a sphere lens is decentered obliquely, the resulting prismatic effect & base direction will be along the _____ meridian of decentration

less accurate, does not take into consideration vertical prism

what are some disadvantages of the sin squared method?