Chapter 4: Prisms

Question 1

What are 3 key features of a prism?

Answer 1

1. Angle, a, between the two plane surface is the refracting angle or apical angle
2. Axis of the prism = line bisecting the refracting angle
3. Opposite surface from the refracting angle is the base of the prism

Question 2

In which direction is light passing through a prism deviated?

Answer 2

towards the base of the prism

Question 3

Which 3 properties determine the angle of deviation of a prism?

Answer 3

1. refractive index of material
2. refracting angle, a
3. angle of incidence of the ray

Question 4

Where is the angle of deviation, D of a prism?

Answer 4

The net change in direction of the ray passing through a prism:

Question 5

When is the angle of deviation, D, of a prism, least?

Answer 5

when the angle of incidence equals the angle of emergence

Question 6

What is the angle of minimum deviation?

Answer 6

the least angle of D, achieved when the angle of incidence = angle of emergence

Question 7

How do you calculate the angle of deviation of a prism?

Answer 7

D = (n-1)a

D=angle of deviation, n=refractive index of prism, a=refracting angle

in a glass prism, n=1.5, therefore D=a/2

Question 8

What are the 3 key properties of the image formed by a prism?

Answer 8

1. erect
2. virtual
3. displaced towards the apex of the prism

Question 9

What is the Prentice position of a prism?

Answer 9

when once surface of the prism is normal to the ray of light so that all the deviation takes place at the other surface of the prism

Question 10

In which position is the power normally specified for a) glass ophthamic prisms and b) for plastic ophthalmic prisms?

Answer 10

1. glass = Prentice position power
2. plastic = power in position of minimum deviation

Question 11

What does 1 prism dioptre of prism power indicate?

Answer 11

produces a linear apparent displacement of 1cm, of an object O, situated at 1m

(therefore if there is 2cm of apparent displacement at 1m, the prism power is 2 D)

Question 12

What is the angle of apparent deviation?

Answer 12

apparent displacement of object O

a prism of 1 prism dioptre power produces an angle of apparent deviation of 0.5 degrees

Question 13

What is a centrad?

Answer 13

same as prism dioptre but image displacement is measured along an arc 1m from the prism rather than linearly

Question 14

What is the angle of deviation of a glass prism with refracting angle 10 degrees?

Answer 14

angle of deviation = (n-1)a

= 0.5 x 10

= 5 degrees

Question 15

What is the prism power of a prism with angle of deviation 5 degrees?

Answer 15

1 prism dioptre = angle of deviation 0.5 degrees

5 degrees / 0.5 = 10

1x 10 = 10 dioptres of power

Question 16

How can you calculate the power from prisms added with one in the horizontal and one in the vertical direction?

Answer 16

vector addition; prism powers considered as diagonals of the triangle and then calculate hypotenuse with pythagoras’ theorem and angle of prism needed is given using trigonometry (soh cah toa)

Question 17

What is a Risley prism?

Answer 17

two prisms of equal power, mounted in front of each other so can be rotated with respect to each other

can be used in conjunction with a Maddox rod to measure phorias

Question 18

What are the 2 ways orthoptists can measure the angle of squint?

Answer 18

1. synoptophore
2. prism cover test

Question 19

How can the angle of squint be measured objectively?

Answer 19

prism cover test

Question 20

How can the angle of squint be measured subjectively?

Answer 20

Maddox rod

Question 21

What are 5 ways prisms can be used to assess squint/ heterophoria?

Answer 21

1. objectively - prism cover test
2. subjectively - Maddox rod
3. likelihood of diplopia after squint surgery
4. measurement of fusional reserve with increasingly powerful prisms
5. Four-dioptre prism test- tests for small degrees of esotropia (microtropia)

Question 22

In addition to assessment of squint and heterophorias, what is another use of diagnostic prisms?

Answer 22

assessment of simulated/fictitious blindness - if placed in front of a seeing eye, eye will move to regain fixation

Question 23

What are 2 uses of therapeutic prisms?

Answer 23

1. corrections of convergence insufficiency
2. relieve diplopia in certain cases of squint

Question 24

How do you prescribe prisms?

Answer 24

the correction is split between the two eyes

the apex of the prism points in the direction of deviation of the eye, i.e. for convergence the prisms must be base-out, for hypertropia they must be base down etc.

Question 25

What are 3 examples of optical instruments that make use of prisms?

Answer 25

1. slit lamp microscope
2. applanation tonometer
3. keratometer

Question 26

What is the effect of a Porro prism?

Answer 26

deviation 180 degrees, image inverted but not transposed left to right

Question 27

What is the function of a dove prism?

Answer 27

no deviation, image inverted but not laterally transposed

Question 28

What is the effect of a right angle prism?

Answer 28

deviation 90 degrees