Prisms

Question 1

What is a prism

Answer 1

a portion of a refracting medium bordered by two plane surfaces which are inclined at a finite angle

Question 2

What is the angle alpha of a prism

Answer 2

The angle α between the two surfaces is called the refracting angle or apical angle of the prism. line bisecting the angle is called the axis of the prism. The opposite surface is called the base of the prism

Question 3

How to describe orientation of prisms

Answer 3

Base in
Base up

Question 4

What is the angle of deviation of a prism determined by

Answer 4

(1) The refractive index of the material of which the prism is made.
(2) The refracting angle, α, of the prism.
(3) The angle of incidence of the ray considered.

Question 5

What happens to the ray of light at each prism surface

Answer 5

obeys Snell’s law at each surface. The ray is deviated towards the base of the prism. The net change in direction of the ray, angle D, is called the angle of deviation

Question 6

When is the angle of deviation of a prism least

Answer 6

when the angle of incidence equals the angle of emergence. Refraction is then said to be symmetrical and the angle is called the angle of minimum deviation
D= alpha/2
In other words, the angle of deviation equals half the refracting angle for a glass prism

Question 7

What is the image formed by a prism

Answer 7

erect, virtual and displaced towards the apex of the prism

Question 8

What is the Prentice position

Answer 8

In the Prentice position one 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 9

Is the deviation of light in the Prentice position greater than that in the position of minimum deviation

Answer 9

Yes because in the Prentice position the angle of incidence does not equal the angle of emergence. Therefore the Prentice position power of any prism is greater than its power in the position of minimum deviation

Question 10

What is 1 Prism dioptre

Answer 10

A prism of one prism dioptre power (1∆) produces a linear apparent displacement of 1 cm, of an object
O, situated at 1 m

Question 11

What is the angle of apparent deviation

Answer 11

The apparent displacement of the object O can also be measured in terms of the angle θ, the angle of apparent deviation Under conditions of ophthalmic usage a prism of 1 prism dioptre power produces an angle of apparent deviation of 1/2°. Thus 1 prism dioptre = 1/2°

Question 12

What is the centrad

Answer 12

(denoted ∇) This unit differs from the prism dioptre only in that the image displacement is measured along an arc 1 m from the prism. The centrad produces a very slightly greater angle of deviation than the prism dioptre, but the difference, in practice, is negligible

Question 13

What do + and - in orthoptist reports mean

Answer 13

+ signifying esotropia and – signifying exotropia

Question 14

What is a fresnel prism

Answer 14

Prisms used in treatment include clip-on spectacle prisms for trial wear. An improvement on these are Fresnel prisms. Fresnel prism consists of a plastic sheet of parallel tiny prisms of identical refracting angle

Question 15

How to prescribe prisms

Answer 15

Generally, when prescribing prisms, the correction is split between the two eyes

Question 16

How to correct convergence using prisms

Answer 16

To correct convergence the prisms must be base-out, e.g. 8∆ base-out R and L

Question 17

How to correct divergence using prisms

Answer 17

Prisms must be base-in, e.g. 6∆ base-in R and L

Question 18

How to correct vertical deviation using prisms

Answer 18

To correct vertical deviation the orientation of the prisms is opposite for the two eyes, e.g.
2∆ base-down RE
2∆ base-up LE for R hypertropia

Question 19

What are practical uses of prisms

Answer 19

Instruments in which prisms are used include the slit lamp microscope, the applanation tonometer and the keratometer

Question 20

What is the image formed by a Dove Prism

Answer 20

No deviation
Image inverted but not laterally transposed

Question 21

What is the image formed by Porro prism

Answer 21

Deviation 180 degrees
Image inverted but not transposed left to right