Prism

Question 1

What is the aim of the introduction to prisms?

Answer 1

To use Snell’s Law to show the principles of prisms, relate angular deviation to prism dioptres, and provide fundamental theory for clinical practice.

Question 2

In a parallel sided block, what is the relationship between the angles of incidence and refraction?

Answer 2

i1’ = i2

Question 3

What happens to the angles of incidence and refraction in a non-parallel sided block?

Answer 3

i1’ ≠ i2

Question 4

What is the total deviation of a ray through a prism represented by?

Answer 4

dTOT = d1 + d2

Question 5

What formula gives the total deviation of a ray through a prism?

Answer 5

dTOT = i1 + i2’ - (i1’ + i2)

Question 6

What is the apical angle of a prism denoted by?

Answer 6

a

Question 7

How is the total deviation simplified for small apical angles in ophthalmic work?

Answer 7

dTOT = i1 + i2’ - a

Question 8

What is the unit commonly used by orthoptists to measure angular deviation?

Answer 8

Prism dioptre (Δ)

Question 9

Fill in the blank: The formula for total deviation of a ray, when a is small, is _______.

Answer 9

dTOT = i1 + i2’ - a

Question 10

True or False: The angle of incidence in ophthalmic work is typically large.

Answer 10

False

Question 11

What does Snell’s Law describe in the context of prisms?

Answer 11

The relationship between angles of incidence and refraction at each surface of the prism.

Question 12

What is the significance of the angle of tilt in a prism?

Answer 12

It affects the path of the ray through the prism.

Question 13

What is the relationship between i1, i2’, and the apical angle for small values of a?

Answer 13

i1 = i2’

Question 14

List the components of the total deviation formula for prisms.

Answer 14

• d1
• d2
• i1
• i2’
• i1’
• i2
• a

Question 15

What should be noted about the values of a in ophthalmic work?

Answer 15

They tend to be small, usually less than 10°.