Chapter 4  (Prisms)

1. The following are true about prism:

a. its orientation is defined by its apex

b. light is deviated towards the apex

c. light with shorter wavelength is deviated more than light with 
    longer wavelength

d. the angle of the prism apex is called the refracting angle

e. the ophthalmic prisms are calibrated according to the Prentice's position
 

Answera.F   b.F   c.T    d.T   e.F
The orientation of a prism is defined by its base. Light is deviated 
towards the base. Light with shorter wavelength is deviated more
than light with longer wavelength by a prism. The refracting angle is 
the angle of the prism apex. The ophthalmic prisms are calibrated 
according to the Prentice's position ie. with one face of the prism perpendicular to the light ray.
. 
 
 

2. The image formed by a prism is:

a. erect

b. magnified

c. laterally inverted

d. virtual

e. deviated towards the apex
 

Answera.T  b.F  c.F  d.T  e.T
The image formed by a prism is erect, virtual and deviated towards the apex.
 

3. The angle of deviation of a prism is determined by:

a. the refracting angle 

b. the angle of incidence of the ray

c. the refractive index of the prism material

d. the width of the base

e. the thickness of the prism

Answera.T   b.T   c.T   d.F   e.F
Three factors determine the angle of deviation: the refracting angle, 
angle of incidence of the ray and the refractive index of the prism material.
 
 

4. The following are true about prisms:
 

a. they can be used to control torsional diplopia

b. they can be used to control diplopia caused by an eye which is 
    deviated out and up

c. the prism power can be calculated form the refracting angle alone

d. a prism with 2^D  will produce a linear displacement of 2 cm 
    of an object situated at 1 m. 

e. a prism with 1^D produce a stronger deviation than one with an 
    angle of apparent deviation of 1⁰
 
 

Answera.F   b.T   c.F   d.T   e.F
Torsional diplopia can not be controlled with prism. In addition
to the refracting angle, the refractive index of the prism is required 
to calculate the prism power. A prism of 1 prism dioptre is equivalent to one with an angle of apparent deviation of 1/2 degree. 

5. The Fresnel prisms:

a. reduce the weight of conventional prisms

b. are made up of a series of small prisms

c. are usually made up of polyvinyl chloride

d. reduce the visual acuity mainly through light scattering at the groove edges

e. are usually applied to the front of patients' glasses
 

Answer
a.T   b.T  c.T  d.F  e.F
Fresnel prisms reduce the weight of conventional lens and used
widely in treating patients with strabismus. They are made up of
a series of small prisms. The most common type are made up of 
polyvinyl chloride. They reduce the visual acuity mainly through 
chromatic aberrations. They are usually applied to the the back of patients' glasses.
 

6. A patient has a 4 ^D deviation of right over left strabismus. The following prisms may be 
    used to correct the vertical diplopia that this patient is experiencing:

a. 4 ^D base down over the right eye

b. 4 ^D base up over the left eye

c. 2 ^D base down over the right eye and 2 ^D base up over the left eye

d. 2 ^D base down over the right eye and 2 ^D base down over the left eye

e. 1 ^D base down over the right eye and 3 ^D base up over the left eye.

Answer
a.T   b.T  c.T  d.F  e.T
To correct a strabismus, the apex of the prism is pointing in the direction of the deviation and the base in the opposite direction. Therefore a patient 
with a right over left strabismus ie. right hypertropia and/or left hypotropia can be corrected with a base down prism over the right eye or a base up prism over the left eye. The power of the prism can be split between the 
two eyes. In such cases, the orientation fo the prisms is opposite for the two eyes. 

 
 

7. A patient has a 8^D right esotropia. The following prisms can be used to correct 
    the deviation:

a. 8^D  base in over the right eye

b. 8 ^D base out over the right eye

c. 8 ^D base in over the left eye

d. 8^D  base out over the left eye 

e. 4^D base out over the right eye and 4^D base out over the left eye. 
 

Answer
a.F   b.T  c.F  d.T  e.T
As mentioned in answer 4, the apex of the prism should be pointing
in the direction of the deviated eye to control the diplopia. Therefore,
the prism should be base out over the right eye or the left eye (unlike 
vertical strabismus, the base orientation of the prisms in horizontal strabismus are in the same direction). If the prisms are divided between the two eyes, the prism dioptre should be 4 with base out over each eye.
 

8. The following are true about the prismatic effect of lenses:

a. if the optical centre of  a myope lens is moved nasally, a base out prism 
    will be induced

b. if the optic centre of a myope lens is moved inferiorly, a base down prism 
    will be induced

c. if the optic centre of a hyperope lens is moved temporally, a base in prism 
    will be induced

d. if the optic centre of a hyperope lens is moved superiorly, a base up prism 
    will be induced.

e. a 2 D base in prism can be produced by shifting the optical centre of a 
    -5.00D myope lens 2 mm temporally

Answera. T   b. F   c. F   d.T   e.F
A myope (concave) lens can be regarded as two prism placed apex 
to apex. 
Decentring a myope lens temporally induces a base in prism; nasally 
induces a base out prism, inferiorly a base up prism and superiorly
a base down prism.
The reverse is true for a hyperope (convex) lens. 
Using the Prentice rule: Prism dioptre = distance from the optic centre (cm) X dioptre power of the lens. The induced prism is 1D.