Prism

1. Prism
By - Shwetali Bhagat .
Bsc. Optometry 3rd yr.

2. Prism- It is simply a refracting medium with triangular
cross section having apex and base.
 Neoclature of prism :
i. Apex - The point where the two refracting surface are meet.
ii. Base - refered to the bottom of prism which opp. to the apex.
iii. Axis - refered to the line bisecting the prism.
iv. Apical Angle - formed B/W two refracting surface of prism. It is also
called as refracting angle.

3. Apex – Tip of the prism where
the two refractive surfaces meet
Apical angle - angle
between two refractive
surfaces ‘α’
Refractive surface – The two
refractive surfaces of a refractive
prism inclined at the apical angle
Reflecting surface – In some prism, internal
ray hitting the 2nd surface is subjected to
total internal reflection (reflecting prisms)
Axis – Line Bisecting The Refractive Angle
Base – Bottom of the prism / side opposite to apex or
optical angle.
Helps in orientation of ophthalmic prisms

4.  Prism – Refraction
Refractive angle / Apical angle - angle
between two refractive surfaces ‘α’
Angle of deviation – Angle
between the incident and the
emergent rays.
 Obeying Snell's Law of refraction light passing
through the prism is deviated to the base.
Where, Prism produce displacement of object seen
through it towards the apex.
 Factors responsible for ‘D’
 Angle of Deviation - refers to the net change in direction of
light ray passing through prism.
 Angle of Minimum Deviation - produced when the angle of
incidence is equal to angle of emergence.

5. Prism and refraction
A
δm
N N
Prism
A is the angle of prism
N is normal on the surface of prism
δm is the deviation of emerging ray from initial direction

6. Dispersion of white light of sun by prism
Red 6600 Å
Orange 6100 Å
Yellow 5800 Å
Green 5500 Å
Blue 4700 Å
Indigo 4400 Å
Violet 4100 Å
A
prism
Spectrum
wavelength λ of the center of color band in angstroms
If a shaft of light entering a prism is sufficiently small such that the coloured edges meet, a spectrum results

7. δR δV
Θ
A
δm ⇒ mean deviation or deviation for yellow
Angle of dispersion θ
θ = δV - δR
Dispersive power ω
θ
δm
ω =
δm
ω =
δV - δR
Deviation by prism
δR ⇒ deviation for red
δV ⇒ deviation for violet
δm

8.  Refraction And Reflection Of Prism

9.  Characteristics of Prism -
 Thickest edge the Base & Thinnest edge the Apex.
 Displace the incident ray towards the base.
 Displace the image towards the apex.
 Does not change vergence of the rays.
 Does not magnify & minify the image.

10.  Prismatic Effect
 Spherical lens
 Spherical lens behave like prisms in all quadrants except the center
 The refractive angle between the lens and the surface incident at the edges.
 Cylinder lens
 Cylindrical lenses have no power along the axis meridian, hence the cylinder can
exert no prismatic effect along its axis.
 Most commonly if correction is not equal in both eyes.
 High myopes
 Aphakia

11.  Types Of Prisms In Clinical Practice
1. Loose Prisms
2. Prisms Bars
3. Trial Prisms
4. Fresnel Prisms
5. Rotating Prisms
6. Risley Double Prisms – 2 rotating Prisms of same strength on a rotating
frame
Prism Bars
Loose Prisms
Rotating
Fresnel Prisms
Prisms

12.  Uses of Prism
 Prisms are used in orthoptic settings, mainly for diagnosis and management
 Diagnosis
 Diagnosis microtropia
 Measurement of angle of deviation
 To correct angle of deviation
 Management
 Eso/ exo deviation of concomitant / incomitant
 Amblyopia, Nystagmus, Malingering, Visual Field defects

13. Prisms in ophthalmic instruments
 Reflecting and dove prism are used in almost all of the of the ophthalmic
instrument and operating micro-scopes
 Using the property of total internal reflection prisms have basically replaced
mirrors in SLB, microscopes, ect.

14. Thank you