Prism

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

Neoclature of prism :

Prism - It is simply a refracting medium with triangular cross section having apex and base.

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.

Refractive surface – The two refractive surfaces of a refractive prism inclined at the apical angle

Axis – Line Bisecting The Refractive Angle

Reflecting surface – In some prism, internal ray hitting the 2nd surface is subjected to total internal reflection (reflecting prisms)

Apex – Tip of the prism where the two refractive surfaces meet

Base – Bottom of the prism / side opposite to apex or optical angle. Helps in orientation of ophthalmic prisms

Apical angle - angle between two refractive surfaces ‘α’

Prism – Refraction

Angle of deviation – Angle between the incident and the emergent rays.

Refractive angle / Apical angle - angle between two refractive surfaces ‘α’ 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.

A

δm

Incident ra

y

Initial d

irectio

n

Emerging ray

A is the angle of prism

N N

N is normal on the surface of prism

δm is the deviation of emerging ray from initial direction

Prism

Prism and refraction

Red 6600 Å Orange 6100 Å Yellow 5800 Å Green 5500 Å Blue 4700 Å Indigo 4400 Å Violet 4100 Å

wavelength λ of the center of color band in angstroms

prism

Dispersion of white light of sun by prism

white lig

ht

A

Spectrum

If a shaft of light entering a prism is sufficiently small such that the coloured edges meet, a spectrum results

δR δVΘ

A

White lig

ht

δm ⇒ mean deviation or deviation for yellow

Angle of dispersion θ

θ = δV - δR

Dispersive power ω

δm

θω =

δm

ω =δV - δR

Deviation by prism

δR ⇒ deviation for red

δV ⇒ deviation for violet

δm

Refraction And Reflection Of Prism

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.

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

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

Trial Prisms

Rotating PrismsFresnel

Prisms

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

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.

Thank you