The prism is a refracting medium .

1. Prism
BY - RAJU RATHORE
B.OPTOMETRY 2nd year 3 SEM
ITM UNIVERSITY RAIPUR

2. Introduction
• A prism is a refracting medium, having
two plane surfaces, inclined at an angle
■Apical angle (refracting angle) of a prism
is the angle between two surfaces . The
greater the angle formed by two surfaces at
the apex, the stronger is the prismatic effect.
■Axis of the prism refers to the line
bisecting the optical angle.

4. ■Base of prism refers to the surface opposite
to
the apical angle. When prescribing a prism, the
orientation is indicated by the position of the
base,
e.g., ‘base-in’ or ‘base-out’ or ‘base-up’.
■Refractionthroughtheprism produces
displacement of the objects seen through it
towards apex (away from the base)

5. ■Power of a prism is measured in prism
dioptres. One prism dioptre (D) produces
displacement of an object by one cm when
kept at a distance of one metre. Two prism
dioptres of displacement is approximately
equal to one degree of arc
■Image formed by a prism is virtual, erect
and
displaced towards its apex

6. Uses in ophthalmology
1. Diagnostic uses include:
• Objective measurement of angle of
deviation
(Prism bar cover test, Krimsky test).
• Measurement of fusional reserve.
• Diagnosis of microtropia.2. Prisms are also used in many ophthalmic
equipments
such as gonioscope, keratometer and
applanation tonometer

7. 3. Therapeutic uses of prisms are:
• To relieve diplopia in patients with
decompensated phorias and small tropias
• For exercises to develop fusional
reserve

8. Lll
Thank You
By- Raju RATHORE
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