SHORT ANSWERS – UNIT 4
Based on: Mr. V. Dinesh, Department of Optometry – *Prismatic Effect* notes
1. Define prism and draw its parts
A prism is a transparent optical element with flat, polished surfaces that refract light. It has two
plane surfaces inclined at an angle to each other, known as the refracting angle. When light passes
through a prism, it deviates or bends from its original path. The main parts of a prism are:
•Apex – the point where two refracting surfaces meet.
•Base – the opposite side of the apex, where light emerges.
•Refracting surfaces – the two plane surfaces that cause light to bend.
Diagrammatically, a prism can be represented as a triangle showing the apex at the top and the
base at the bottom.
2. Find the prismatic effect at a point 8mm above and 4mm
inwards from the optical center of the lens (Power:
-3.00DS/-2.00DC × 90)
To calculate the prismatic effect, we use **Prentice’s Rule**:
Formula: P = (c × F) / 10
Where P = prism in prism diopters (∆), c = decentration in mm, F = lens power in diopters.
For -3.00DS/-2.00DC × 90:
•Horizontal meridian (power at 180°): -3.00D
•Vertical meridian (power at 90°): -5.00D
Vertical prism = (8 × 5) / 10 = 4∆ Base Down (because minus lens, above OC → base down)
Horizontal prism = (4 × 3) / 10 = 1.2∆ Base In (because minus lens, inward → base in)
Resultant prism = √(4² + 1.2²) ≈ 4.18∆ with base direction slightly down and in.
3. Explain the Prentice rule with example
Prentice’s Rule states that the prismatic effect (P) induced by decentration of a lens is directly
proportional to the decentration (c) and the power (F) of the lens.
Formula: P = (c × F) / 10
Example: For a +4.00D lens decentered 6mm below the optical center:
P = (6 × 4)/10 = 2.4∆ Base Up (since it’s a plus lens and point is below OC).
4. What is decentration? State its effect on eye
Decentration is the displacement of the optical center of a lens away from the pupil or visual axis.
When the optical center is not aligned with the pupil, a prismatic effect is induced.
Effects on the eye:
•Image displacement causing the object to appear shifted.
•Eye strain due to the eye’s effort to realign.
•Diplopia (double vision) in severe cases.
5. What is Fresnel prism? Mention its clinical uses
