ABO NOCE Basic Opticianry Ophthalmic Optics and Principles Questions and Answers 3 — Questions and Answers
Question 1: Prentice's Rule states that prism power (Δ) equals lens power (D) multiplied by which factor?
- The lens thickness in mm
- The decentration in centimeters (Correct answer)
- The vertex distance in mm
- The base curve in diopters
Correct answer: The decentration in centimeters
Prentice's Rule: Δ = D × c, where c is the decentration in centimeters.
Question 2: Which surface of a lens is defined as the back vertex power measurement used clinically to verify prescriptions?
- Front surface power
- Back vertex power (Correct answer)
- Base curve power
- True power
Correct answer: Back vertex power
Back vertex power (BVP) is the standard clinical measurement taken from the back surface of a lens and used to verify prescriptions.
Question 3: A lens with a base curve of +6.00 D and a back surface of −3.50 D has an approximate power of:
- +9.50 D
- +2.50 D (Correct answer)
- +3.50 D
- +6.00 D
Correct answer: +2.50 D
Approximate lens power ≈ front surface + back surface = +6.00 + (−3.50) = +2.50 D.
Question 4: Which optical principle explains why higher index lenses can be made thinner than lower index lenses for the same prescription?
- Snell's Law of refraction
- Prentice's Rule
- Lensmaker's equation (Correct answer)
- Brewster's angle
Correct answer: Lensmaker's equation
The Lensmaker's equation shows that higher index materials refract more strongly, allowing less curvature (and thus thinner lenses) to achieve the same power.
Question 5: What is the vergence of light rays that are parallel (coming from optical infinity)?
- +1.00 D
- Infinite vergence
- 0.00 D (Correct answer)
- −1.00 D
Correct answer: 0.00 D
Parallel rays from optical infinity have zero vergence (neither converging nor diverging).
Question 6: In ophthalmic optics, what does the term 'transposition' refer to?
- Converting plus cylinder form to minus cylinder form (or vice versa) (Correct answer)
- Rotating the axis of a cylinder lens
- Changing the base curve of a lens
- Adjusting pantoscopic tilt
Correct answer: Converting plus cylinder form to minus cylinder form (or vice versa)
Transposition is the mathematical process of rewriting a prescription from plus cylinder form to minus cylinder form or vice versa without changing its optical effect.
Question 7: A -4.00 D lens placed 12 mm from the eye. If moved to 0 mm (contact lens), the effective power at the cornea is approximately:
- -3.82 D (Correct answer)
- -4.00 D
- -4.20 D
- -3.60 D
Correct answer: -3.82 D
Effective power = F / (1 − d×F) = −4.00 / (1 − 0.012 × (−4.00)) = −4.00 / 1.048 ≈ −3.82 D.
ABO NOCE Basic Opticianry Ophthalmic Optics and Principles Questions and Answers 3
Prentice's Rule states that prism power (Δ) equals lens power (D) multiplied by which factor?