NOCE Ophthalmic Optics and Lenses 3 — Questions and Answers
Question 1: What type of aberration causes light rays at different distances from the optical axis to focus at different points?
- Spherical aberration (Correct answer)
- Chromatic aberration
- Coma
- Field curvature
Correct answer: Spherical aberration
Spherical aberration occurs because peripheral rays focus at a different point than paraxial rays due to the spherical lens surface.
Question 2: A high-minus lens is best fabricated with which base curve strategy to minimize edge thickness?
- Flatter base curve (Correct answer)
- Steeper base curve
- Plano base curve
- Same as vertex power
Correct answer: Flatter base curve
Flatter base curves on minus lenses reduce edge thickness by allowing the front surface to be less curved.
Question 3: The Abbe value (V-number) of a lens material describes its:
- Resistance to chromatic dispersion (Correct answer)
- Scratch resistance
- UV absorption
- Impact resistance
Correct answer: Resistance to chromatic dispersion
The Abbe value measures how much a material disperses light into its spectral components; higher values mean less chromatic aberration.
Question 4: A patient with -5.00 D prescription switches from glass (n=1.523) to polycarbonate (n=1.586). The new lenses will be:
- Thinner at the edges (Correct answer)
- Thicker at the edges
- The same thickness
- Thinner only at the center
Correct answer: Thinner at the edges
Higher index materials require less curvature to achieve the same power, resulting in thinner edges for minus lenses.
Question 5: Prentice's Rule states that prism power at any point equals:
- Distance from optical center (cm) × lens power (D) (Correct answer)
- Distance from optical center (mm) × lens power (D)
- Distance from optical center (cm) × lens power (D) / 10
- Lens power (D) / distance from optical center (cm)
Correct answer: Distance from optical center (cm) × lens power (D)
Prentice's Rule: Prism (Δ) = distance in cm × power in diopters (P = cF).
Question 6: Which of the following lens materials has the highest impact resistance?
- Polycarbonate (Correct answer)
- CR-39
- Crown glass
- High-index 1.74
Correct answer: Polycarbonate
Polycarbonate is the most impact-resistant ophthalmic lens material and is required for safety eyewear.
Question 7: When transposing -2.00 -1.50 × 090, the result is:
- -3.50 +1.50 × 180 (Correct answer)
- +2.00 -1.50 × 180
- -3.50 -1.50 × 090
- -2.00 +1.50 × 090
Correct answer: -3.50 +1.50 × 180
To transpose: new sphere = sum of sphere and cylinder (-2.00 + -1.50 = -3.50), new cylinder = opposite sign (+1.50), new axis = 90° away (180).
NOCE Ophthalmic Optics and Lenses 3
What type of aberration causes light rays at different distances from the optical axis to focus at different points?