ABO NOCE Basic Opticianry Ophthalmic Optics and Principles Questions and Answers 4 — Questions and Answers
Question 1: Which type of lens aberration causes the image of a point source off the optical axis to appear as a comet-shaped blur?
- Spherical aberration
- Coma (Correct answer)
- Astigmatism of oblique incidence
- Field curvature
Correct answer: Coma
Coma produces a comet-shaped blur for off-axis point sources because rays through different zones of the lens converge to different points.
Question 2: The Abbe number (V-number) of a lens material is used to measure which optical property?
- Surface reflectance
- Chromatic dispersion (Correct answer)
- Index of refraction
- Lens transmittance
Correct answer: Chromatic dispersion
The Abbe number indicates the degree of chromatic dispersion; a higher Abbe number means less chromatic aberration.
Question 3: A patient has 2Δ base-in prism OD and 2Δ base-in prism OS. What is the total prism effect?
- 0Δ
- 2Δ base-in
- 4Δ base-in (Correct answer)
- 4Δ base-out
Correct answer: 4Δ base-in
Base-in prisms in both eyes add together for horizontal vergence demand, totaling 4Δ base-in.
Question 4: What is the purpose of a lenticular lens design in high-power prescriptions?
- To increase chromatic aberration
- To reduce lens weight and thickness by limiting the optical zone (Correct answer)
- To increase the field of view
- To eliminate the need for anti-reflective coating
Correct answer: To reduce lens weight and thickness by limiting the optical zone
Lenticular designs place full prescription power only in a central optical zone, reducing overall lens weight and thickness for high-power corrections.
Question 5: Which formula correctly describes Snell's Law of refraction?
- n₁ × cos θ₁ = n₂ × cos θ₂
- n₁ × sin θ₁ = n₂ × sin θ₂ (Correct answer)
- n₁ / sin θ₁ = n₂ / sin θ₂
- n₁ + θ₁ = n₂ + θ₂
Correct answer: n₁ × sin θ₁ = n₂ × sin θ₂
Snell's Law states n₁ sin θ₁ = n₂ sin θ₂, relating the indices of refraction and angles at a refracting interface.
Question 6: A lens has a front surface power of +8.00 D and a back surface power of −4.50 D. Using the thick lens formula, the reduced distance (t/n) introduces a power correction. If t/n = 0.003 m, approximately how much additional power does this add?
- +0.11 D (Correct answer)
- +0.96 D
- +0.04 D
- +0.25 D
Correct answer: +0.11 D
The thick lens correction ≈ (t/n) × F1 × F2 = 0.003 × 8.00 × 4.50 = 0.108 ≈ +0.11 D added power.
Question 7: A patient's spectacle prescription is −5.00 DS. Using a vertex distance of 13 mm, the approximate contact lens power needed is:
- -4.66 D (Correct answer)
- -5.34 D
- -5.00 D
- -4.80 D
Correct answer: -4.66 D
Effective power = F / (1 − d×F) = −5.00 / (1 + 0.013 × 5.00) = −5.00 / 1.065 ≈ −4.69 D, closest to −4.66 D.
ABO NOCE Basic Opticianry Ophthalmic Optics and Principles Questions and Answers 4
Which type of lens aberration causes the image of a point source off the optical axis to appear as a comet-shaped blur?