ABO NOCE Basic Opticianry Ophthalmic Optics and Principles 2 — Questions and Answers
Question 1: Which lens form has the same power on both surfaces but with one surface convex and the other concave?
- Plano-convex
- Biconvex
- Meniscus (Correct answer)
- Biconcave
Correct answer: Meniscus
A meniscus lens has one convex and one concave surface, giving it a crescent-like cross-section.
Question 2: What is the term for the bending of light as it passes from one medium to another of different optical density?
- Reflection
- Refraction (Correct answer)
- Diffraction
- Dispersion
Correct answer: Refraction
Refraction is the change in direction of light as it crosses the boundary between two media with different indices of refraction.
Question 3: If a lens has a back vertex power of +3.00 D and a front vertex power of +2.75 D, which power is used when verifying the prescription with a lensometer?
- Front vertex power
- Back vertex power (Correct answer)
- The average of both
- Either power can be used
Correct answer: Back vertex power
The back vertex power is the standard used for lensometer verification because it represents the effective power at the back surface closest to the eye.
Question 4: What happens to the critical angle when light travels from glass (n=1.5) into air (n=1.0)?
- It increases as n increases
- It is approximately 41.8° (Correct answer)
- It equals 90°
- No critical angle exists for this direction
Correct answer: It is approximately 41.8°
Using sin(c) = n2/n1 = 1.0/1.5, the critical angle for glass-to-air is approximately 41.8°.
Question 5: A prism with apex angle 10° and refractive index 1.5 produces approximately how much deviation in diopters (prism diopters)?
- 5Δ
- 10Δ (Correct answer)
- 15Δ
- 20Δ
Correct answer: 10Δ
Prentice's approximation: P = a(n-1) × 100, so 10 × 0.5 = 5°, but in prism diopters at thin prism approximation, deviation ≈ 10Δ for an apex of 10° with n=1.5.
Question 6: What is the vergence of light rays that are parallel to the optical axis before entering a lens?
- Positive vergence
- Negative vergence
- Zero vergence (Correct answer)
- Infinite vergence
Correct answer: Zero vergence
Parallel rays have zero vergence because they neither converge nor diverge; they come from optical infinity.
Question 7: In ophthalmic optics, the term 'base curve' most commonly refers to which lens surface?
- The back concave surface
- The front convex surface (Correct answer)
- The optical center
- The edge of the lens
Correct answer: The front convex surface
The base curve is the front (convex) surface curve of an ophthalmic lens, which determines the lens form.
ABO NOCE Basic Opticianry Ophthalmic Optics and Principles 2
Which lens form has the same power on both surfaces but with one surface convex and the other concave?