NOCE Ophthalmic Optics and Lenses 4 — Questions and Answers
Question 1: A progressive addition lens (PAL) fitting cross is placed at what anatomical landmark?
- Pupil center (Correct answer)
- Corneal reflex
- Lower lid margin
- Temporal canthus
Correct answer: Pupil center
The fitting cross of a PAL is aligned with the pupil center to ensure the distance optical center is correctly positioned.
Question 2: What is the refractive index of standard CR-39 plastic lens material?
- 1.499 (Correct answer)
- 1.523
- 1.586
- 1.600
Correct answer: 1.499
CR-39 (allyl diglycol carbonate) has a refractive index of approximately 1.499.
Question 3: Slab-off prism is used to correct vertical imbalance caused by:
- Anisometropia when reading through off-center lenses (Correct answer)
- Horizontal phoria at distance
- Prism vergence at near
- Base curve differences between lenses
Correct answer: Anisometropia when reading through off-center lenses
Slab-off (bicentric grinding) compensates for differential vertical prism induced by anisometropic lenses when the eyes depress for near tasks.
Question 4: A lens with a power of +4.00 D has a focal length of:
- 25 cm (Correct answer)
- 4 cm
- 40 cm
- 250 cm
Correct answer: 25 cm
Focal length (meters) = 1 / power (D), so 1/4.00 = 0.25 m = 25 cm.
Question 5: The phenomenon where a lens splits white light into its component colors is called:
- Chromatic dispersion (Correct answer)
- Spherical aberration
- Reflection
- Birefringence
Correct answer: Chromatic dispersion
Chromatic dispersion occurs because different wavelengths of light are refracted by different amounts, separating white light into colors.
Question 6: An antireflective (AR) coating works by:
- Destructive interference of reflected light waves (Correct answer)
- Absorbing light at the lens surface
- Increasing the refractive index of the lens
- Polarizing reflected light
Correct answer: Destructive interference of reflected light waves
AR coatings use thin film layers where reflected waves are out of phase and cancel each other through destructive interference.
Question 7: What does the term 'lenticular lens' describe?
- A high-power lens with a central optical zone and non-optical carrier (Correct answer)
- A bifocal with a lenticular segment
- A lens designed for contact lens correction
- A lens with a prism ground into the carrier
Correct answer: A high-power lens with a central optical zone and non-optical carrier
A lenticular lens has the prescription ground only in a central optical zone (bowl), with a non-prescription carrier that reduces thickness and weight in high-power lenses.
NOCE Ophthalmic Optics and Lenses 4
A progressive addition lens (PAL) fitting cross is placed at what anatomical landmark?