NOCE Ophthalmic Optics and Formulas 4 — Questions and Answers
Question 1: Using Snell's law, if light strikes glass (n = 1.50) at 30° from the normal in air (n = 1.00), what is the approximate angle of refraction?
- 10°
- 19.5° (Correct answer)
- 30°
- 45°
Correct answer: 19.5°
n₁ sin θ₁ = n₂ sin θ₂ → sin θ₂ = sin 30° / 1.50 = 0.333, so θ₂ ≈ 19.5°.
Question 2: What is the critical angle for total internal reflection in a material with n = 1.50?
- 30.0°
- 41.8° (Correct answer)
- 48.6°
- 60.0°
Correct answer: 41.8°
sin θc = 1/n = 1/1.50 = 0.667; θc = arcsin(0.667) ≈ 41.8°.
Question 3: A lens material with an Abbe value of 30 has more chromatic aberration than a material with an Abbe value of:
- 20
- 25
- 28
- 58 (Correct answer)
Correct answer: 58
Higher Abbe values indicate less chromatic dispersion; 58 > 30, so that material has less aberration.
Question 4: The index of refraction of a material is defined as:
- The ratio of the speed of light in vacuum to the speed of light in the medium (Correct answer)
- The ratio of the speed of light in the medium to the speed in vacuum
- The angle at which total internal reflection occurs
- The reciprocal of the focal length
Correct answer: The ratio of the speed of light in vacuum to the speed of light in the medium
n = c / v, where c is the speed of light in vacuum and v is the speed in the medium.
Question 5: Standard CR-39 plastic (allyl diglycol carbonate) has an index of refraction of approximately:
- 1.49 (Correct answer)
- 1.53
- 1.60
- 1.74
Correct answer: 1.49
CR-39 has an index of approximately 1.499 (commonly cited as 1.49 or 1.50).
Question 6: Compared to n = 1.50 for the same prescription, a lens made from n = 1.74 material will be:
- Thicker and heavier
- Thinner and lighter (Correct answer)
- The same thickness but lighter
- Thicker but lighter
Correct answer: Thinner and lighter
Higher index materials allow thinner lens profiles for the same power, reducing both thickness and weight.
Question 7: The Abbe number (V) is calculated as V = (nD − 1) / (nF − nC). This formula quantifies:
- The index of refraction at the sodium D wavelength only
- Chromatic dispersion — higher values mean less dispersion (Correct answer)
- The critical angle for total internal reflection
- Surface reflectance of a lens material
Correct answer: Chromatic dispersion — higher values mean less dispersion
The Abbe number measures how much a material disperses light by wavelength; higher V means less chromatic aberration.
NOCE Ophthalmic Optics and Formulas 4
Using Snell's law, if light strikes glass (n = 1.50) at 30° from the normal in air (n = 1.00), what is the approximate angle of refraction?