COT Optics and Lensometry 4 — Questions and Answers
Question 1: According to Snell's Law, when light passes from a medium with a lower index of refraction to a higher index, it:
- Speeds up and bends away from the normal
- Slows down and bends toward the normal (Correct answer)
- Maintains the same speed and direction
- Reflects entirely back into the first medium
Correct answer: Slows down and bends toward the normal
Snell's Law states that light bends toward the normal when entering a denser (higher-index) medium and slows down.
Question 2: A patient presents with glasses marked OD: +1.50 -0.75 x 045. What is the power in the 135° meridian?
- +1.50 D
- +0.75 D (Correct answer)
- -0.75 D
- +2.25 D
Correct answer: +0.75 D
The 135° meridian is 90° away from the axis (045°), so it carries sphere + cylinder = +1.50 + (-0.75) = +0.75 D.
Question 3: What is the unit used to measure prismatic effect in spectacle lenses?
- Diopter
- Prism diopter (Δ) (Correct answer)
- Meter angle
- Radian
Correct answer: Prism diopter (Δ)
Prismatic effect in ophthalmic optics is measured in prism diopters (Δ), where 1Δ = 1 cm deviation per meter.
Question 4: Using Prentice's Rule, what is the prismatic effect induced when a patient looks through a point 4 mm below the optical center of a +3.00 D lens?
- 0.75 prism diopter
- 1.00 prism diopter
- 1.20 prism diopters (Correct answer)
- 12.0 prism diopters
Correct answer: 1.20 prism diopters
Prentice's Rule: P = d (cm) × F = 0.4 cm × 3.00 D = 1.20 prism diopters.
Question 5: Which surface of a spectacle lens does the lensmeter measure when the lens rests against the lens stop?
- Front vertex power
- Back vertex power (Correct answer)
- Mean through power
- Equivalent power
Correct answer: Back vertex power
Standard lensometry measures back vertex power because the back surface of the lens rests against the lensmeter stop.
Question 6: A high-index lens material with n=1.74 will be thinner than a standard CR-39 lens (n=1.50) because:
- It is lighter in weight
- Higher index requires less curvature to achieve the same power (Correct answer)
- It absorbs more UV light
- It has better impact resistance
Correct answer: Higher index requires less curvature to achieve the same power
Higher index materials bend light more efficiently, allowing flatter (less curved) surfaces that produce thinner lenses for the same prescription.
Question 7: If the axis drum of the lensmeter reads 55°, in which direction does the cylinder axis of the lens run?
- Horizontally at 0°
- At 55° from horizontal (Correct answer)
- At 125° from horizontal
- Vertically at 90°
Correct answer: At 55° from horizontal
The axis drum directly indicates the axis of the cylinder in standard optometric notation, so 55° on the drum = 55° axis.
COT Optics and Lensometry 4
According to Snell's Law, when light passes from a medium with a lower index of refraction to a higher index, it: