COT Optics and Lensometry 2 — Questions and Answers
Question 1: When performing lensometry on a bifocal lens, where should you position the lens to read the distance power?
- Center the optical center of the add segment
- Center the distance portion of the lens in the lensmeter stop (Correct answer)
- Place the reading segment against the lensmeter stop
- Tilt the lens 10 degrees toward the lensmeter
Correct answer: Center the distance portion of the lens in the lensmeter stop
The distance portion of the bifocal is centered in the lensmeter stop to obtain the distance prescription.
Question 2: A lens with a cylinder power of -1.50 at axis 90 is equivalent in cross-cylinder notation to which combination?
- +1.50 sphere combined with -1.50 at axis 180
- Plano combined with -1.50 at axis 90 (Correct answer)
- -1.50 sphere combined with +1.50 at axis 90
- Plano combined with +1.50 at axis 90
Correct answer: Plano combined with -1.50 at axis 90
A pure cylinder lens has plano power in one meridian and cylinder power in the perpendicular meridian.
Question 3: What does the term 'index of refraction' describe?
- The curvature of a lens surface
- The ratio of the speed of light in a vacuum to its speed in a given medium (Correct answer)
- The amount of light absorbed by a lens material
- The focal length of a converging lens
Correct answer: The ratio of the speed of light in a vacuum to its speed in a given medium
Index of refraction (n) is defined as the speed of light in a vacuum divided by the speed of light in the medium.
Question 4: Which prism diopter measurement describes a prism that displaces an image 1 cm at a distance of 1 meter?
- 0.5 prism diopter
- 1 prism diopter (Correct answer)
- 2 prism diopters
- 10 prism diopters
Correct answer: 1 prism diopter
By definition, 1 prism diopter deviates a ray of light 1 cm per 1 meter of distance.
Question 5: A patient's glasses have a base curve of 6.00 D on the front surface and a back surface power of -4.00 D. What is the approximate total lens power?
- +10.00 D
- +2.00 D (Correct answer)
- -2.00 D
- +6.00 D
Correct answer: +2.00 D
Approximate total power equals front surface power plus back surface power: +6.00 + (-4.00) = +2.00 D.
Question 6: Which instrument is used to verify that a finished spectacle lens matches the prescribed prescription?
- Keratometer
- Lensmeter (lensometer) (Correct answer)
- Retinoscope
- Phoropter
Correct answer: Lensmeter (lensometer)
The lensmeter (lensometer or focimeter) measures the back vertex power of a finished spectacle lens.
Question 7: When transposing +2.00 -1.00 x 180 to minus cylinder form, what is the result?
- +1.00 +1.00 x 090 (Correct answer)
- +1.00 -1.00 x 090
- +2.00 +1.00 x 090
- -1.00 +2.00 x 180
Correct answer: +1.00 +1.00 x 090
To transpose: new sphere = algebraic sum (+2.00 + (-1.00) = +1.00), new cylinder = sign change (+1.00), new axis = 90° (180° ± 90°).
COT Optics and Lensometry 2
When performing lensometry on a bifocal lens, where should you position the lens to read the distance power?