COT Optics and Lensometry 5 — Questions and Answers
Question 1: Which aberration causes peripheral rays passing through a lens to focus at a different point than paraxial rays, reducing image sharpness?
- Coma
- Spherical aberration (Correct answer)
- Distortion
- Field curvature
Correct answer: Spherical aberration
Spherical aberration occurs because peripheral rays refract more than paraxial rays, creating multiple focal points along the axis.
Question 2: A concave (minus) lens causes light rays to:
- Converge toward a real focal point
- Diverge as if coming from a virtual focal point (Correct answer)
- Have no effect on light direction
- Produce a real, inverted image
Correct answer: Diverge as if coming from a virtual focal point
A concave lens diverges light, and the divergent rays appear to originate from a virtual focal point on the same side as the incoming light.
Question 3: When using the lensmeter to locate the optical center of a lens, what phenomenon indicates that you are centered on the optical center?
- Maximum brightness of the mires
- No prismatic displacement of the mire target (Correct answer)
- The mires appear at their sharpest focus
- The axis wheel reads zero
Correct answer: No prismatic displacement of the mire target
At the optical center, no prism is present, so the mire target is centered in the reticle without displacement.
Question 4: What is the recommended sequence for lensometry of a progressive addition lens?
- Read near zone first, then distance zone
- Read distance zone first, then move to near zone to calculate add power (Correct answer)
- Read only the corridor power
- Read the intermediate zone only
Correct answer: Read distance zone first, then move to near zone to calculate add power
Standard procedure reads the distance power first from the upper portion, then the near power from the lower reading zone to calculate the add.
Question 5: Which law states that the angle of incidence equals the angle of reflection when light strikes a smooth surface?
- Snell's Law
- The Law of Reflection (Correct answer)
- Prentice's Rule
- Lambert's Law
Correct answer: The Law of Reflection
The Law of Reflection states that the angle of incidence equals the angle of reflection, both measured from the normal to the surface.
Question 6: A toric (astigmatic) lens has two principal meridians. What is the angular relationship between these two meridians?
- They are parallel
- They are perpendicular (90° apart) (Correct answer)
- They are 45° apart
- They can be at any angle
Correct answer: They are perpendicular (90° apart)
The two principal meridians of a toric lens are always perpendicular to each other, corresponding to the maximum and minimum power meridians.
Question 7: A spectacle prescription reads -2.00 +1.50 x 090. What is the power in the 090° meridian?
- +1.50 D
- -2.00 D (Correct answer)
- -0.50 D
- +3.50 D
Correct answer: -2.00 D
The 090° meridian is the cylinder axis meridian, which carries only the sphere power of -2.00 D with no cylinder contribution.
COT Optics and Lensometry 5
Which aberration causes peripheral rays passing through a lens to focus at a different point than paraxial rays, reducing image sharpness?