Optics and Lensometry Flashcards

Which aberration causes peripheral rays passing through a lens to focus at a different point than paraxial rays, reducing image sharpness?

Answer: Spherical aberration

Spherical aberration occurs because peripheral rays refract more than paraxial rays, creating multiple focal points along the axis.

A concave (minus) lens causes light rays to:

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.

When using the lensmeter to locate the optical center of a lens, what phenomenon indicates that you are centered on the optical center?

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.

What is the recommended sequence for lensometry of a progressive addition lens?

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.

Which law states that the angle of incidence equals the angle of reflection when light strikes a smooth surface?

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.

A toric (astigmatic) lens has two principal meridians. What is the angular relationship between these two meridians?

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.

A spectacle prescription reads -2.00 +1.50 x 090. What is the power in the 090° meridian?

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.