Optics and Lensometry Flashcards

According to Snell's Law, when light passes from a medium with a lower index of refraction to a higher index, it:

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.

A patient presents with glasses marked OD: +1.50 -0.75 x 045. What is the power in the 135° meridian?

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.

What is the unit used to measure prismatic effect in spectacle lenses?

Answer: Prism diopter (Δ)

Prismatic effect in ophthalmic optics is measured in prism diopters (Δ), where 1Δ = 1 cm deviation per meter.

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?

Answer: 1.20 prism diopters

Prentice's Rule: P = d (cm) × F = 0.4 cm × 3.00 D = 1.20 prism diopters.

Which surface of a spectacle lens does the lensmeter measure when the lens rests against the lens stop?

Answer: Back vertex power

Standard lensometry measures back vertex power because the back surface of the lens rests against the lensmeter stop.

A high-index lens material with n=1.74 will be thinner than a standard CR-39 lens (n=1.50) because:

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.

If the axis drum of the lensmeter reads 55°, in which direction does the cylinder axis of the lens run?

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.