Ophthalmic Optics and Formulas Flashcards

Transpose +2.00 −1.50 × 090 into plus cylinder form. What is the result?

Answer: +0.50 +1.50 × 180

New sphere = +2.00 + (−1.50) = +0.50; new cylinder = +1.50; new axis = 090 + 90 = 180.

A +10.00 D lens is moved 10 mm farther from the eye. What is the approximate effective power at the eye?

Answer: +11.11 D

F_eff = F / (1 − d×F) = 10 / (1 − 0.010 × 10) = 10 / 0.90 ≈ +11.11 D.

Using the simplified sag formula s ≈ y²/(2r), what is the sag when y = 30 mm and r = 200 mm?

Answer: 2.25 mm

s = (30²) / (2 × 200) = 900 / 400 = 2.25 mm.

What is the correct minus cylinder transposition of −3.00 +2.00 × 045?

Answer: −1.00 −2.00 × 135

New sphere = −3.00 + 2.00 = −1.00; new cylinder = −2.00; new axis = 045 + 90 = 135.

What is the spherical equivalent of −2.00 −1.50 × 180?

Answer: −2.75 D

SE = sphere + (cylinder / 2) = −2.00 + (−1.50 / 2) = −2.00 − 0.75 = −2.75 D.

For the prescription −2.00 −1.00 × 090, what is the power in the 180° meridian?

Answer: −3.00 D

With cylinder axis at 090, maximum power acts at 180°: −2.00 + (−1.00) = −3.00 D.

When light travels from air (n = 1.00) into glass (n = 1.50) at an oblique angle, it bends toward the normal because:

Answer: The speed of light decreases in glass

Light slows down in a denser medium, causing it to refract toward the normal per Snell's law.