Ophthalmic Optics and Formulas Flashcards

Using Prentice's rule, how much prism is induced when a patient looks 8mm below the optical center of a +3.00 D lens?

Answer: 2.4 prism diopters

Prentice's rule: P = d × F = 0.8 cm × 3.00 = 2.4 prism diopters.

What is the focal length of a +4.00 diopter lens?

Answer: 25 cm

Focal length f = 1/D = 1/4.00 = 0.25 meters = 25 cm.

Light diverging from a point source located 50 cm away has a vergence of:

Answer: -2.00 D

Diverging light has negative vergence; V = -1/0.50 m = -2.00 D.

A lens has a front surface power of +8.00 D and a back surface power of -4.00 D. What is its approximate total power?

Answer: +4.00 D

Approximate total power = +8.00 + (−4.00) = +4.00 D (thin lens approximation).

A prism with its base at 270° will displace the image in which direction?

Answer: Upward

The image is displaced toward the prism apex; base at 270° (down) means the apex is up, so the image shifts upward.

Using the lensmaker's equation with n=1.50, r₁=+200 mm, and r₂=+400 mm, what is the approximate lens power?

Answer: +1.25 D

F = (n−1)(1/r₁ − 1/r₂) = 0.50 × (5.00 − 2.50) = +1.25 D.

If a patient needs 2 prism diopters base-in for each eye, what is the total convergence demand reduced?

Answer: 4 prism diopters

Base-in prism in both eyes reduces convergence demand; 2Δ BI OD + 2Δ BI OS = 4Δ total.