Optics and Refraction Flashcards

An object is placed 50 cm in front of a +4.00 D thin lens. Where is the image formed?

Answer: 50 cm behind the lens

Object vergence is -2.00 D, so image vergence is -2.00 + 4.00 = +2.00 D, forming a real image 50 cm behind the lens.

A -5.00 D spectacle lens is decentered 3 mm from the visual axis. Per Prentice's rule, how much prism is induced?

Answer: 1.5 prism diopters

Prentice's rule: prism = decentration (cm) x power (D) = 0.3 x 5 = 1.5 prism diopters.

What is the spherical equivalent of +2.00 -3.00 x 090?

Answer: +0.50 D

Spherical equivalent = sphere + half the cylinder = +2.00 + (-1.50) = +0.50 D.

What is the correct plus-cylinder transposition of +1.00 -2.00 x 180?

Answer: -1.00 +2.00 x 090

Add sphere and cylinder for the new sphere, flip the cylinder sign, and rotate the axis 90 degrees.

A patient is corrected with a +10.00 D spectacle lens at a 12 mm vertex distance. What contact lens power is approximately required?

Answer: +11.25 D

Effective power = F/(1 - dF) = 10/(1 - 0.012 x 10) ≈ +11.36 D, so about +11.25 D.

According to Knapp's law, axial ametropia corrected by a spectacle lens placed at the anterior focal point of the eye produces what retinal image size?

Answer: Same as that of an emmetropic eye

Knapp's law states that a lens at the anterior focal point (~15 mm) yields an emmetropic-sized retinal image in axial ametropia.

When refining cylinder axis with a Jackson cross cylinder in a minus-cylinder refraction, which way should the axis be rotated?

Answer: Toward the minus axis of the preferred cross-cylinder position

In minus-cylinder technique, rotate the trial cylinder axis toward the red (minus) dot of the preferred flip position.