Optics and Refraction Flashcards

A patient's refraction is -4.50 +1.00 x 090. To correct this refractive error, what type of lens is required?

Answer: A sphero-cylindrical concave lens

The spherical component of the refraction is -4.50 diopters, which indicates myopia. Myopia is corrected with a concave (minus power) lens. The "+1.00 x 090" component indicates astigmatism, requiring a cylindrical correction, making the overall lens a sphero-cylindrical concave lens.

A patient has a steep vertical corneal meridian and a flat horizontal meridian. This type of astigmatism is known as:

Answer: With-the-rule astigmatism

With-the-rule (WTR) astigmatism is the most common type, where the vertical corneal meridian is steeper (has more power) than the horizontal meridian. This is typically corrected with a minus cylinder lens at or near the 180-degree axis.

Snell's Law (n1 * sin(θ1) = n2 * sin(θ2)) is a fundamental principle in optics that describes:

Answer: The refraction of light as it passes between two different media.

Snell's Law describes the relationship between the angles of incidence (θ1) and refraction (θ2) and the refractive indices (n1, n2) of two different isotropic media. It governs how much a light ray bends when it crosses the boundary from one medium to another, which is the fundamental basis for how lenses work.

What is the primary physiological cause of presbyopia?

Answer: Decreased elasticity and hardening of the crystalline lens.

Presbyopia is the age-related loss of accommodation. The primary cause is the progressive hardening (sclerosis) and loss of elasticity of the crystalline lens, which prevents it from changing shape sufficiently to focus on near objects, even with normal ciliary muscle contraction.

During retinoscopy on an uncorrected myopic eye, the examiner working at a standard distance will observe:

Answer: "Against" motion when working outside the far point.

The far point of a myopic eye is a real point in space in front of the eye. When the retinoscope is positioned outside this far point (as is typical at a standard working distance), the light reflex will move in the opposite direction to the streak, which is termed "against" motion.

The effect of vertex distance is most clinically significant when prescribing spectacles for which of the following refractive errors?

Answer: -12.00 D sphere

Vertex distance is the distance between the back surface of the spectacle lens and the front of the cornea. Its effect on the effective power of the lens is negligible for low powers but becomes clinically significant for powers greater than +/- 4.00 diopters. For a high myope (-12.00 D), a small change in vertex distance can significantly alter the power delivered to the eye.

A Jackson Cross Cylinder (JCC) is used during subjective refraction to:

Answer: Refine the axis and power of the astigmatic correction.

The Jackson Cross Cylinder is a lens that combines an equal-power plus and minus cylinder with axes 90 degrees apart. It is used to refine both the axis of the correcting cylinder by straddling the estimated axis and the power of the cylinder by aligning its axes with the cylinder's axis.