Ophthalmic Optics and Formulas Flashcards

Using Snell's law, if light strikes glass (n = 1.50) at 30° from the normal in air (n = 1.00), what is the approximate angle of refraction?

Answer: 19.5°

n₁ sin θ₁ = n₂ sin θ₂ → sin θ₂ = sin 30° / 1.50 = 0.333, so θ₂ ≈ 19.5°.

What is the critical angle for total internal reflection in a material with n = 1.50?

Answer: 41.8°

sin θc = 1/n = 1/1.50 = 0.667; θc = arcsin(0.667) ≈ 41.8°.

A lens material with an Abbe value of 30 has more chromatic aberration than a material with an Abbe value of:

Answer: 58

Higher Abbe values indicate less chromatic dispersion; 58 > 30, so that material has less aberration.

The index of refraction of a material is defined as:

Answer: The ratio of the speed of light in vacuum to the speed of light in the medium

n = c / v, where c is the speed of light in vacuum and v is the speed in the medium.

Standard CR-39 plastic (allyl diglycol carbonate) has an index of refraction of approximately:

Answer: 1.49

CR-39 has an index of approximately 1.499 (commonly cited as 1.49 or 1.50).

Compared to n = 1.50 for the same prescription, a lens made from n = 1.74 material will be:

Answer: Thinner and lighter

Higher index materials allow thinner lens profiles for the same power, reducing both thickness and weight.

The Abbe number (V) is calculated as V = (nD − 1) / (nF − nC). This formula quantifies:

Answer: Chromatic dispersion — higher values mean less dispersion

The Abbe number measures how much a material disperses light by wavelength; higher V means less chromatic aberration.