2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 1: Geometric Optics
Prisms
Critical Angle; Total Internal Reflection
Most applications of Snell’s law envision light traveling from a less dense (lower n) medium, such as air, into a more dense (higher n) medium, such as water or glass. In this situation, as we have seen, the light bends toward the surface normal. If the direction of light transmission is reversed, the light will bend away from the surface normal as it enters the less dense medium. However, if the angle of incidence exceeds the critical angle specified by the formula sin θcrit = n2/n1, Snell’s law cannot be satisfied, as no angle has a sine greater than 1.0. The result is total internal reflection (TIR)—the light is totally reflected at the interface and cannot pass from the denser medium to the less dense medium (Figure 1-8). For an interface between air and an aqueous tissue, the critical angle is about 48°. This prevents a view of the anterior segment angle from the exterior of the eye without optical tricks, such as the use of a goniolens (Figure 1-9).
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.