Flat Refracting Surfaces—Snell’s Law
The optics of a flat refracting interface, such as the surface of a still pool of water or a flat slab of glass, are easy to describe. As light passes from a “less dense” medium (lower refractive index, greater speed of light) to a denser medium (higher refractive index, lesser speed of light), light rays bend toward the line perpendicular to the interface at the point of entry (the surface normal, or just the “normal line”), according to Snell’s law (Figure 1-1):
1 sin θ1 = n2 sin θ2
Figure 1-1 Snell’s law. The angle of incidence θ1 is defined by the incident ray and the surface normal (dashed line). The angle of refraction θ2 is defined by the refracted ray and the surface normal. The 4 variables n1, n2, θ1, and θ2 are related by Snell’s law, n1 sin θ1 = n2 sin θ2. When n1 < n2 (ie, the speed of light is reduced when rays cross the interface from left to right), the refracted ray bends toward the surface normal; when n1 > n2, the refracted ray bends away from the normal.
(Illustration developed by Edmond H. Thall, MD.)
Figure 1-2 The fisherman must throw the spear in front of the virtual fish to hit the actual fish.
(Illustration developed by Kevin M. Miller, MD, rendered by Jonathan Clark, and modified by Neal H. Atebara, MD.)
If light travels from a denser medium to a less dense medium, light rays bend away from the surface normal.
This redirection of light at a flat interface produces an apparent object displacement, such as that seen when you look into a body of water (Figure 1-2) or that provided by an ophthalmic prism.
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.