E1 Light Propagation and Basic Geometric Optics

Topic

Light emanating from an object travels in a straight line—following what this model calls an optical ray—within a homogeneous medium (a medium whose optical properties are uniform throughout). Each point on the object simultaneously emits numerous optical rays in various directions; as long as these rays remain within the same homogeneous medium, they continue to travel in a straight line without deviating, regardless of the distance covered.

An optical ray ceases to travel in a straight, undeviated line when it reaches an interface: the boundary where one homogeneous medium ends and a different one—with a distinct refractive index—begins. The refractive index of a homogeneous medium describes the extent to which the medium slows down the light traveling within it, relative to the speed that light would have in a vacuum; the higher the refractive index, the slower the light travels within that medium. The angle at which an optical ray strikes an interface—measured relative to the line perpendicular to the interface at the point of impact—is known as the angle of incidence.

When an optical ray crosses an interface into a homogeneous medium with a refractive index different from that of the medium it is leaving, the ray changes direction exactly at the point of crossing; this change in direction is called refraction. The greater the difference between the refractive indices of the originating and destination media, the greater the change in direction the ray undergoes upon refraction; if a ray enters a medium with a higher refractive index, it bends toward the line perpendicular to the interface, whereas if it enters a medium with a lower refractive index, it bends away from that perpendicular line. At each interface, an optical ray does not refract entirely into the new homogeneous medium: a portion of the ray reflects back into the original medium without crossing the interface—a phenomenon known as reflection—while the remainder crosses the interface and refracts into the new medium—a phenomenon known as transmission. The greater the difference in the refractive indices of the two media meeting at an interface, the larger the portion of the ray reflected at that interface and the smaller the portion transmitted to the other side.

When light emanating from an object passes through a series of interfaces—such as those formed by the cornea and the lens before reaching the retina—the refraction experienced by each ray at each interface collectively determines where that ray is ultimately directed. It is the combination of all rays originating from the same point on the object—refracted in the same manner at each interface—that determines whether or not they converge again at a single point on the retina. Consequently, this model involving optical rays, refraction, reflection, and transmission serves as the foundation for all subsequent calculations regarding image formation and the design specifications required for a lens to ensure the image forms exactly where needed.

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