Light Propagation, Reflection, and Refraction: A Comprehensive Guide

Posted on Oct 30, 2024 in Physics

Propagation of Light

Light emitted from sources travels through various media. Substances are classified as opaque, transparent, or translucent based on their interaction with light. Opaque substances block light, while transparent substances, like water, glass, and air, allow light to pass through in a single direction. Translucent substances, such as vellum or frosted glass, scatter light, preventing clear image formation.

Light in Homogenous Media

In a homogenous, transparent medium, light travels in straight lines—a fundamental law of geometric optics. To simplify the study of optical phenomena, light sources are often considered point sources emitting light rays. A collection of rays is a beam. Beams from distant sources are considered parallel, while beams from near sources are conical.

Velocity and Refractive Index

Light’s speed is constant in a given homogenous, transparent medium but varies between media. The speed of light in a vacuum is approximately 300,000 km/s. In other transparent media, light travels slower. The absolute refractive index (n) of a medium is the ratio of light’s speed in a vacuum (c) to its speed in the medium (v): n = c/v. A higher refractive index indicates slower light speed.

Relative Refractive Index

The relative refractive index (n12) compares the velocities (v1 and v2) in two different media: n12 = v1/v2 or n12 = n2/n1. An n12 less than 1 means light travels faster in the second medium.

Reflection of Light

Reflection occurs when light changes direction upon striking a surface. Objects are visible due to light reflection. Reflection can be regular (smooth surface) or diffuse (rough surface). Regular reflection, like in a mirror, preserves the beam’s shape. Diffuse reflection scatters the rays.

Laws of Reflection

  1. The incident ray, normal, and reflected ray lie in the same plane.
  2. The angle of incidence equals the angle of reflection.

Refraction of Light

Refraction is the change in light’s propagation direction when it passes obliquely between two transparent media with different refractive indices. Many optical instruments rely on refraction.

Laws of Refraction

  1. The incident ray, normal, and refracted ray lie in the same plane.
  2. Snell’s Law: n1 * sin(e1) = n2 * sin(e2), where n is the refractive index and e is the angle relative to the normal.

Implications of Snell’s Law

If n1 > n2, then e2 > e1 (refraction away from the normal). If n1 < n2, then e2 < e1 (refraction towards the normal). A medium’s refractive index measures its refraction. Denser media have slower light speeds and higher refractive indices.