Ray diagrams, total internal reflection and optical communication

Ray diagrams, total internal reflection and optical communication

A ray diagram is a geometric model, so every line has a meaning. Draw a straight boundary, a normal at the point of incidence, incident and refracted or reflected rays, and arrows showing propagation. For total internal reflection, the incident ray travels from higher to lower refractive index. At the critical angle, the refracted ray is exactly along the boundary; above it, no refracted ray emerges and the ray reflects internally. Optical fibres use repeated total internal reflection to guide light, but practical signal quality can also depend on absorption, bends and coupling losses.

Work it through

Glass with n = 1.5 has light speed v = c/n = 3.0 × 10⁸ / 1.5 = 2.0 × 10⁸ m s⁻¹. A ray from glass to air can undergo total internal reflection at sufficiently high incidence angle; a ray from air into glass cannot. In a diagram, label the inside medium before deciding whether TIR is possible.

Mastery target

Draw a correct ray diagram and apply refractive-index direction, speed and critical-angle conditions to a physical application such as an optical fibre.