Total Internal Reflection

Definition

The complete reflection of a light ray back into a denser medium when it strikes the boundary with a rarer medium at an angle greater than the critical angle.

Key points

• It occurs only when light travels from a denser to a rarer medium (for example glass to air or water to air) and the angle of incidence exceeds the critical angle.
• The critical angle is the angle of incidence at which the refracted ray grazes the boundary at 90°; beyond it, no light refracts out and all of it reflects back.
• Optical fibres rely on it to carry light signals over long distances with very little loss, the backbone of high-speed internet and telecommunication.
• It explains the brilliance and sparkle of diamonds (which have a very small critical angle) and the formation of mirages on hot roads.
• Endoscopes used in medicine and periscopes (with prisms) also use total internal reflection.

Why it matters for CAPF

Total internal reflection, the critical angle, optical fibres, mirages, and the sparkle of diamonds are recurring optics facts, and optical-fibre communication links to secure data networks.

Common confusion

It happens only going from a denser to a rarer medium, and only above the critical angle; ordinary partial reflection happens at any angle. A mirage is caused by refraction through air layers leading to total internal reflection, not by an actual water surface.

Related concepts

concept optical fibre, concept electromagnetic spectrum

Parent note

physics everyday