Evanescent Wave
Definition
A wave that fades exponentially instead of traveling — it appears just past a boundary during total internal reflection.
Technical definition
When the wave number becomes imaginary (e.g., beyond a TIR interface), the field decays as e^(−κx) with penetration depth ~λ/2π. It carries no net energy but enables tunneling.
Example
Fingerprint sensors use evanescent waves — your ridges disturb the decaying field at the glass surface, revealing the print pattern.
Common confusion
Evanescent waves are not 'failed' waves. They are legitimate solutions of the wave equation, and quantum tunneling is their close cousin.