Understanding Frustrated Total Internal Reflection
Frustrated Total Internal Reflection (FTIR) occurs when an evanescent wave, which forms during total internal reflection (TIR) at a boundary between a denser and a less dense medium, interacts with a second medium placed extremely close to the reflecting interface. This interaction allows light to 'tunnel' across the small gap, effectively 'frustrating' the complete reflection that would normally happen and causing partial transmission.
Key Principles and Components of FTIR
When light undergoes TIR, it generates an evanescent wave that does not propagate but penetrates a very short distance (typically a few wavelengths) into the less dense medium. If a second denser medium is introduced within this evanescent wave's penetration depth, the wave couples with it. This coupling transfers energy, allowing a portion of the light to be transmitted through the thin gap into the second medium, even though the critical angle for TIR was met.
A Practical Example of FTIR
Consider a light beam traveling through a prism, internally reflecting off one face at an angle greater than its critical angle (Total Internal Reflection). If you bring a second identical prism almost touching that reflecting face, some of the light will jump the tiny air gap and enter the second prism, rather than being entirely reflected within the first. This 'leaking' of light is a direct observation of Frustrated Total Internal Reflection.
Importance and Applications in Technology
FTIR is a crucial principle underlying various optical technologies. It is widely used in optical sensors, such as modern fingerprint scanners, where the contact of a finger ridge allows light transmission due to FTIR, while valleys cause TIR. It also finds applications in optical switches, modulators, and specific fiber optic components, enabling precise control over light pathways based on proximity and contact.