- Does the evanescent wave transport energy across the interface?
- No, under the ideal conditions modeled here (perfect dielectrics, infinite plane wave), the time-averaged energy flow normal to the interface is zero. The evanescent field stores reactive energy near the surface. However, if a third medium (like a prism or a fluorescent molecule) is brought close to the interface, it can couple to this field, allowing energy transfer in a process called frustrated total internal reflection (FTIR).
- Why is the penetration depth important in real-world technology?
- The precise, exponential decay of the evanescent field makes it an exquisite probe of surfaces. In Total Internal Reflection Fluorescence (TIRF) microscopy, it selectively excites fluorescent molecules within ~100 nm of a cell's membrane, providing exceptional background rejection. In optical fiber sensors, changes in the evanescent field due to external substances alter the guided light, enabling detection of chemicals or biological agents.
- What is a key limitation of this simplified model?
- This model assumes an infinite plane wave and a perfectly smooth interface. In reality, laser beams are finite, which means some light can 'tunnel' across a small gap even during TIR (the Goos-Hänchen shift). Furthermore, if the rarer medium is absorptive (has a complex refractive index), the evanescent wave can transfer energy and heat the medium, a principle used in attenuated total reflection (ATR) spectroscopy.
- How does the wavelength of light affect the evanescent wave?
- The penetration depth d_p is directly proportional to the incident wavelength λ. For a given angle and refractive indices, red light (longer λ) will penetrate farther into the rarer medium than blue light (shorter λ). This scaling is explicit in the equation d_p = λ / (4π √(n₁² sin²θ_i - n₂²)).