This diagram illustrates the phenomenon of total internal reflection, where a standing wave is formed in a dense medium and an evanescent wave is created in a rarer medium.

•Standing Wave: This wave is formed in the denser medium ( •n 1

𝑛 1

) due to the interference between the incident and reflected waves. The wave oscillates in time, but its peak amplitude profile remains stationary in space. The wavelength of the standing wave is denoted by  •λ e x

𝜆 𝑒 𝑥

. •Evanescent Wave: This is a non-propagating electromagnetic field that exists in the rarer medium ( •n 2

𝑛 2

) near the interface. Its amplitude,  •E

𝐸

, decreases exponentially with distance ( •z

𝑧

) from the interface. The characteristic distance over which the amplitude falls to  •1 / e

1 / 𝑒

of its value at the interface ( •E 0

𝐸 0

) is called the penetration depth, denoted by  •d p

𝑑 𝑝

. The evanescent wave does not carry energy away from the interface but its presence is crucial for the reflection process.

Quantum tunneling occurs when a wave, acting as a potential barrier, forms an evanescent field that decays exponentially but still allows a portion of the wave to penetrate and emerge on the other side with reduced amplitude, a phenomenon that applies to both quantum particles and classical waves like light