# What is an evanescent field?

**URL:** <https://forum.ofscn.net/t/what-is-an-evanescent-field/1826>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [June 3, 2026, 2:08pm UTC](https://forum.ofscn.net/t/what-is-an-evanescent-field/1826 "2026-06-03T14:08:49Z")\
**Posts on this page:** 2\
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**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\
**Post date:** [June 3, 2026, 2:08pm UTC](https://forum.ofscn.net/t/what-is-an-evanescent-field/1826/1 "2026-06-03T14:08:49Z")

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I heard that light runs a little outside of glass, is that true? Can it be used to measure liquids?

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**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\
**Post date:** [June 3, 2026, 2:09pm UTC](https://forum.ofscn.net/t/what-is-an-evanescent-field/1826/2 "2026-06-03T14:09:19Z")

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This is a classic and interesting physics phenomenon. **“Light travels a little bit outside the glass” is true**. In optical engineering, this phenomenon is called the **evanescent field** (also translated as decaying wave or evanescent wave). It is not only a real physical existence but is also widely applied in modern optical liquid sensors.

Here is a rigorous academic explanation from the perspectives of physical principles, liquid measurement mechanisms, and practical engineering applications:

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### I. What is the “Evanescent Field”? (Physical Essence)

When light undergoes **total internal reflection (TIR)** at the interface between a medium with a higher refractive index n\_1 (e.g., the core glass of an optical fiber) and a medium with a lower refractive index n\_2 (e.g., the cladding of an optical fiber, air, or liquid), the light energy is not instantaneously and 100% cut off and reflected back at the interface.

In wave optics, light, as an electromagnetic wave, must have a portion of its electromagnetic field penetrate into the second medium to satisfy the continuity conditions of Maxwell’s equations at the boundary. This portion of the electromagnetic field propagates along the interface but decays **exponentially rapidly** in the direction perpendicular to the interface (outward from the glass). This electromagnetic field, which does not radiate energy outwards and exists only within a very short distance from the interface, is the **evanescent field**.

Its electromagnetic field intensity decays with increasing penetration distance z. The **penetration depth** d\_p of the evanescent field (i.e., the depth at which the amplitude decays to 1/e of its value at the interface) can be calculated using the following formula:

d\_p = \frac{\lambda}{2\pi \sqrt{n\_1^2 \sin^2\theta - n\_2^2}}

_Where: \lambda is the wavelength of the incident light, \theta is the angle of incidence, and n\_1 and n\_2 are the refractive indices of the high and low refractive index media, respectively._

Under normal circumstances, this penetration depth d\_p is in the **nanometer to micrometer range** (typically a few hundred nanometers to about 1 micrometer). This means that light does indeed