2.5 - Evanescent Wave
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Understanding Total Internal Reflection
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Today, we will explore total internal reflection and its implications. Can anyone tell me what total internal reflection is?
Isn't it when light hits a surface at an angle greater than the critical angle and reflects completely back?
Exactly! Great job, Student_1. So, when that happens, what kind of wave exists just beyond the surface?
Is it the evanescent wave?
Yes! The **evanescent wave**. Remember, this wave does not carry energy away from the interface, but it plays a significant role in optical devices. Can anyone think of an application of evanescent waves?
Fiber optics! They use light that reflects internally.
Absolutely right! In fiber optics, the evanescent wave helps to couple light into the fiber. This concept is quite crucial in modern optical technology.
Characteristics of Evanescent Waves
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Let's dig deeper into some characteristics of evanescent waves. Can anyone describe how this wave decays?
I think the field decays exponentially as you move away from the interface?
Correct, Student_4! The intensity of the wave decreases rapidly, leading to negligible energy carrying. What does this imply for applications like fiber optics?
It means that the wave can still interact with surrounding mediums, facilitating coupling?
Exactly! This interaction is essential for precise control of light in these applications.
Applications of Evanescent Waves
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Letβs explore applications again. Why are evanescent waves significant in fiber optics?
They allow light to be contained within optical fibers without losing too much intensity.
Exactly! They enable efficient transmission of data over long distances. Frustrated total internal reflection also utilizes these waves when light couples to external media. Can anyone think of a scenario of this?
Something like sensing? Where you want to detect changes in an external medium?
Great connection! The ability of evanescent waves to interact with nearby substances makes them invaluable in sensor technologies.
Introduction & Overview
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Quick Overview
Standard
This section discusses the characteristics of evanescent waves, which exist just beyond the boundary interface during total internal reflection. These waves do not carry energy away but can play crucial roles in applications such as fiber optics and frustrated total internal reflection.
Detailed
Evanescent Wave
An evanescent wave is a unique phenomenon that arises during total internal reflection of light at an interface between two media of different refractive indices. When light travels from a denser medium to a rarer one and exceeds the critical angle, it reflects entirely back into the denser medium. However, interestingly, even at this interface, there exists a field in the rarer medium that does not propagate energy away from the interface. This field decays exponentially with distance from the boundary and is vital for various optical applications, including fiber optics and frustrated total internal reflection. Understanding this concept is essential for grasping how light can be manipulated in technological contexts.
Audio Book
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Evanescent Wave Definition
Chapter 1 of 2
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Chapter Content
Even when light undergoes total internal reflection, a non-zero field exists just beyond the interface in the rarer medium.
Detailed Explanation
An evanescent wave is a wave that appears at the interface of two different optical media when total internal reflection occurs. When light hits the boundary between a denser medium (like glass) and a rarer medium (like air) at a certain high angle, instead of passing through, it reflects back entirely into the denser medium. However, interestingly, just at the surface, a small portion of the wave continues into the rarer medium, creating what's called an evanescent wave. This wave doesn't propagate far into the rarer medium but has a defined existence close to the interface.
Examples & Analogies
Imagine a trampoline with a person bouncing on it. The highest point of the person's jump represents total internal reflection; while they donβt go through the trampoline (like light reflecting), the energy exerted on the trampoline creates ripples that extend briefly into the surrounding air, similar to how the evanescent wave exists just outside the reflective boundary.
Characteristics of Evanescent Waves
Chapter 2 of 2
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Chapter Content
This field:
β Does not carry energy away
β Decays exponentially
β Enables frustrated TIR and fiber optics coupling
Detailed Explanation
Evanescent waves have distinct features. First, they do not transport energy away from the boundary, which means that despite their existence, they won't contribute to energy transfer in the medium. Secondly, the intensity of the evanescent wave decays exponentially with distance from the interface, meaning that the wave becomes weaker very quickly as you move away from where it was created. Lastly, evanescent waves play an important role in technologies like fiber optics. They can couple light from one medium to another with efficiency, allowing signals to be transmitted through fibers without significant loss of information.
Examples & Analogies
Consider dipping a stick into a pool of water. The stick shows signs of bending due to refraction at the water's surface (like the evanescent wave). If you measure the bending, it diminishes rapidly as you move further from the surface, similar to how the evanescent wave's intensity decreases with distance. This bending is akin to how evanescent waves might work in fiber optics, allowing light to bend and travel efficiently through the medium.
Key Concepts
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Evanescent Wave: A non-propagating wave that exists just beyond the surface of a medium, decaying exponentially.
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Total Internal Reflection: Occurs when light reflects entirely within one medium as the angle exceeds the critical angle.
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Critical Angle: The specific angle of incidence beyond which total internal reflection occurs.
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Fiber Optics: Technology that leverages total internal reflection and evanescent waves for efficient light guiding.
Examples & Applications
In fiber optics, light is guided through glass fibers due to total internal reflection, with evanescent waves allowing efficient light coupling.
A sensor that uses frustrated total internal reflection can detect changes in a surrounding medium through its interaction with evanescent waves.
Memory Aids
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Rhymes
Evanescent waves don't go far, near the surface - thatβs where they are!
Stories
Imagine a party at the shoreline where guests can't leave the sand; the light shines but is trapped inside, creating waves that are close at hand.
Memory Tools
Every Time I Reflect - Evanescent Waves Abound! (ETIR-EWAB)
Acronyms
EWR - Evanescent Waves Reflect, indicating they don't propagate energy.
Flash Cards
Glossary
- Evanescent Wave
A wave that is present in the medium just beyond the boundary of a total internal reflection but does not carry energy away from the interface.
- Total Internal Reflection
The phenomenon where light is completely reflected within a medium when it hits a boundary at an angle greater than the critical angle.
- Critical Angle
The minimum angle of incidence at which total internal reflection occurs.
- Frustrated Total Internal Reflection
A process where evanescent waves enable light to be coupled with an external medium.
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