Principles of Optical Fiber

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Let’s explore refraction and reflection of light.
• What happens when light is shone diagonally upward from underwater?
• Add some color to the water (such as paint) and shine a laser pointer into it. By changing the angle between the laser beam and the water surface, we can observe how the light behaves.
• When the angle with the water surface is large, part of the light escapes into the air while some is reflected.
  However, when the angle is small, all the light is reflected back into the water and does not escape into the air. This phenomenon is known as total internal reflection.
• When light passes between two different materials, the following relationship applies (see the diagram on the left):
  \[ \frac{\rm{sin}θ_1}{\rm{sin}θ_2}=\frac{n_2}{n_1} \] where θ₁ is the angle of incidence, θ₂ is the angle of refraction, n₁ is the refractive index of the incident medium (e.g., 1.33 for water), and n₂ is the refractive index of the refracted medium (e.g., 1.00 for air).

• The angle of incidence at which light transitions between being refracted into the air and being totally reflected within the water is called the critical angle θc. For the water–air boundary, the critical angle
  θc is 49°, which corresponds to an angle of 41° relative to the water surface. This means that if the angle between the water surface and the light is less than about 41°, the light will not escape into the air but will be totally internally reflected and continue traveling within the water.
• This principle of total internal reflection is the basis for optical fibers. An optical fiber consists of a core with a high refractive index surrounded by cladding with a lower refractive index. Light traveling within the core is reflected entirely at the core–cladding boundary, enabling efficient transmission of optical signals over long distances.
• This video was produced with the support of the JSPS Grant-in-Aid for Scientific Research (18K03956).

[Keywords]	Refraction and reflection of light, total internal reflection
[Related items]	Water as Optical Fiber

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