Total Internal Reflection
Overview: Total Internal Reflection, also known as TIR is what makes internet connections possible. TIR occurs at something called a critical angle where light cannot escape from one medium to another and therefore bounces back and forth within the medium. With very minimal data loss you can send information via light waves through fiber optics because the light will stay contained within the fiber.
Supplies: Laser pointer, acrylic rod with stands, fiber optics, laser waterfall
Objectives: What does light do when it hits a surface? How does light reflect? What is total internal reflection?
Setup:
- Set up the acrylic rod on the stands
How to run the demo:
- Shine a green laser pointer through the flat end of the rod.
- Change the angle of the laser pointer up and down until you see the light bounce off the sides of the rod. This is the critical angle for Total Internal Reflection (TIR).
- Repeat the above experiment with coiled fiber optics. Touch the laser pointer to one end of the fiber and see that the light comes out the other end! (This may take a little wiggling, but it demonstrates that the light stays inside the fiber even when the fiber is coiled up into any shape.)
Reminder: This demonstration can be done with eye-safe laser pointers, but if you have people directing the lasers themselves be sure to practice general safety so no one looks directly into the laser.
What’s Happening?
The laser light stays within the acrylic rods due to Total Internal Reflection (TIR). This can occur when light reaches a boundary between materials with a higher index of refraction (acrylic, in this case) and a lower index of refraction (air). If the light approaches this boundary at the critical angle, then the light will reflect and stay within the material.
This is a demonstration of how optical fibers work! Light reflects along the inside of the fiber, so the light stays completely contained within the fiber and can be guided and detected at the other end.
Try this:
- The green laser will be more visible in a darker area, so if you cannot see the bounces within the acrylic rod, try blocking overhead lights with a cloth.
- TIR is also demonstrated in the Laser Waterfall, and Snell’s Window.
Learn more: (external links)