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Shine a laser into a bent glass rod. Does the light follow the bend?
Glowing Glass Rod lab · Last updated: 16 October 2026
A laser beam shines into one end of a curved glass rod. Instead of escaping at the bend, the light follows the curve and glows out of the far end. You explore how, and when the trick stops working.
What you'll discover
- Light inside glass can bounce off the inside surface without escaping.
- This total internal reflection only happens when light hits the surface at a shallow enough slant.
- Bend the rod too sharply and some light leaks out.
The science in plain words
When light travels from glass towards air, it bends away from the normal as it leaves. The more slanted the ray, the more it bends. At one particular angle, called the critical angle, the light bends so much that it skims along the surface. For ordinary glass this is about 42 degrees from the normal.
If the ray hits the surface at an even greater slant, it cannot get out at all. Instead it reflects completely back into the glass, as if from a perfect mirror. This is total internal reflection.
In a glass rod or fibre, light meets the walls at a shallow slant, so it bounces from side to side all the way along, following gentle curves. If the rod bends too tightly, some rays hit the wall too steeply and leak out.
Optical fibres are hair-thin strands of very pure glass that use this effect to carry light signals for great distances. Undersea fibre cables carry most of the internet traffic between continents. Doctors use bundles of fibres in endoscopes to see inside the body without surgery. Sparkling diamonds also owe their glitter partly to total internal reflection.
Where it fits
- India, NCERT: Total internal reflection goes beyond the NCERT syllabus for classes 6 to 10; the lab builds on refraction in Class 10 Chapter 9: Light: Reflection and Refraction.
- United States: Grades 9 to 12
Common questions
How does light travel through an optical fibre?
Light inside the fibre hits the walls at a shallow angle and is totally reflected back inside, again and again. It zigzags along the fibre, even round gentle bends, without escaping.
What is total internal reflection?
It happens when light inside a material like glass or water hits the surface at more than the critical angle. Instead of leaving, all of it reflects back inside.
What are optical fibres used for?
They carry internet and phone signals as flashes of light, including across oceans in undersea cables. Doctors also use them in endoscopes to look inside the body.
Related labs
- Bending Light: Shine a ray into a glass block. How much does it bend?
- Candle, Lens and Screen: Slide the screen until the candle is sharp. Why is its picture upside down?
- Rainbow and Beyond: Slide from ultraviolet through the rainbow to infrared. Which light makes the star glow?
See all physics labs or every Taz science lab.
Try it yourself
The Glowing Glass Rod lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.