Key relationship
sin c = n₂/n₁ for n₁>n₂; TIR if incidence > c.
Physics · Class 12
TIR and fibre guidance
Ray Optics & Optical Instruments
Total internal reflection needs both a higher starting index and incidence beyond the critical angle. Repeated reflections can guide light through an ideal optical fibre.
Open the interactive experiment →TIR and fibre guidance
முழு அக எதிரொளிப்பிற்கு உயர்ந்த ஒளிவிலகல் எண்ணிலிருந்து தாழ்ந்த எண்ணுக்குச் செல்ல வேண்டும்; படுகோணம் மாறுநிலைக் கோணத்தை விடப் பெரியதாக வேண்டும்.
Key notes, relationships and model limits
Remember
Total internal reflection needs both a higher starting index and incidence beyond the critical angle. Repeated reflections can guide light through an ideal optical fibre.
Common trap
A large incidence angle alone is insufficient when light enters a denser medium.
Model limits
Lossless interface; light travels from n₁ to n₂. At the critical angle the transmitted ray is tangent, not total reflection. Fibre cladding must have lower index than core.
What you will learn
Recognize the TIR conditions and connect them to fibre guidance.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
At30° the ray refracts; at60° it reflects internally.
Why this matters
The model connects tir and fibre guidance with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
For glass1.5 to air1, the critical angle is approximately?
- 60°
- 41.81°
- 90°
Read the explanation
c=sin⁻¹(1/1.5)=41.81°.
Course connection: Class XII Chapter 9. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.