Total Internal Reflection
Beyond the critical angle, light cannot leave the denser medium.
About this topic
When light travels from a denser to a less dense medium at an angle greater than the critical angle, given by sin θc = n₂/n₁, it cannot escape and is totally reflected. For water-air it's about 49°, for glass-air about 42°.
Optical fibres use this to carry light signals over long distances with little loss, forming the backbone of the internet, and doctors use bundles of fibres in endoscopes to look inside the body. Prisms in binoculars and periscopes use it to reflect light more efficiently than mirrors, and diamonds sparkle because their high index gives a small critical angle of about 24 degrees.
Worked example: for glass of index 1.5 surrounded by air, sin θc = 1/1.5 = 0.667 so θc ≈ 42 degrees. A ray hitting the boundary at 50 degrees stays entirely inside the glass, whereas one hitting at 30 degrees partly escapes. Underwater, you can see a mirror-like surface when you look up at a shallow angle.
Two conditions are needed: the light must go from higher to lower index, and the angle of incidence must exceed θc. A misconception is that total internal reflection can happen going into a denser material, which is impossible. Another is that some light is still lost; unlike a metal mirror, the reflection is essentially perfect.
Key ideas
- Needs n₁ > n₂
- Optical fibres rely on it
- Diamonds sparkle
Quick check
Use cases: where Total Internal Reflection helps
- Homework: review the key idea behind Total Internal Reflection before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Total Internal Reflection without notes.
- Visual learning: play with the interactive lab to see how Total Internal Reflection changes when you adjust its inputs.
- Teaching a friend: explain Total Internal Reflection in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Total Internal Reflection?
Beyond the critical angle, light cannot leave the denser medium.
What will I learn in Total Internal Reflection?
Needs n₁ > n₂. Optical fibres rely on it. Diamonds sparkle.
Is Total Internal Reflection part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Total Internal Reflection lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Total Internal Reflection?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.