Total Internal Reflection
Total internal reflection occurs when light hits a boundary inside a denser medium above the critical angle and reflects completely.
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Total internal reflection occurs when light hits a boundary inside a denser medium above the critical angle and reflects completely.
- Needs light moving to a less dense medium
- The angle must exceed the critical angle
- Used in optical fibres
🧠 Quick quiz
Three questions. Score 2 or more to complete the topic.
What is Total Internal Reflection?
Within Physics, which covers forces, energy, waves, electricity, magnetism, light and space explained through motion and experiments, this topic explains Total Internal Reflection. Physics topics explain how things move, push, heat up and carry energy, so each one here pairs a definition with a live model you can change.
Check that you can explain Total Internal Reflection in your own words and give one example from everyday life. If you can do that, you have understood the core idea.
Understanding Total Internal Reflection explains how the physical world behaves, from sport and transport to electricity and medicine.
Total Internal Reflection in detail
Total internal reflection (TIR) happens when light travelling in a denser medium hits the boundary with a less dense medium at an angle greater than the critical angle, and all of it reflects back inside. The critical angle satisfies sin c = 1/n.
TIR is the basis of optical fibres, which carry telephone and internet signals as light pulses over long distances with little loss, and of endoscopes used to look inside the body. It also makes reflective prisms in binoculars and cat's-eyes, and contributes to the sparkle of diamonds, which have a small critical angle of 24°.
How the interactive helps you understand Total Internal Reflection
Watch the light rays in the interactive. Use any sliders or buttons it offers to change the values and see how the light rays responds.
Before you change anything, predict the result. Then change one value at a time and check whether your prediction was right. This is a quick way to build a correct mental model of Total Internal Reflection.
Real-life uses and examples
- Use Total Internal Reflection to make sense of a practical and its results.
- Answer “explain” questions by linking Total Internal Reflection to a clear cause and effect.
- Predict what a ball, car or circuit will do with Total Internal Reflection before you do the calculation.
Common mistakes to avoid
- Mixing up Total Internal Reflection with a related quantity that has different units.
- Forgetting that some quantities, such as force or velocity, also have a direction.
- Learning the definition without testing it. Use the quiz and the interactive to check that you can apply the idea.
Frequently asked questions about Total Internal Reflection
What is Total Internal Reflection?
Total internal reflection occurs when light hits a boundary inside a denser medium above the critical angle and reflects completely.
What are the most important points about Total Internal Reflection?
The main points are: Needs light moving to a less dense medium; The angle must exceed the critical angle; Used in optical fibres.
Why is Total Internal Reflection useful?
Understanding Total Internal Reflection explains how the physical world behaves, from sport and transport to electricity and medicine.
How can I practise Total Internal Reflection?
Open the interactive at the top of this page and change the controls one at a time. Predict the result before each change, then take the three-question quiz. A score of two or more marks the topic as complete on this device.
Which topics are related to Total Internal Reflection?
Related topics in Physics include Dispersion and Colour, Static Electricity, Resistance and Ohm's Law.