Part 2

Total Internal Reflection (TIR): The Physics behind Optical Fiber

About the Author:
Prof. Chandrakant Kali (M.Sc., M.Ed.) is a retired Physics professor with over 50 years of teaching experience. Through Physics Prana, he simplifies complex Physics concepts using clear explanations, practical examples, and easy-to-understand diagrams, making learning enjoyable for students and lifelong learners.

1.Introduction:

       In Part 1 of this Optics series, we learned about light, reflection, and refraction. We discovered that light changes its direction whenever it enters a different medium because its speed changes.

But have you ever wondered:

• Why does a diamond sparkle so brilliantly?
• How does an endoscope allow doctors to see inside the human body?
• How can light travel through a thin glass fiber for hundreds of kilometers without escaping?

The answer lies in one fascinating optical phenomenon known as Total Internal Reflection (TIR).

   This principle has revolutionized modern communication and made Optical Fiber Technology possible.

2.What is Total Internal Reflection (TIR)?

       When light travels from a denser medium to a rarer medium, it bends away from the normal.

As the angle of incidence increases, the angle of refraction also increases.

      At a particular angle, the refracted ray travels exactly along the boundary between the two media.

This special angle is called the Critical Angle.

    If the angle of incidence becomes greater than the Critical Angle, the light does not emerge into the second medium. Instead, it is completely reflected back into the denser medium.

This phenomenon is known as Total Internal Reflection (TIR).

3.Understanding the Critical Angle:

   The Critical Angle (i_c) is the angle of incidence in the denser medium for which the angle of refraction becomes 90°.

Beyond this angle, refraction is no longer possible.

The critical angle satisfy the condition,

sini_c = n₂ / n₁ 

Where

n₁ is refractive index of denser medium

n₂ is refractive index of rarer medium

4.Conditions for Total Internal Reflection:

Two conditions must be satisfied:

   1. Light must travel from a denser medium to a rarer medium.

Examples

• Glass → Air
• Water → Air
• Diamond → Air

  2. Angle of Incidence

The angle of incidence must be greater than the Critical Angle.

If both conditions are satisfied,

Total Internal Reflection occurs.

5.Fig. C explains the complete process of Total Internal Reflection:

Fig. C

Case A

When Light strikes normally then No bending occurs.

Case B

 For small angle of incidence, Light refracts into air.

Case C

By increasing angle incidence . There is increase Refraction angle.

Case D

At the Critical Angle i_c, The refracted ray travels along the surface.

Case E

At Angle greater than Critical Angle (i  > i_c ) , No refraction. Complete reflection inside the glass.

This is Total Internal Reflection.

6.Why Does Total Internal Reflection Occur?

        When light tries to move from a denser medium into a rarer medium, it accelerates.

Beyond the Critical Angle, there is no physically possible refracted path.

Therefore, the entire light energy is reflected back into the denser medium.

Unlike an ordinary mirror, almost 100% of the light energy remains inside the medium.

7.Everyday Examples of TIR:

  7.1. Diamond Sparkle:

Diamonds have a very high refractive index.

Light undergoes multiple total internal reflections before emerging.

This gives diamonds their exceptional brilliance.

 7. 2. Mirage:

On a hot summer day, roads often appear wet.

This illusion occurs because light undergoes refraction and, under suitable atmospheric conditions, total internal reflection in layers of air.

 7.3. Right-Angle Prism:

Many binoculars and periscopes use prisms instead of mirrors because total internal reflection produces brighter images.

 7.4. Endoscope:

Doctors use flexible optical fibers inside endoscopes.

Light travels through these fibers by repeated total internal reflection.

This allows doctors to examine internal organs without major surgery.

 7.5. Optical Fiber:

The most important application of TIR is optical fiber communication.

Billions of light pulses travel through optical fibers every second with very little loss of energy.

8.Advantages of Total Internal Reflection:

• Nearly 100% reflection
• Very little energy loss
• Produces brighter images
• No mirror coating required
• High efficiency in optical devices

9. Applications of Total Internal Reflection:

• Optical Fiber Communication
• Medical Endoscopy
• Binoculars
• Periscopes
• Laser Systems
• Diamond Cutting
• Optical Sensors
• Scientific Instruments

10. Did You Know?

• More than 99% of international internet traffic travels through optical fiber cables laid under the oceans.
• These cables depend entirely on Total Internal Reflection.

11. Examination Tips:

Remember these important points.

✔ Definition of Critical Angle

✔ Conditions for TIR

✔ Difference between Reflection and Total Internal Reflection

✔ Applications of TIR

✔ Optical Fiber works on TIR

12. Key Takeaways:

• Total Internal Reflection occurs only when light travels from a denser medium to a rarer medium.
• The angle of incidence must be greater than the Critical Angle.
• TIR produces almost complete reflection.
• Optical fibers depend entirely on Total Internal Reflection.
• TIR is widely used in medicine, communication, and scientific instruments.

13. Frequently Asked Questions:

What is Total Internal Reflection?

It is the complete reflection of light back into a denser medium when the angle of incidence exceeds the Critical Angle.

What is the Critical Angle?

The angle of incidence in the denser medium, for which the angle of refraction becomes 90° is called Critical angle.

What are the conditions for TIR?

1. Light must travel from a denser medium to a rarer medium.
2. The angle of incidence must be greater than the Critical Angle.

Why is TIR important?

It enables optical fiber communication and improves the efficiency of many optical instruments.

Where is TIR used?

TIR is used in Optical fibers, prisms, binoculars, endoscopes, lasers, and scientific instruments.

14. Conclusion:

    Total Internal Reflection is one of the most remarkable phenomena in Optics. It allows light to remain trapped inside a denser medium with almost no loss of energy, making it invaluable in modern technology.

      From the sparkle of a diamond to high-speed internet and minimally invasive medical procedures, TIR has transformed the way we communicate, diagnose diseases, and explore the world.

     In the next and final part of this series, we will explore Optical Fiber Technology in detail. You will learn about the **construction of optical fibers, the role of the core and cladding, the path of light inside the fiber, different types of optical fibers, their advantages, and their wide-ranging applications in communication, medicine, military systems, AI data centers, and future technologies.

• Part 1: Light, Reflection and Refraction – The Foundation of Optics
• Part 2: Total Internal Reflection (TIR): The Physics Behind Optical Fiber
• Part 3: Optical Fiber: Construction, Working Principle and Applications