Total Internal Reflection (TIR) is a special light phenomenon where a ray of light bounces back completely instead of passing through a surface. This happens when light moves from a denser medium to a rarer medium at a certain angle.
Understanding TIR is important for SSC, Banking, RRB NTPC, UPSC, and State exams, as it is used in optical fibers, diamonds, periscopes, and medical instruments. In this guide, we will cover the definition, conditions, and real-world applications of Total Internal Reflection in a simple way.
Read now to master this important physics topic for exams!
What is Total Internal Reflection?
Total Internal Reflection (TIR) is a phenomenon in which a light ray traveling from a denser medium to a rarer medium is completely reflected back into the denser medium instead of refracting (passing through). This happens when the light strikes the boundary at an angle greater than the critical angle.
Simple Explanation of TIR
Total Internal Reflection (TIR) is an amazing light phenomenon that happens when light moves from a denser medium (like water or glass) to a less dense medium (like air).
Normally, light bends away when it enters a rarer medium. This bending is called refraction. But at a special angle, known as the critical angle, something interesting happens! Instead of bending, the light stays inside the denser medium and reflects completely.
It’s like skipping a stone on water—the light ray grazes the surface and then bounces back. This reflection happens without any mirrors or barriers!
Why is Total Internal Reflection Important?
- Mirages in Deserts: TIR makes distant roads or sand look like water on hot days.
- Fiber Optic Cables: Used in high-speed internet and medical devices, these cables guide light using TIR, ensuring minimum signal loss.
- Shiny Diamonds: Diamonds sparkle because light gets trapped inside and reflects multiple times.
✅ Also Read: Image formed by Plane Mirror
Diagram of Total Internal Reflection
Consider the following ray diagram of total Internal Reflection _
Here,
AO1 is the incident ray
O1B’ is a partially reflected ray, and
O1B is a partially transmitted ray
Now, let’s see what happens if we gradually increase the value of the angle of incidence.
When we gradually increase the angle of incidence, then at a certain angle (ic= ∠AO3N), the refracted ray (∠O3D) is bent so much away from the normal that it passes through the surface at the interface between the two media. This phenomenon is called the Grazing effect as the bending refracted ray grazes the interface between the two media.
In the grazing effect phenomenon, the angle of refraction is 90 deg. The angle of incidence ∠AO3N is called the critical angle (ic) for the given pair of media.
If the incidence angle is increased further (e.g., the ray AO4), refraction is stopped, and the incident ray is reflected. This is called total internal reflection.
Critical angle of some transparent media for air
| Substance medium | Critical angle | Refractive index |
|---|---|---|
| Diamond | 24.41 | 2.42 |
| Dense flint glass | 37.31 | 1.62 |
| Crown glass | 41.14 | 1.52 |
| Water | 48.75 | 1.33 |
Conditions for Total Internal Reflection
There are two conditions for the total internal reflection to take place. They are:
- Light Needs to Go From “Denser” to “Rarer”: For total internal reflection (TIR) to happen, the light ray must travel from a denser medium (like glass or water) to a rarer medium (like air).
- Hitting the Critical Angle: There’s a specific angle, called the critical angle, at which total internal reflection occurs.
Total Internal Reflection in nature
Mirages: The Cool Trick of Light
Have you ever seen a shimmering pool of water on a hot road or desert, only to find no water when you get closer? This mysterious effect is called a mirage, and it’s caused by total internal reflection (TIR)!
What is a Mirage?
A mirage is an optical illusion where light bends, creating the illusion of water or a distant object. This happens because light bends when it travels through air layers with different temperatures. The trick behind it is total internal reflection, a special way light behaves at certain angles.
The Science of a Mirage
Here’s how it works:
On a hot day, the ground gets heated by the sun, causing the air right above it to become very hot and less dense than the cooler air higher up.
When light travels from the cooler air to the hotter, less dense air, it bends. If the angle is just right (called the critical angle), the light doesn’t pass through but gets reflected back toward the observer, creating the illusion of water on the ground.
Where Do Mirages Appear?
- Deserts: Mirages are most common in hot deserts.
- On the Road: You might also spot them on hot summer days while driving on a road, especially on highways.
For instance, you may notice a distant road patch looking wet from afar. But when you arrive, there’s no water! That’s a mirage in action.
How Does a Mirage Happen in a Hot Desert?
- Hot Ground: On hot summer days, the air near the ground becomes much hotter than the air above.
- Different Densities: Cool air is thicker than hot air. As the temperature changes from the ground to the sky, the air’s density also changes.
- Light Bending: When light travels from the cool air (above) to the hot air (below), it bends. If the angle is large enough, the light reflects off the hot air layer, causing the illusion of water on the ground.
Why Do We See a Mirage?
A mirage happens because of how light bends and reflects at the right angle. This optical illusion makes it seem like there’s water, but it’s just light being tricked by the different air layers.
Key Takeaways:
Mirages are most common in hot deserts and on hot roads on summer days.
A mirage is an optical illusion caused by total internal reflection (TIR).
It occurs when light passes through air layers with different temperatures, causing the light to bend and reflect.
Total Internal Reflection in Diamond
Have you ever wondered why diamonds sparkle so brightly, even more than glass, even when cut in similar ways? The secret to a diamond’s dazzling sparkle lies in a fascinating optical phenomenon known as total internal reflection (TIR).
The Science Behind Diamond Sparkle
When light travels from one material (like diamond) into another (like air), it bends. However, something special happens when light hits the surface between the diamond and the air at a specific angle, called the critical angle.
If the angle is just right, instead of the light bending and passing through the surface, it reflects back inside the diamond—this is called total internal reflection!
Why Diamonds Sparkle More Than Glass
Diamonds have a smaller critical angle (around 24.4°) compared to glass, which has a critical angle of around 42°. This means that light entering a diamond is more likely to hit the critical angle and get trapped inside the diamond due to TIR. As the light bounces around within the diamond, it interacts with the many facets (the flat surfaces cut into the diamond) and reflects off at various angles. These reflections are what create the stunning sparkle that diamonds are known for.
Total Internal Reflection in Action
Because diamonds have a smaller critical angle, light undergoes more internal reflections, which means that more light stays inside the diamond, bouncing off its surfaces and intensifying the brilliance. The facets of the diamond help to direct the light and enhance the sparkling effect.
Why Glass Sparkles Less
In contrast, glass has a larger critical angle, so light passes through the glass more easily and is less likely to reflect internally. As a result, glass doesn’t capture and reflect as much light as diamonds, leading to a less brilliant sparkle.
Key Takeaways:
- Diamonds sparkle more because light is trapped inside through total internal reflection (TIR).
- Diamonds have a smaller critical angle than glass, which makes light more likely to bounce inside and reflect off the facets.
- The sparkling effect comes from the interaction of light with the diamond’s facets, creating dazzling reflections.
Total internal reflection in diamonds is like a tiny prison for light, trapping it and making sure it bounces around to give you the sparkle that catches everyone’s eye!
Total Internal Reflection in Prism
Prisms are designed to bend light by specific angles, such as 90° or 180°, using the principle of total internal reflection (TIR). In fact, many prisms rely on TIR to bend and reflect light efficiently, which is why they are often used in optical devices like binoculars, cameras, and telescopes.
How Does TIR Work in a Prism?
When light enters a prism, it bends as it travels through the prism’s material. If the angle of incidence is high enough (greater than the critical angle), light undergoes total internal reflection at the prism’s internal surfaces. This means that instead of passing through the surface and escaping, the light is reflected back inside the prism.
Inverting Images with Prisms
In addition to bending light, prisms are often used to invert images without changing their size. By using the correct angles and employing TIR, a prism can reflect light at 90° or 180°, flipping the image in the process. This is why prisms are commonly used in optical instruments to view images correctly.
Critical Angle Requirement
For a prism to effectively use total internal reflection, the critical angle for the material of the prism must be less than 45°. This ensures that light inside the prism will reflect instead of refract through the surface. As shown in Table 9.1, both crown glass and dense flint glass have critical angles lower than 45°, making them suitable for use in optical prisms.
Key Points:
- Prisms use total internal reflection (TIR) to bend light by angles like 90° or 180°.
- Prisms can also invert images without changing their size.
- The critical angle for the material must be less than 45° to ensure total internal reflection occurs.
- Crown glass and dense flint glass are commonly used for prisms due to their low critical angles.
This use of total internal reflection in prisms allows for precise manipulation of light, making prisms valuable in many optical devices and applications.
Application of Total Internal Reflection
Total internal reflection (TIR) plays a crucial role in several modern technologies and optical devices. Below are some of the important applications of TIR:
Optical Fibers
Data Transmission
- Optical fibers are widely used for transmitting data in the form of light signals. Electrical signals are converted into light by transducers, and the light signals are transmitted through the optical fibers, which use TIR to keep the light trapped inside and direct it along the fiber. This allows for high-speed communication over long distances with minimal signal loss.
Visual Examination of Internal Organs
- Optical fibers are used as light pipes in medical diagnostics, allowing doctors to perform procedures like endoscopies. They facilitate the visual examination of internal organs such as the esophagus, stomach, and intestines. The light travels through the fiber and illuminates the area, providing a clear view without the need for large incisions.
Decorative Lamps
- A bundle of fine plastic fibers can be arranged to create decorative lighting effects. One end of the fiber bundle is fixed over an electric lamp, while the other end is shaped into a fountain-like structure.
- When the lamp is turned on, light travels from the bottom of each fiber, and at the free end, it appears as dots of light, creating a sparkling effect.
Diagnostic Tools (Endoscopy)
- Endoscopy uses optical fibers to examine the internal parts of the body. For example, it helps doctors see the inside of the stomach and duodenum to diagnose medical conditions without major surgery. The fiber optic system provides high-quality imaging and precise lighting for accurate diagnosis.
Sparkling Brilliance of Diamonds
- The stunning sparkle of diamonds is made possible by TIR. When light enters a diamond, it undergoes multiple total internal reflections, causing it to bounce around and reflect off the diamond’s facets. This effect enhances the brilliance and creates the characteristic sparkle of diamonds.
Optical Instruments (Periscope, Binoculars)
- Periscopes and binoculars use TIR to bend light at 90° or 180°, allowing users to see objects at a distance. In a periscope, light reflects multiple times within a prism, while in binoculars, TIR ensures the image is magnified and aligned correctly.
Cat’s Eye Reflectors
- Cat’s eye reflectors are used on roads to make them visible at night. These reflectors bounce light from a car’s headlamps back toward the driver. The use of TIR in these reflectors ensures maximum reflection, making the road easily visible even in the dark.
Single-Lens Reflex Camera (SLR)
- In a Single-Lens Reflex (SLR) camera, a pentaprism uses TIR to reflect light several times. This helps correct image inversions caused by the lens and aligns the image with the viewfinder, allowing photographers to see the scene exactly as it will appear in the final photograph.
Key Takeaways:
- Optical fibers enable data transmission and medical procedures like endoscopy.
- TIR is responsible for the sparkle of diamonds and decorative lighting effects.
- Optical instruments like periscopes and binoculars use TIR to manipulate light.
- Cat’s eye reflectors and SLR cameras utilize TIR to improve visibility and image alignment.
