1. How Colladon's 1841 Experiment Showed Light Following a Stream of Water
In 1841, in Geneva, a Swiss scientist named Colladon performed an experiment with a jet of water. He shone light through the fast-flowing stream. The report was published the next year, in 1842. It was said that the light traveled along with the curve of the water [4]. Later, Tyndall demonstrated a similar effect, which made the phenomenon widely known. The exact year of Tyndall’s demonstration varies by source, so it is not specified here [4].
To control the light, there was no need to arrange mirrors. The water itself became the path for the light. But why was the light trapped inside the water? The answer to that question appears in later sections [4].
2. What the Core and Cladding Layers of a Fiber Optic Cable Do
A fiber optic cable is made of glass fibers with two layers. The center is called the "core," and the outer layer that surrounds it is called the "cladding" [1]. Light travels through the core in the center.
In a standard single-mode fiber, the core has a diameter of about 9 micrometers (1 micrometer is 1/1000 of a millimeter). The cladding has a diameter of 125 micrometers [3]. This means the core is only about one-fourteenth the thickness of the cladding.
Both layers are made of quartz glass. However, they are made so that light travels through them slightly differently. This "slight difference" is the main character in the next section [1].
3. How Total Internal Reflection Keeps Light Inside the Fiber
When you skip stones on a pond, the stone bounces forward only when it hits the water surface at a shallow angle. Something similar happens to light inside a fiber optic cable. Light that hits the boundary between the core and the cladding at a shallow angle does not go outside. Instead, it is all bounced back inside. This is called total internal reflection [2].
For total internal reflection to happen, two conditions must be met. First, light must travel from a material with a higher refractive index to one with a lower refractive index. Second, it must hit the boundary at a fairly shallow angle (where the angle measured from the perpendicular line is greater than the critical angle) [2]. The refractive index of the core is made higher than that of the cladding for this reason.
A common misunderstanding is that there is a mirror pasted on the inside of the glass. In reality, there is no mirror. The light is trapped only by the difference in refractive index between the two glasses [2]. However, just trapping the light is not enough for it to reach far distances. The weakening of light inside the glass becomes the problem in the next section.
4. Why Dirty Glass, Not Shape, Was the Real Problem for Light Signals
Even with a path for the light, there was another wall. The light became weak as it traveled through the glass. At that time, fibers made by bundling glass together lost about 1 decibel of light for every 1 meter they traveled [6]. After 10 meters, the light intensity was 1/10. After 100 meters, it was 1/10 billion.
In 1966, Kao and Hockham argued that the loss in glass fibers could be reduced to less than 20 decibels per kilometer [5]. They thought the large losses at that time were caused by impurities mixed into the glass, not by the structure of the glass itself [5].
20 decibels means the light intensity becomes 1/100. Even after traveling 1 kilometer, 1 percent of the light remains [5]. This remaining amount seems small when looking at numbers alone. However, previous bundled fibers lost about 1 decibel per meter. This meant they needed to reduce the loss by about 980 decibels per kilometer [6]. Kao did not "invent" the fiber optic cable. This paper showed a path that made it possible [5].
5. How Corning Glass Works Made a Fiber Below 20 Decibels in 1970
In August 1970, Maurer, Schultz, Keck, and others at Corning Glass Works in the USA made a fiber optic cable that was below 20 decibels. On August 7, Keck measured a 29-meter sample. The loss was 17 decibels per kilometer [6]. The light that traveled 1 kilometer would have about 2 percent remaining, according to calculations.
The cladding was pure silica. The core was silica with titanium added [6]. Impurities were intentionally and carefully added to the core to adjust the refractive index. They used a method called vapor deposition to layer the glass without creating air gaps that scatter light [7].
"Reducing impurities" and "adding impurities for refractive index" are different things. They worked on both tasks at the same time: removing unwanted dirt while precisely adding only the necessary components. Half of the 2009 Nobel Prize in Physics was awarded to Kao for groundbreaking work on light transmission for fiber optic communication [5].
6. How Light Flashes Carry Digital Signals as 1 and 0
In fiber optic communication, it is explained that digital signals are sent using short flashes of light (light pulses). The state where light is on is 1, and the state where it is off is 0. The arrangement of these 1s and 0s replaces letters, sounds, and images. It is said that the light is made using semiconductor lasers.
Because light gets weaker little by little, optical amplifiers that strengthen the light or repeaters that recreate the signal are sometimes placed along long distances. Some repeaters take the received light pulses, turn them into electrical signals to read whether they are 1 or 0, and then send them out again with a laser. It seems that which method is used depends on the communication system.
The content of this section was confirmed within the scope of summary articles. It is good to check with materials from communication companies or public institutions for more details.
7. How to Test the Fiber Optic Idea with Water at Home
You can check the mechanism of fiber optics with water around you.
Ask an adult to make a small hole in the side of a plastic bottle. Cover the hole, fill the bottle with water, and test it in the bathtub or over the kitchen sink. Make the surroundings dark. Shine a flashlight from behind the hole. Open the hole and look at the stream of water. If the stream of water glows, that is a home version of the 1841 experiment [4].
SAFETY NOTE: Do not shine the flashlight directly into your eyes. A normal flashlight is fine, but do not use laser pointers or similar devices.
Communication companies and the Ministry of Internal Affairs and Communications publish explanations about fiber optic lines. Try to find out how detailed the explanation of the 1 and 0 flashes is.