Light and Information Transfer

Once students have explored mechanical waves, they move onto light waves, modeling how light interacts with optical fibers by observing how a beam of light can be reflected inside a stream of water. Students then model a data communication system to compare how digital and analog signals encode and transmit information.

• Light is a form of kinetic energy that travels through space and matter, and light waves are patterns of light energy that move at a constant speed until meeting matter.
• Light waves travel in a straight line, but their path can change when they interact with matter.
• People can design technologies that have specific properties to control how light moves.
• Optical fibers are long strands of pure glass that are as thin as a human hair. They are grouped in bundles called fiber optical cables that transmit information digitally.
• Fiber optic cables are commonly used to transmit information in communication systems. A communication system is a group of interacting components that function together to transmit or exchange information between people and equipment.

• How are light waves similar to seismic waves and other mechanical waves (such as sound and water waves)?
• How are light waves different from mechanical waves?
• What evidence supports the argument that light waves can travel through a vacuum?
• How does reflection change the path of light waves?
• How is reflection different from refraction?
• How is absorption different from both reflection and refraction?
• Why are optical fibers designed with a transparent core and a reflective cladding?
• How do fiber optic cables transmit digital information?
• Why do people who design fiber optic communication systems need to understand how light interacts with matter?
• Why do fiber optic communication systems usually use digital signals?

Misconception: Light cannot be used to send information.

Fact: Light is commonly used to send digital information through fiber optics.

Absorption: the interaction of light energy with matter that results in the energy changing to thermal energy

Communication system : a group of interacting components that function together to transmit or exchange information between people and equipment

Light : a form of kinetic energy that travels through space and matter

Light wave : a pattern of light energy that moves at a constant speed until meeting matter

Reflection : occurs when light bounces off of the surface of an object

Refraction : occurs when light passes from one medium to another and changes its direction

“Listening” with Light

One morning, a car drove slowly on a side road in Texas. Thousands of kilometers away, a computer screen in England filled with a series of red lines, similar to the jagged lines of a seismograph’s readings. The red lines represented ground vibrations made by the sound of the car moving over the surface.

The vibrations caused by the car in Texas were sent to the computer in England by light signals sent through specially designed tubes called fiber optic cables.

Fiber Optics Transmit Light

Fiber optics, also called optical fibers, are long strands of pure glass that are as thin as a human hair. They are grouped in bundles called fiber optical cables that transmit light signals from one place to another. To transmit means to pass on.

Fiber optic cables work because of how light interacts with different kinds of matter. All light is a form of kinetic energy that travels through space and matter. Light is complex, and there is still much that scientists don’t know about it. There are different models of light that scientists use to better understand how light moves and how it interacts with matter. One model describes how light moves in straight-line paths called light rays. Whenever you see a narrow beam of light, it is actually a bundle of many parallel light rays.

Fiber Optics Communication

Fiber optic cables are commonly used to transmit information in communication systems. A communication system is a group of interacting components that function together to transmit or exchange information between people and equipment.

In a fiber-optic communication system, information is sent digitally through the cables. This means the information is translated into a numerical value, most often ones and zeroes.

Different patterns of light wave pulses represent different combinations of ones and zeroes Digital technology is different from analog technology because in analog technology, the information is recorded or used in its original form. A transmitter at one end of the fiber-optic cable sends the information digitally. A receiver on the other end picks up the information. The digital information is translated back into its original form, such as sound or light. Fiber optic cables are commonly used in telephone communications.

Think back to the computer screen in England filled with a series of red lines when the truck moved over the ground in Texas. That computer screen was a receiver in a communications system that used a fiber-optic cable. Pulses of laser light were sent through the cable. When the ground vibrated, it caused extremely small vibrations in the cable. These vibrations caused small changes in how the light moved through the cable. The company that owns the computer screen has designed special signal-processing techniques to analyze what the sounds are likely to be, such as a truck, a person walking, or the wind.

Analog vs. Digital Communications

Digital signals are more reliable than analog. This is because the recording does not become less precise over time. As long as the numbers can be read, you will always get exactly the same wave. Because of this, fiber optic communication systems most commonly use digital signals.

Think about someone speaking into a telephone. Their voice produces an analog signal in the form of a sound wave. That sound wave enters the telephone, where it is converted to digital pulses of light. This signal is made up of pulses of ones and zeros. These pulses then move through the optical fiber to the receiver, where they are changed back into an analog signal in the form of sound waves that the person at the other end can hear.

As the signal travels, it picks up a lot of noise. This is called interference. However, because the sound wave was broken down into discrete numbers, the converter can easily convert the sound wave back to the same pattern as the original signal.

In contrast, if the system used just analog signals, it would be difficult to separate out the interference because there are many possible frequencies and amplitudes the wave could have. The end result is a representation of the original signal. This can result in information being lost or garbled when it comes through. Digital calls are also more secure because the digital code encrypts the conversation so outside listeners cannot follow what is being said.

Kinds of Communication Systems

There are many kinds of communication systems that transmit information. When you use GPS on your phone, you are using a communication system that transmits information about your location through a kind of light wave called a microwave. When you listen to the radio, you are using a communication system that transmits sounds through radio waves.

In this lesson, students model how light interacts with materials in optical fiber by experimenting with how a beam of light can produce a reflection phenomena inside a stream of water. Students then engineer a model data communication system to compare the effectiveness of digital and analog signals to encode and transmit information.