The digital revolution

Television delivered through cable or fiber optics requires high data ratesWhile bringing a world of news and entertainment to millions, AM radio and similar technologies have limitations: Interference can add an annoying hiss and reception quality can vary. Today’s technology, however, allows us to reliably digitize, store, and exchange nearly perfect copies of speech, music, and even high-definition video with people across the globe. Many forms of modern communication use infrared (IR) and visible light, which are EM waves that vibrate much more rapidly than radio waves. With frequencies of many trillions of hertz (terahertz, or THz), such waves can easily convey billions of bytes (or gigabytes) of data each second—making it possible to create the impression of smooth, realistic motion across a large screen. Read the text aloud
Whereas radio transmissions are usually described as waves, visible and infrared forms of light are best thought of as particles, since their wavelengths are microscopically small. (The near-infrared photons most commonly used in telecommunications have wavelengths of 1,550 nm or 1.55×10−6 m, roughly 1/100th the width of a human hair.) Furthermore, visible and IR radiation usually are detected by devices that rely on the photoelectric effect, which you read about on page 1287. When a near-infrared or visible photon smacks into such a device, it knocks an electron loose, thereby creating electric current. This collision can most easily be understood by thinking about the light as a stream of individual photons. Read the text aloud
Fiber optics are based on the concept of total internal reflection
Visible and near-infrared light can’t wiggle through clouds and fog the way radio waves can. Yet such light has fantastically high frequencies and information-packing power. How can we use light to transmit data from city to city and country to country? By guiding it from place to place along as smooth a road as possible. And that road is a bundle of optical fibers, which are spaghetti-like strands of glass. (Optical fibers are so ubiquitous that you can now find them in novelty stores.) Light enters an optical fiber much like it enters the lens in your eye: by refraction from air into the fiber material. Once inside, the light is prevented from escaping from the fiber by a series of total internal reflections, until it reaches the far end of the fiber. The internal reflections off the inside surface of the fiber can only happen if the light enters the fiber at a suitably small angle of incidence! Read the text aloud Show Fine-tuning optical fibers
Most long-haul optical-fiber networks rely on near-infrared light with a wavelength of 1,550 nm, or 1.55×10−6 m. This light travels at a speed of 2.04×108 m/s in the fiber’s core. What is the core’s index of refraction for this particular type of light?
  1. 1.3×1014
  2. 316
  3. 1.47
  4. 0.68
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Near-infrared light has a much shorter wavelength than radio waves. Which of the following must be true?
  1. Near-infrared light travels slower than radio waves in a vacuum.
  2. Near-infrared light travels faster than radio waves in a vacuum.
  3. Near-infrared light has a lower frequency than radio waves.
  4. Near-infrared light has a higher frequency than radio waves.
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