Read the article, "Microwaves," and identify its audience (technical or general), purpose, message, style, and tone.

Microwaves

Much of the world around us is in motion. A wave-like motion. Some waves are big like tidal waves and some are small like the almost unseen footprints of a waterspider on a quiet pond. Other waves can't be seen at all, such as an idling truck sending out vibrations our bodies can feel. Among these are electromagnetic waves. They range from very low frequency sound waves to very high frequency X-rays, gamma rays, and even cosmic rays.

Energy behaves differently as its frequency changes. The start of audible sound- somewhere around 20 cycles per second-covers a segment at the low end of the electromagnetic spectrum. Household electricity operates at 60 hertz (cycles per second). At a somewhat higher frequency we have radio, ranging from shortwave and marine beacons, through the familiar AM broadcast band that lies between 500 and 1600 kilohertz, then to citizen's band, FM, television, and up to the higher frequency police and aviation bands.

Even higher up the scale lies visible light with its array of colors best seen when light is scattered by raindrops to create a rainbow.
Lying between radio waves and visible light is the microwave region-from roughly one gigahertz (a billion cycles per second) up to 3000 gigahertz. In this region the electromag- netic energy behaves in special ways.

Microwaves travel in straight lines, so they can be aimed in a given direction. They can be reflected by dense objects so that they send back echoes-this is the basis for radar. They can be absorbed, with their energy being converted into heat-the principle behind microwave ovens. Or they can pass tftrougft some substances that are transparent to the energy-this enables food to be cooked on a paper plate in a microwave oven.

Microwaves for Radar

World War II provided the impetus to harness microwave energy as a means of detecting enemy planes. Early radars were mounted on the Cliffs of Dover to bounce their microwave signals off Nazi bombers that threatened England. The word radar itself is an acronym for RAdio Detection And Ranging.

Radars grew more sophisticated. Special-purpose systems were developed to detect air- planes, to scan the horizon for enemy ships, to paint finely detailed electronic pictures of harbors to guide ships, and to measure the speeds of targets. These were installed on land and aboard warships. Radar-especially shipboard radar-was surely one of the most significant technological achievements to tip the scales toward an Allied victory in World War II.

Today, few mariners can recall what it was like before radar. It is such an important aid that it was embraced universally as soon as hostilities ended. Now, virtually every commercial vessel in the world has one, and most larger vessels have two radars: one for use on the open sea and one, operating at a higher frequency, to "paint" a more finely detailed picture, for use near shore.

Microwaves are also beamed across the skies to fix the positions of aircraft in flight, obviously an essential aid to controlling the movement of aircraft from city to city across the nation. These radars have also been linked to computers to tell air traffic controllers the altitude of planes in the area and to label them on their screens.

A new kind of radar, phased array, is now being used to search the skies thousands of miles out over the Atlantic and Pacific oceans. Although these advanced radars use microwave en- ergy just as ordinary radars do, they do not depend upon a rotating antenna. Instead, a fixed antenna array, comprising thousands of elements like those of a fly's eye, looks everywhere. It has been said that these radars roll their eyes instead of turning their heads.

High-Speed Cooking

During World War II Raytheon had been selected to work with M.I.T. and British scientists to accelerate the production of magnetrons, the electron tubes that generate microwave en- ergy, in order to speed up the production of radars. While testing some new, higher-powered tubes in a laboratory at Raytheon's Waltham, Massachusetts, plant, Percy L. Spencer and several of his staff engineers observed an interesting phenomenon. If you placed your hand in a beam of microwave energy, your hand would grow pleasantly warm. It was not like put- ting your hand in a heated oven that might sear the skin. The warmth was deep-heating and uniform.

Spencer and his engineers sent out for some popcorn and some food, then piped the energy into a metal wastebasket. The microwave oven was born.

From these discoveries, some 35 years ago, a new industry was born. In millions of homes around the world, meals are prepared in minutes using microwave ovens. In many processing industries, microwaves are being used to perform difficult heating or drying jobs. Even print- ing presses use microwaves to speed the drying of ink on paper.

In hospitals, doctors' offices, and athletic training rooms, that deep heat that Percy Spencer noticed is now used in diathermy equipment to ease the discomfort of muscle aches and pains.

Telephones Without Cable

The third characteristic of microwaves-that they pass undistorted through the air-makes them good messengers to carry telephone conversations as well as live television signals- without telephone poles or cables-across town or across the country. The microwave signals are beamed via satellite or by dish reflectors mounted atop buildings and mountain- top towers.

Microwaves take their name from the Greek mikro meaning very small. While the waves themselves may be very small, they play an important role in our world today: in defense; in communications; in air, sea, and highway safety; in industrial processing; and in cooking. At Raytheon the applications expand every day.