Read the following passage below and answer the questions that follow on the basis of what is stated/ implied in that passage.
A microwave oven is a kitchen appliance employing microwave radiation primarily to cook or heat food. Microwave ovens have revolutionized cooking since their use became widespread in the 1970s.
Cooking food with microwaves was discovered by Percy Spencer while building magnetrons for radar sets at Raytheon. He was working on an active radar set when he noticed a strange sensation and saw that a peanut candy bar he had in his pocket started to melt. Although he was not the first to notice this phenomenon, as the holder of 120 patents, Spencer was no stranger to discovery and experiment and realized what was happening. The radar had melted his candy bar with microwaves. The first food to be deliberately cooked with microwaves was popcorn, and the second was an egg (which exploded in the face of one of the experimenters). In North America, microwave popcorn is now one of the most commonly cooked items in microwave ovens, virtually to the exclusion of other home cooking methods such as hot air and oil popping. Most microwaves sold in North America today have a specific “popcorn button” which is solely used to cook premeasured packages of popcorn, ostensibly to make it easier for consumers to microwave popcorn without worrying about burning it or leaving a lot of kernels unpopped. The standard time for the “popcorn” setting on most microwaves is about three minutes.
On 8 October 1945 Raytheon filed a patent for Spencer’s microwave cooking process and in 1947; the company built the first microwave oven, the Radarange. It was almost 6 feet (1.8 m) tall and weighed 750 pounds (340 kg). It was water-cooled and produced 3000 watts, about three times the amount of radiation produced by microwave ovens today. An early commercial model introduced in 1954 generated 1600 watts and sold for $2,000 to $3,000. Raytheon licensed its technology to the Tappan Stove company in 1952. They tried to market a large, 220 volt, wall unit as a home microwave oven in 1955 for a price of $1,295, but it did not sell well. In 1965 Raytheon acquired Amana, which introduced the first popular home model, the countertop Radarange in 1967 at a price point of $495.
In the 1960s, Litton bought Studebaker’s Franklin Manufacturing assets, which had been manufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Litton then developed a new configuration of the microwave, the short, wide shape that is now common. The magnetron feed was also unique. This resulted in an oven that could survive a no-load condition indefinitely. The new oven was shown at a trade show in Chicago and helped begin a rapid growth of the market for home microwave ovens. Sales figures of 40,000 units for the US industry in 1970 grew to one million by 1975. Market penetration in Japan, which had learned to build less expensive units by re-engineering a cheaper magnetron, was more rapid.
A number of other companies joined in the market, and for a time most systems were built by defence contractors, who were the most familiar with the magnetron. Litton was particularly well known in the restaurant business. By the late 1970s, the technology had improved to the point where prices were falling rapidly. Formerly found only in large industrial applications, “microwaves” were increasingly becoming a standard fixture of most kitchens. The rapidly falling price of microprocessors also helped by adding electronic controls to make the ovens easier to use. By the late 1980s, they were almost universal, and current estimates hold that nearly 95% of American households have a microwave.
A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Most molecules are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore vibrate as they try to align themselves with the alternating electric field induced by the microwave beam. This molecular movement creates heat. Microwave heating is most efficient on liquid water, and much less so on fats, sugars, and frozen water. Microwave heating is sometimes incorrectly explained as the resonance of water molecules, which only occurs at much higher frequencies, in the tens of gigahertz.
Most microwave ovens allow the user to choose between several power levels, including one or more defrosting levels. In most ovens, however, there is no change in the intensity of the microwave radiation; instead, the magnetron is turned on and off in cycles of several seconds at a time. This can actually be observed when microwaving airy foods like Krembos (An Israeli confection): it blows up during heating phases, while it deflates when the magnetron is turned off.
The cooking chamber itself is a Faraday cage enclosure which prevents the microwaves from escaping into the environment. The oven door is usually a glass panel for easy viewing but has a layer of conductive mesh to maintain the shielding. Because the size of the perforations in the mesh is much less than the wavelength of 12 cm, the microwave radiation can not pass through the door, while visible light (with a much shorter wavelength) can.
Professional chefs generally find microwave ovens to be of limited usefulness. On the other hand, people who are lacking in free time, or not comfortable with their cooking skills, can use microwave ovens to reheat stored food (including commercially available pre-cooked frozen dishes) in only a few minutes.
The central theme of the passage is: