Five scientists who have extended this century's great physics revolution into practical machinery and ideas have won the this year's Nobel prizes in physics and chemistry.

Two Americans and a Swede won the physics prize, while an American and a Japanese shared the prize in chemistry. This continues America's 30-year domination of the annual awards from the Royal Swedish Academy of Sciences, with six Americans among the winners in this year's science prizes. Four of the Americans are foreign-born.

The chemistry prize was won by Roald Hoffmann, 44, of Cornell University and Kenichi Fukui, 63, of Japan's Kyoto University. The physics prize was shared by Arthur Schawlow, 60, of Stanford University, Nicolaas Bloembergen, 62, of Harvard University, and Kai Siegbahn, 63, of Sweden's Uppsala University.

The $180,000 physics prize was split in half this year, with Schawlow and Bloembergen sharing half for their work in developing laser techniques to sort out the atomic properties of substances, a field of science called laser spectroscopy.

In this science, the pure light from a laser is aimed at a substance. By noting how the light is absorbed or scattered by the atoms in the substance, the researchers can determine what atoms are present. Schawlow said that it is like listening to a distant, unseen bell. By listening, he explained, "we can tell if it is big or little, we can even tell the shape. We can study the light from atoms and molecules in detail -- we learned almost everything we know about them that way."

Since Schawlow also was one of the original inventors of the laser, he has been mentioned for the Nobel prize for many years. The other techniques developed by Schawlow have extended the reach of the laser down to fine atomic detail. This makes it possible in the future to, as Schawlow put it, "seek out very small numbers of atoms and molecules -- even as few as one -- and to learn about their properties and how they come together to form chemical compounds."

This is especially important in a world where cancer-causing substances are present in water, food, and other everyday substances in the tiniest imaginable amounts. They are very difficult to detect. Also, the components of helpful substances, such as a mother's milk, are not known in detail because of the problem of identifying substances in it. In industry and in drug manufacturing, a very slight contamination of chemicals can produce disastrous results, and the ability to identify contaminants is critical to producing safe drugs and workable chemical products.

"We may be able to use this method for ultrasensitive analysis to detect the tiniest traces of all kinds of substances," Schawlow said. "We might find traces of harmful pollutants, or helpful impurities that we never even suspected before."

The critical area in this work, and the great advance which Schawlow and his colleagues have made, is in what is called "tuning" the lasers.

Shooting light at atoms makes the atoms emit light in a special signature pattern of spectral lines, or colors of light. These identifying patterns in the past have been blurred by the random motion of the atoms being studied. This blurring is eliminated by filtering out all of the atoms except those moving from side to side across the light.

The Stanford team of Schawlow and Theodor Hansch helped develop this technique and now have made it applicable to practically any atom or molecule by using a broadly "tunable" laser that utilizes filters and focusing devices, sharpening its breadth by several million times.

Bloembergen, a native of the Netherlands who left there after World War II to study in the United States, was cited for developing a technique in which he was able to focus three high-intensity laser beams through a substance and produce a fourth beam. By carefully controlling this beam, it can be produced at any desired wavelength -- an important advance that will allow lasers to be made at many different wavelengths of light, some suitable for use in surgery.

Also critical in this process is the fact that the way the fourth beam is produced can tell much about the structure of the material that the three lasers are focused on. Knowing how laser light affects a material and how this effect can be controlled is critical to work using high-intensity lasers. One example would be in using lasers to ignite nuclear fusion, a technique that some hope will be a key to America's energy future.

Kai Siegbahn of Sweden is the son of a Nobel prize-winner of 1924, who won the prize then for work in a related field to the one in which his son took the prize yesterday.

Siegbahn is one of the chief pioneers in electron spectroscopy, which involves shooting ultraviolet light at atoms and studying the electrons that are showered out from the atoms as a result.

The importance of this is similar to that for Schawlow's work: finding out what substances are present in everyday materials.

The prize in chemistry was given for "explaining chemistry to chemists," as one Cornell colleague of winner Roald Hoffmann put it. Hoffmann was born in Poland and fled the persecution by the Nazis in his country, eventually making his way to the United States after World War II.

With his award, Fukui became the first Japanese to win the prize in chemistry. Fukui said he was stunned because, "every year at this time my colleagues tell me I've been nominated for the Nobel prize. But in the past I didn't get it, so this year I didn't take it seriously."

Although quantum mechanics, the great mathematical edifice of modern physics, can explain in detail the reactions between chemicals, until the work of Hoffmann and Fukui it was of little practical application.

Hoffmann and Fukui developed a set of simple rules from quantum mechanics that allows chemists to predict the outcome of an entire class of chemical reactions.

The case Hoffmann worked with as an example was the synthesis of vitamin B12. Before his work, putting together the proper series of chemicals under the right temperature did not always produce the vitamin.

Making the vitamin was done by trial and error and could take 50, 100, or even more tries.

Hoffman's work sorted out the difficulties, and allows chemists to direct the outcome of the reactions involved in producing the vitamin.