1. White light seems to be a combination of all colors. The energy that comes from a source of light is not limited to the kind of energy you can see. Heat is given off by a flame or an electric light. On a cloudy day it is possible to get a sunburn even though you feel cool. Visible light and the kinds of energy that produce warmth and sunburn am examples of electromagnetic energy.
2. The sun is 93 million miles from the earth. Yet we can use energy from the sun because electromagnetic energy travels through space.
3. Many other kinds of energy are also types of electromagnetic energy. Radio, television, and radar signals travel from transmitters to receivers as low-energy electromagnetic waves. Infrared （红外线的） radiation is an electromagnetic wave. When it is absorbed by matter, heat is produced. Waves of infrared and visible light have more, energy than waves of radio, television, or radar. Ultraviolet rays （紫外线） and X-rays are electromagnetic waves with even greater amounts of energy. Infrared radiation is used in cooking food and heating buildings. Sunlight and electric lights are part of our requirements for normal living. Ultraviolet radiation is useful in killing certain disease organisms. X-rays and gamma rays have so much energy that they travel right through solid objects. They can be used to detect and treat cancer. X-rays are used in industry to find hidden cracks in metal, and in medicine to reveal broken bones.
4. Usually we use electricity to generate electromagnetic energy. The source of most of our energy is the sun. Heat from the sun causes water to evaporate. When the water falls to the earth as rain, some of it is trapped behind dams and then used to operate electric generators. Other generators are powered by coal, but the energy stored in coal came from the sun, too.
5. Until recently, the source of the tremendous amount of energy given off by the sun was a puzzle. If the sun depended on chemical reactions, it would have used up all its energy long ago, Experiments with electromagnetic radiation led to the theory that mass can be converted into energy. About forty years after the theory was proposed, nuclear energy was harnessed （利用） by man. Chemical energy comes from electron （电子） rearrangement. Nuclear energy comes from a change in the nucleus of an atom. Compared with chemical reactions, nuclear reactions release millions of times more energy per pound of fuel. We now believe that the sun's energy comes from the nuclear reactions in which hydrogen is changed into helium （氦）.
6. Nuclear energy is beginning to compete with coal as an economical source of power to generate electricity. It is also being used to operate engines in large ships. Scientists continue to seek new and better methods of obtaining and using energy.
23 Paragraph 3 .
24 Paragraph 4 .
25 Paragraph 5 .
26 Paragraph 6 .
A Nuclear Reactions as' the Lasting Source of the Sun's Energy
B The Most Important Source of Energy
C Types of Electromagnetic Energy
D The Machines Used for Energy Generation
E Seeking New Sources of Energy
F The Use of Ultraviolet Radiation in Medicine
27 One can get a sunburn even .
28 Infrared radiation can produce heat .
29 X-rays and gamma rays can be used to detect and treat cancer .
30 Chemical energy is generated .
A when it is absorbed by matter
B when it is cloudy,
C because they can pass through solid objects
D when the sunrays are fierce
E when a change in the nucleus of an atom takes place
F when electron rearrangement takes place
23. C 24. B 25. A 26. E
27. B 28. A 29. C 30. F