ElectromagneticEnergy

1 White light seemsto be a combination of all colors.The energy that comes from a source of lightis

not limited to thekind of energy you can see.Heat is given off by a flame or an electric light.Ona cloudy

day it is possibleto get a sunburn even though you feel cool.Visible light and the kind of energythat produce

warmth and sunburnare examples of electromagnetic energy.

2 The sun is 93million miles away from the earth.Yet we can use energy from the sun becauseelec-

tromagnetic energytravels through space.

3 Many other kindsof energy are also types of electromagnetic energy.Radio,television,and radarsignals

travel fromtransmitters to receivers as low-energy electromagnetic waves. Infrared（红外线的）radiation is an

electromagneticwave.When it is absorbed by matter,heat is produced.Waves of infrared andvisible light

have more energythan waves of radio , television , or radar. Ultraviolet rays（紫外线）and X-rays are electro-

magnetic waves witheven greater amounts of energy.Infrared radiation is used in cooking food andheating

buildings.Sunlightand electric lights are part of our requirements for normal living. Ultravioletradiation is

useful in killingcertain disease organisms.X-rays and gamma rays have so much energy that theytravel right

through solidobjects.They can be used to detect and treat cancer. X-rays are used inindustry to find hidden

cracks in metal,andin medicine to reveal broken bones.

4 Usually we useelectricity to generate electromagnetic energy.The source of most of our energyis the

sun.Heat from thesun causes water to evaporate.When the water falls to the earth as rain,some ofit is

trapped behind damsand then used to operate electric generators.Other generators are powered bycoal,but

the energy stored incoal came from the sun,too.

5 Until recently,thesource of the tremendous amount of energy given off by the sun was a puzzle.If

the.sun depended onchemical reactions,it would have used up all its energy long ago.Experimentswith

electromagneticradiation led to the theory that mass can be converted into energy.About fortyyears 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 withchemi-

calreactions,nuclear reactions release millions of times more energy per pound offuel.We now believe that

the sun's energycomes from the nuclear reactions in which hydrogen is changed into helium（氦）.

6 Nuclear energy isbeginning to compete with coal as an economical source of power to generateelec-

tncity.It is alsobeing used to operate engines in large ships.Scientists continue to seek newand better

methods of obtainingand using energy.Paragraph3_____________

A.Nuclear Reactionsas the Lasting Source of the Sun's Energy

B.The Most ImportantSource of Energy

C.Types ofElectromagnetic Energy

D.X-rays Are Used toDetect and Treat Cancer

E.Seeking NewSources of Energy