ElectromagneticEnergy

1 White light seemsto be a combination of all colors.The energy that comes from a source of lightis not limited to the kind of energy you can see.Heat is given off by a flame

or an electriclight.On a cloudy day it is possible to get a sunburn even though you feelcool.Visible light and the kinds of energy that produce warmth and sunburn are

examples ofelectromagnetic energy.

2 The sun is 93million miles from the earth.Yet we can use energy from the sun becauseelectromagnetic energy travels through space.

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

waves ofradio,television,or radar.Ultraviolet rays（紫外线）and X-rays are electromagnetic waves with even greater amounts ofenergy.Infrared radiation is used in cooking

food and heatingbuildings. Sunlight and electric lights are part of our requirements for normalliving.Ultraviolet radiation is useful in killing certain diseaseorganisms.X-rays

and gamma rays haveso much energy that they travel through solid objects. They can be used todetect and treat cancer.X-rays are used in industry to find hidden cracks in

metal,and inmedicine to reveal broken bones.

4 Usually we useelectricity to generate electromagnetic energy.The source of most of our energyis the sun.Heat from the sun causes water to evaporate.When the water falls

to the earth asrain,some of it is trapped behind dams and then used to operate electricgenerators.Other generators are powered by coal,but the energy stored in coalcame

from the sun,too.

5 Until recently,thesource of the tremendous amount of energy given off by the sun was a puzzle.Ifthe sun depended on chemical reactions,it would have used up all its

energy longago.Experiments with electromagnetic radiation led to the theory that mass canbe converted into energy.About forty years after the theory was proposed ,nuclear

energy was harnessed（利用）by man.Chemical energy comes fromelectron （电子）rearrangement.Nuclear energy comes from achange in the nucleus of an

atom.Compared withchemical reactions,nuclear reactions release millions of times more energy perpound of fuel.We now believe that the sun's energy comes from the

nuclear reactions inwhich hydrogen is changed into helium（氦）.

6 Nuclear energy isbeginning to compete with coal as an economical source of'power to generateelectricity.It is also being used to operate engines in large ships.Scientists

continue to seek newand better methods of obtaining and using energy.Infrared radiationcan produce heat______.

A.when it isabsorbed by matter

B.when it is cloudy

C.because they canpass through solid objects

D.when the sunraysare fierce.

E.when a change inthe nucleus of an atom takes place