ELECTROMAGNETIC WAVES

 

 

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Along about the year 1900 physics changed – a lot.  Ten or fifteen years earlier most physicists thought they knew just about everything worth knowing.  About that many years later, they realized that they knew virtually nothing, and all that they knew before was just an approximation good for certain special circumstances.  Now, at about the year 2000, they have learned far more than anyone would have dreamed possible 100 years earlier, especially about the atomic world, the nuclear world, and the world of sub nuclear particles. But it is clear that much more could be learned.  What is not clear is whether we have the means to learn very much more.  Some people insist that a so-called “theory of everything” will be available in the near future.  Others are not so sure.

 

The physics known before 1900, now called “classical physics”, was based on Newton’s Laws of Motion, some related concepts such as energy and thermodynamics, and a large amount of collected knowledge about electromagnetism and optics.  Most people accepted that atoms exist although there were a few holdouts.  The structure of the atom was still unknown, but it was thought to be only a matter of time until that problem was solved.

 

There were too many surprises in store to count.  Some of them involved the nature of light.  This had apparently been solved.  A number of experiments had long since suggested that light is a wave, and this suggestion seemed to be on very solid ground – solid enough that we still speak of “light waves” even though this was the subject of some of the surprises.  One problem was that no one could describe the exact nature of these waves.  A wave is a disturbance in a medium such as a spring or the air.  The disturbance moves from place to place, but in the case of light waves, no one could say what the medium consisted of or just what kind of disturbance it could support. No one could say just what was waving.

 

But in the late 1800’s, the theory of electromagnetism had predicted that something called electromagnetic waves should exist, and that certain types of these waves should behave just like light.  A recent theory of electromagnetism by James C. Maxwell of Scotland gave an apparently complete description of them. Therefore it was thought that the nature of light waves was finally understood.  The same theory predicted that radio waves should exist, and Hertz soon experimentally discovered them (see the article on reserve in the library about Hertz in Asimov’s Encyclopedia). 

 

In order to describe some of the changes and surprises that followed, it is going to be necessary to discuss electromagnetic waves and other kinds of waves in a bit more detail.  So look at the picture below, which shows a picture of a wave.

 

Think of this train of waves moving to the right at some speed.  They could be waves on the surface of the ocean, waves in a spring that is stretched out, light waves, or radio waves.  The peaks are called “crests” and the valleys are called “troughs”.  The distance from one crest to another is called a wavelength.  A distance equal to a wavelength would also be measured from one trough to another, and also in some other ways. 

 

 

 

 

One wavelength
 

trough
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


As the wave train moves to the right, new waves appear at the left.  If you watched a single wavelength appear, from one crest to the next, then you could notice the time it takes for that wavelength to appear. That time is called one “period”.  

 

If you watched the waves for one second you could count the number that comes into view from the left in that time.  The number of wavelengths appearing in one second is called the “frequency”.  Suppose that 60 wavelengths come into view in one second.  Then the frequency would be 60 cycles per second.  It is common to call a cycle per second by the name “hertz” named after the same physicist mentioned above for experimentally discovering radio waves.  So the frequency would be 60 hertz. 

 

The waves of visible light are electromagnetic waves of a very short wavelength – a few tenths of a micrometer.  The word “micro” always means “one millionth”, a very small fraction.  This is part of the same naming system in which “milli” means “one thousandth”.  So a millimeter is one thousandth of a meter.  If you look at a meterstick, a millimeter is represented by the smallest division on the stick.  There are a thousand of them on the meterstick. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Now imagine one of those millimeters divided into one thousand equal pieces.  If that happens, then each one of the thousand divisions is a micrometer.  This is true because one-thousandth times one-thousandth is one millionth.  A typical bacterium is around a micrometer or a few micrometers in size.

 

A wave of visible light has a wavelength of a few tenths of a micrometer.  There is actually a range of wavelengths that the eye can detect from about 0.4 micrometers to around 0.7 micrometers.  The long wavelengths are seen as red, and the shortest are seen as violet.  The other colors of the spectrum have intermediate wavelengths.

 

 

 

 

Different colors of visible light have different wavelengths as depicted here for light in the air or vacuum.  The distance scale is in micrometers, and the colors represented by the different wavelengths are represented approximately by the line colors in the picture to the right.

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


There are electromagnetic waves with wavelengths shorter than violet, and they are called ultraviolet.  They have become well known for causing sunburn, suntans, skin cancer, and other skin effects.   The wavelengths of these waves range from the violet wavelengths down to about a nanometer.  That prefix “nano” means one billionth, which is one thousandth of one millionth.  So imagine one of the micrometers described above, and divide it into 1000 equal pieces.  Each of these pieces is a micrometer.  A nanometer is about ten times the size of a typical atom.

 

Below a nanometer, the electromagnetic waves are called x-rays, with well-known medical applications.  They are also used in testing the strength of materials, and in scientific research about the structure of molecules and crystals.  X-rays are shot at the crystal or other target, and although the mathematics is complicated, the structure of the target can be deduced from the way in which the x-rays bounce (the proper scientific term is “scatter”). For example, the structure of the genetic material DNA was discovered by means of interpreting x-ray scattering results.

 

At wavelengths on the average shorter than x-rays, electromagnetic waves are called gamma rays.  Gamma rays are a type of radioactivity, and they cause much damage to living tissue.  It is below about 0.1 nanometer that the name “gamma ray” is used.  But there is a lot of overlap between the x-ray range and the gamma ray range; they are not much different. So, although it is basically good to get medical x-rays when medically necessary, such x-rays should not be overdone or taken for trivial reasons.

 

There are also electromagnetic waves at longer wavelengths.  From red light (about 0.7 micrometers) to around a millimeter, these waves are called “infrared”.  The human eye cannot see infrared, but humans feel infrared as heat.  It is also true that humans and other objects at about the same temperature emit infrared.  In other words, everything around you emits at least some infrared.  If our eyes could see it, then everything would be glowing, including you.  There is, of course, infrared sensitive film.  Also some kinds of night vision instruments picks up infrared emissions and then displays them as visible light.

 

The next range, from about a millimeter to about a meter, is called the microwave region.  These waves also have many well-known uses including communication, radar, and ovens.

 

Above the microwave region, electromagnetic waves are called radio waves, and they are used for all sorts of communication.  The wavelengths involved range from about a meter on up to very long wavelengths.

 

In summary, we have been describing the electromagnetic spectrum, in other words the entire range of wavelengths for electromagnetic waves.  It is divided into the different regions described above.  Here they are again, from largest wavelength to smallest:

 

 

                                                                                                Radio

Wavelength increases in this direction.
 

Frequency increases in this direction
 
                                                                                                Microwave

                                                                                                Infrared

                                                                                                Visible

                                                                                                Ultraviolet

                                                                                                X-ray

                                                                                                Gamma Ray

 

 

 

 

Of course, the frequency has to increase for smaller wavelengths.  To see why, first remember that all of these wavelengths move with the speed of light, which is 186,000 miles per second in vacuum and only a little smaller in air.  At this rate, light gets from the moon to the earth in very close to a time of 1.3 seconds.  That is also true of radio communication inducing a short delay in communication with anyone on the moon.  Light from the sun reaches the earth in about 8.3 minutes.  Therefore anything that happens on the sun will not become known on the sun for that amount of time.  Light from the same event would not reach the planet Pluto for about 5.5 hours, and it would not reach the nearest star (other than the sun) for about 4 years.

 

Now look at the two waves pictured below.

 


  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In this picture, one wave has a wavelength 10 times the other.  If they are both moving at the same speed, then ten of the small wavelengths will enter from the left in the same time that one of the large waves.  So it has ten times the frequency.  In the same way, electromagnetic waves of smaller wavelength have a higher frequency. 

 

In will develop in the articles to follow that a higher frequency corresponds to a higher energy.  So, other things being equal, the short end of the electromagnetic spectrum (ultraviolet, x-ray, gamma ray) will hit harder and do more damage than the longer wavelengths.

 

The next two articles will explore the energy of these waves as well as the basic nature of them, their relationship to electric forces, and their relationship to electric charges.