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Name: Katherine
Status: educator
Grade: 9-12
Country: USA
Date: Spring 2012

Why are some regions of the electromagnetic spectrum shown to overlap on illustrations? For example, x-rays and gamma rays; infrared and microwaves; and microwaves and radio waves are typically shown to overlap on illustrations in high school textbooks. I thought that the electromagnetic spectrum was divided into regions by wavelength, but clearly this is not the case. Some resources I've found suggest that the overlap has to do with the source of the energy. That's fine for waves generated on Earth, but how then do you tell the boundary between an x-ray and gamma ray generated by the sun? I can't imagine when the source is the sun, the regions overlap.

Hi Kate,

The electromagnetic spectrum is a spectrum, a continuum. The sources for the energy may vary and one source may be more specific for a certain region than another. There are no boundaries in the spectrum, except for our naming them.

That is a part of the problem. The continuum is a long, expansive spectrum! In an effort to understand it, many scientists of many disciplines have been involved. Therefore, generally a team of scientists would concentrate on a small part of the spectrum and study it.

Then another team, equally competent and quite possibly from another angle of study entirely, studied an adjacent part. And so on as we learned about the spectrum and all of its properties. For the sake of simplicity, certain regions were named and have remained. The regions are named very loosely and vary widely among texts. The purpose for the regions is to help people relate the spectrum to things they have heard about.

When a scientist needs to know the energy of a part of the spectrum we indicate wavelength.

Please do not go crazy over regions named by a text. There are so many interesting phenomena in the expanse of the spectrum, you may wish to relate to the whole continuum and relish in it.

Hope this helps with the continuum!

Peter E. Hughes, Ph.D. Milford, NH

Hi Kate, great question.

There are a few different answers to your question, but the overarching reason is that these labels are not based on some fundamental scientific principal, but more on the history of our understanding of the spectrum. There are no hard-and-fast boundaries, and so different information sources may differ somewhat (overlap) or group different wavelengths into different groups. (and this is not the only subject where the history of the science plays a strong role in how current ideas are explained and new ideas are developed!)

Let me explain a little more --

Today, we have a (somewhat) comprehensive view of the electromagnetic spectrum. But that was not always the case. Early scientists performed experiments and made observations, and tried to explain those observations. Light has been known since before mankind, and has been studied for thousands of years. Magnetism was also known to the ancients -- but they did not know the two were related (that did not happen until the 1800's). br>
As science progressed, other forms of electromagnetic radiation (EM) were discovered. The way people discovered them is by observing how the EM interacted with matter. It just so happens that different EM interacts differently with matter, and so early scientists had no idea that EM was all related -- they just had a collection of strange effects. Thus, different EM was classified according to how it was observed (how it interacted with matter). These classifications persist today -- light is visible, infrared heats things up, .... etc.

Over time, science realized that EM was all connected and slowly put the pieces together. There was a ton of experimentation in the late 19th century (Bose, Hertz, Marconi, Maxwell, Faraday... big names you may have heard of). Some EM has been very well studied (like light). However, some EM remains less studied (the EM with tricky interactions with matter, that are harder to experiment around) -- like terahertz.

In the end, the labels on the EM spectrum reflect a history of discovery, as well as common physical traits (how the EM interacts with matter, which relates to how it was discovered and studied).

Hope this helps, Burr Zimmerman

Greetings Kate,

To be perfectly honest, I think all of our divisions of the electromagnetic spectrum are relatively arbitrary and based off historical precedence. In some cases where the radiation was discovered it was not obvious at the time that it was "electromagnetic radiation" but rather something different (x-rays and gamma rays). As such, we may not always agree on exactly where to draw the line (for instance) between hard x-rays and gamma rays. Likewise, different textbooks may show different divisions depending upon who wrote the material and where they take the information (not to mention the occasional mistake). One place I see lists below 120keV as hard x-ray and above that as gamma. Another lists the boundary at 124keV. It's just arbitrary and part of a naming convention (that we don't all agree upon). Using that 120keV as an example though, for most things there is very little difference between a 119keV photon and a 121keV photon.

Take optical wavelengths as another example. I doubt everyone sees exactly the same spectrum. Thus we can use an average to say where the transition from infra-red to red light occurs.

cheers, Michael Pierce

Katherine, It is important to remember that the only difference between x-rays and gamma rays is frequency (and wavelength). There is no exact boundary at which something stops being an x-ray and starts being a gamma ray. They are electromagnetic waves, just like radio waves and light waves. At one time, they were not known to be even related. What defined the kind of wave was how it is produced. The different references linger. Really, x-rays and gamma rays are essentially the same thing.

Dr. Ken Mellendorf Physics Instructor Illinois Central College

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