The Electromagnetic Spectrum


Q1:How can we distinguish different elements of the EM spectrum? Will two different spectrums have the same wavelength? How about when two light ranges overlap?

Q2:I'm reading some of the old e-mail questions I've asked you and find one which I am not sure about the answer. The questions is 'how can we distinguish the different elements in the EM spectrum?' At first I thought that we can distinguish the different spectra by their frequencies but you told me that EM spectra overlap each other. Then it seems that there will be some shared frequency. So is there any other methods to do the distinguishment?


Q1. A spectrum is not an object found in nature, it is an artifact defined by people to conveniently organize the study of electromagnetic waves. We speak of the radio spectrum, the microwave spectrum, the infra-red spectrum etc. Different authorities have somewhat different ideas about where the boundaries between the various spectra are. They indeed may overlap.

The EM spectrum itself is subdivided into the spectra I mentioned above, and others, so the EM spectrum overlaps all others. To complicate the issue, some spectra are subdivided into bands.

Q2. The word "spectrum" originally applied only to visible light separated into its color components. It comes from the same root as the words, inspect and spectator. As people came to understand that visible light was only one component of a broader electomagnetic spectrum, we began to use the word spectra, the plural of spectrum, to refer to different frequency ranges within the whole electro-magnetic spectrum. Any of the spectra we may name is defined as a range of frequencies, so to answer your question... a spectrum is defined only by frequency.

The difficulty is that spectra are not like kilograms or meters for example, where we have universal agreement on those units. Spectra are somewhat more loosely defined. Everybody agrees that the microwave spectrum lies above radio and below the infra red, but there are some disagreements on exactly where the boundaries are. It is not a big issue. The loose definitions work well enough for us.