Amongst the different ways that the electromagnetic spectrum can be classified, one of the easiest, at least to me, is to measure it in electronvolts. This is especially true of photons that are going to interact energetically with ordinary matter, which is usually visible light and above. An electronvolt is the amount of kinetic energy that an electron gains when propelled by a field of one volt, or conversely the amount of kinetic energy needed to push an electron against a field of one volt. It is possible to imagine a photon as striking an electron, and if the photon is energetic enough, pushing the electron against an electric current, or out of an energetically favorable situation, such as a comfortable orbit in an atom.

This is helped along by the fact that in the range of visible light, the numbers connect somewhat to a scale we might be familiar with. The equations are rather complicated, and I can't claim to understand them totally, but by taking this equation, and multiplying it by the speed of light, we get an equation where 1240 nm=1 ev, and where moving down in wavelength gives you a corresponding increase in energy. If the analogy is not too anthropomorphic, an electron volt is about equal to one dollar. Most common chemical interchanges is in the region of a few electron volts, just as dollars are our common walking around money. Ultraviolet light is in the 10-100 electronvolt range, making it something like large purchases, but not too unusual. Above that, we get into X-Rays and gamma rays, with electron volts anywhere from thousands to millions. And much like with money, you don't see that amount floating around casually very often. Also, the first ability for a photon to engage in pair production occurs around one million electron volts, which makes sense...you would have to be a millionaire to afford something as exotic as antimatter.

Although my hastily sketched comparison chart of electronvolts with dollars is somewhat whimsical, the important thing is to remember that electronvolts are a permissible, and sometimes very useful way to measure the energy of a photon.