What is electromagnetic radiation?
Electromagnetic (EM) radiation is a form of energy, i.e., it has the ability to perform work. Okay, but can we be a little more physical? Yes, we can. To understand what EM radiation is, consider a simple analogy.
Suppose that we lived in the days before radio and television. An earthquake occurs off the west coast of North America. If we lived in the western US in Oregon, how would we know that the earthquake occured? Well, if the earthquake was large enough, a tsunami would be generated which moved toward us. We would be made aware of the existence of the earthquake by the tsunami which smashes into the coast of Oregon.
In this scenario we note that the ocean was disturbed by an earthquake. The disturbance generated a wave which moved away from the point of the disturbance and it was the wave which carried the information that the earthquake (disturbance) occured. EM radiation is much like the tsunami in that EM radiation carries information that a disturbance was made.
Recall that there are only four types of forces which exist in the Universe. (This is neat because if we can understand these four ways in which things in the Universe interact with each other, then we can understand everything which happens in the Universe!) The four are the gravitational, electrical, strong (nuclear), and weak forces. EM radiation is due to the electrical force.
Suppose that I have two electrical charges sitting in the Universe. For definiteness, let one charge be positive and one be negative so that the two charges attract each other. The strength of the attraction is
Force = constant x q x Q / (separation squared)
So, for fixed charges q and Q, the strength of the attraction is determined by how far apart the charges sit. If they are close then the attraction is strong. If they are far apart then the attraction is weak. Now let us perform an experiment. Suppose that I take charge Q and jiggle it around. The jiggling will cause the distance between the two charges to vary. This means that the strength of the electrical attraction between the two charges will vary. But now, let me ask the question,
How does the charge q know that I am jiggling charge Q around?
Or stated in a different (and leading) way, how does charge q know that I am disturbing the electrical attraction (field) due to charge Q? Well, what happens is that by moving around charge Q, I cause a disturbance which generates a wave (an electromagnetic wave) which propagates through the Universe carrying the information that I made an electrical disturbance. Physically, this is all that electromagnetic radiation is.
But now what about the electromagnetic spectrum? What does this mean? We need to make a few definitions (observations) on waves right now to answer this question.
The faster that I jiggle the charge around the shorter the wavelength of the wave. That is, the smaller the distance between the crests of the ripple. In an arbitrary manner, we can electromagnetic radiation of different wavelengths by different names. For example,
Later on, I will go into the nature of electromagnetic radiation. The preceding discussion was in a sense an operational definition of electromagnetic radiation; it defined electromagnetic radiation by the way it behaved (and by what it did).
The true nature of electromagnetic radiation is a subtle and interesting topic.