ELECTROMAGNETIC RADIATION

Question: What is electromagnetic radiation?

Electromagnetic (EM) radiation is a form of energy, i.e., it has the ability to perform work. Fine but let's be a little more physical. Consider a simple analogy.

Suppose that we lived in the time before radio and television. An earthquake occurs off the coast of Oregon every 300 or so years caused by the motion of the Juan de Fuca plate:



Question: How would we know the earthquake occurred?

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 smashed into Oregon.

In this scenario the ocean was disturbed by the earthquake. The disturbance created a wave which moved from the point of the disturbance to us. The wave carried the information that an earthquake occured.

Question: What does this have to do with EM radiation?

Electromagnetic Radiation: A Wave-like Phenomenon

There are four ways in which things interact in the Universe (we say that there are 4 different types of forces which exist in nature). 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 forces are the gravitational, electrical, strong (nuclear), and weak forces.

Suppose that there are two electrical charges sitting in the Universe. For definiteness, let one charge be positive and the other be negative so that the two charges attract each other.


The strength of the electrical attraction is

Attraction = constant x q x Q / D^2

For charges q and Q, the strength of the attraction is determined by how far apart the charges are.

Now, grab charge Q and jiggle it around. The jiggling causes the distance (and therefore attraction) between the two charges to vary, i.e.,



But, just as in the earthquake scenario, we can ask, How does the charge q know that I am jiggling charge Q?

Stated in a different (and leading) way, how does charge q know that I am altering the electrical attraction (field)? What happens is that by moving charge Q around, I generate a disturbance which launches a wave (an electromagnetic wave) into the Universe. The wave tells the Universe I generated an electrical disturbance. This disturbance is referred to as electromagnetic radiation.

Electromagnetic Spectrum

What does this mean? In order to discuss electromagnetic radiation we need to make a few definitions.



The faster I jiggle the charge around the shorter the wavelength of the wave, that is, the shorter the distance between the crests of the ripples. We arbitarily define the different types of electromagnetic radiation by their wavelengths. In order of increasing wavelength:



Comment--Our discussion (so far) has been on an operational level; it defined electromagnetic radiation by the way it traveled. The true nature of electromagnetic radiation is a subtle and rich topic.