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, different kinds of waves would
be generated. For example, the ground could shake. Also
a tsunami could be generated which
would move toward us. We would be made aware of the existence of the
earthquake by the shaking and tsunamis which smash into Oregon.
In this scenario the ocean and crust were disturbed by the earthquake. The disturbance created waves which moved from the point of the disturbance to us. The waves carried the information that an earthquake occured.
Question: What does this have to do with EM radiation?
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 forces
then we can understand
everything which happens in the Universe! The four forces are the
gravitational, electrical, strong (nuclear), and weak forces.
Let's consider electrical interactions.
Suppose 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 / D2
For charges q and Q, the strength of the attraction is determined
by distance between the charges. Now, suppose I grab
charge Q and jiggle it
around. What happens?
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 leading manner, 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 (in a manner
analogous to the seimic waves and tidal waves generated by
the offshore earthquake). This wave is known as an
electromagnetic wave.
The wave
tells the Universe I generated an electrical disturbance.
The overall phenomenon is referred to as electromagnetic (EM)
radiation.
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 forces then we can understand everything which happens in the Universe! The four forces are the gravitational, electrical, strong (nuclear), and weak forces.
Let's consider electrical interactions. Suppose 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
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 leading manner, 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 (in a manner analogous to the seimic waves and tidal waves generated by the offshore earthquake). This wave is known as an electromagnetic wave. The wave tells the Universe I generated an electrical disturbance. The overall phenomenon is referred to as electromagnetic (EM) radiation.
Question: What do we mean by electromagnetic spectrum?
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. Note that the wave pattern moves at the same speed c; the speed of light c = 300,000 km per second, regardless of the speed of the jiggling. This combined with the increase or decrease in the wavelength induced by the speed of the jiggling says that the rate at which peaks pass by changes (the frequency of the wave changes). The mathematical relationship between wavelength, W and frequency, f is given by
The energy carried by the wave is given by
We arbitarily define the different types of electromagnetic radiation by their wavelengths. In order of increasing frequency (decreasing wavelength):
