# ``LET THERE BE ELECTROMAGNETIC RADIATION"

### Light can be thought of as a WAVE

`` " = ``Wavelength'' = distance between crests (or any two identical spots). The units are distance units like ``feet'' or ``meters''.

``f'' or `` '' = ``frequency'' = Number of times a boat goes up and down per unit time. The units are 1/(time), e.g. 1/second. Sometimes this is stated as ``cycles/second''.

v = speed of the wave. Units of distances/time.

The boat goes up and down with the waves. What about the water?

Q. Just what is traveling in the direction of the waves?

Energy and information. Waves are a disturbance traveling through a medium (although E-M radiation can travel in a vacuum).

There are other kinds of waves (ocean waves are sometimes called ``gravity waves'' or ``surface waves'')

• Sound Waves: are density/pressure waves.

• The speed of sound in air (on a cool day) is ~1000 feet/second (this is the basis of the old ``distance of the thunderstorm'' trick). Do the air molecules travel at this speed? In a higher-density medium (for example railway tracks) it travels faster.

### So, what does this have to do with LIGHT?

• An electrically charged particle has an electric field extending to infinity. If this charge is accelerated , the position of the field center changes and that information propogates out as a ``kink'' in the field lines. A magnetic field is also induced at the kink. This traveling kink is an electro-magnetic wave.

• The SPEED with which the information that the charge accelerated travels outward is called the Speed of Light:

Michael Bolte
Wed Jan 14 23:06:17 PST 1998