Electromagnetic Radiation
Introduction
The picture of the atom that we have so far is that of a small dense nucleus containing protons and neutrons surrounded by electrons in the space around the nucleus. Although the nucleus determines the mass of the atom and also the number of electrons needed to give the atom a neutral charge, the nucleus does not play a role in chemical reactions.   When two or more atoms join together to form a compound the two nuclei stay relatively far apart.  Only the electrons in the outermost area of the atom come in close contact.  The chemical properties of an element are therefore determined by the electrons.  How the electrons are distributed is called the atoms's electronic configuration.   The clues that we have about how the electrons are arranged comes from a study of light emitted when the atoms are excited or energized. We must therefore learn a little about light itself.

Two forms of electromagnetic radiation that you have encountered are heat and visible light. These are not the only forms however and in chemistry we deal with the whole range of energies called the electromagnetic spectrum of which heat and light are only mere portions.

     Electromagnetic energy is energy carried through space or matter by means of waves. These waves are very much like the waves in water.  However what oscillates up and down in water waves is a physical substance called H2O.  What oscillates up and down in electromagnetic radiation is energy.  Each oscillation is called one cycle.  That is the wave going from peak to peak or trough to trough.  A successive series of these oscillations is called electromagnetic radiation or more popularly a light wave.  The number of cycles per second that pass or hit you are called the lights frequency.

      Frequency is identified by the Greek letter "v" pronounced "new".   Frequency can be used to describe other events. For example you go to school 5 days a week or you pay your cable TV bill once per month. Frequency describes how often an event occurs.  In the metric system, the unit of time is the second, so frequency is given as the unit "per second" which is 1/second or 1 second-1.

                                                            1 Hz = 1 s-1
 

Waves crashing on a beach may have a frequency of 1 Hz or 1 wave per second.  Paying your bills would have a frequency of  1 payment/month.
 
      The spaces between light waves are even.  The distance between peaks is called the wavelength and is symbolized by the Greek letter "  " which is pronounced "lambda". Wavelength is a distance and so the unit of measurement for wavelength is the meter.

If we multiply the wavelength by the frequency we get the velocity or speed of the light waves.
The speed of light is a constant known to be 3.0 x 108 m/s or 3.0 x 108 m.s-1.

Because the speed of light is an important physical constant it is designated by the sysmbol "c".

                                               c = 3.00 x 108 m.s-1

The speed of light =  the distance between each wave    X    the frequency of the waves.

                                                          OR

                                                c =  lambda  X  v


What does this concept mean?  Let's use a store analogy.  You are sent to the store with a fixed amount of cash, "c". You can buy 5 small cans for this amount or 2 large cans for the same amount.*

                      ie.   cash  =  number of cans  X  size of can

                            cash =  5small       OR          cash =  2   X    large


Do you see the implications here. Since we have a fixed constant, if the number of cans is a large number then the size must be correspondingly small. If the number of cans is a small number then the size of the cans must be correspondingly large.

*  For the sake of the economists we will assume that the amount of food in 5 small cans equals the amount of food in the 2 large cans and so there is no point in arguing about the economics of buying large versus small. For the sake of any environmentalists who worry so much about the amount of metal in 5 cans versus two that will eventually get sent to a landfill let me say that this was just an example question meant to illustrate a principle.


When it comes to light waves this means that if the wavelength is large, then the frequency must be small. If the wavelength is small then the frequency must be large.

                                        c = lambda X v   or          c =lambda v

Go to Electromagnetism Frequency, Wavelength & Speed of Light Worksheet