ELECTROMAGNETIC WAVES

The "classical" description of radio waves, light, infrared radiation, ultraviolet light ...........

( By "classical", physicists mean nonquantum explanation. Here, it is Maxwell's model we are using.)

When a charge accelerates, it will (must) radiate. (By accelerate, I mean ANY CHANGE IN VELOCITY.)

In a sense, all charges are aware of every other charge in existance - by Coulomb's Law, all charges sense the force exerted on them by all other charges. (The ramifications of this are extraordinary - think of the number of charged particles there are in the Universe!!)

If, say, an electron is shaken ( accelerated) when does another electron know about it?? The second "knows" about it when the shake is signalled to it. The electric field of the first finally changes where the second charge is. The change arrives as a change in field strength and it is delayed in its arrival because the change takes a while to get there.

How can such a change in field strength go from one place to another? A WAVE is set up in the field of the accelerating electron which progresses at the 3 x 10⁸ ms^-1 in a vacuum - ie a radio wave, light etc!!

For the first electron - think of the electrons being accelerated in a radio transmitter, while the second (receiver) electron is an electron in the antenna of your bedroom radio receiver.

Of course life is never easy, Induction tells us that waving charges around creates magnetic fields while changing magnetic fields induce emfs and electric fields. The wave which is set up is a mixture of a magnetic and electrostatic waves which DO NOT NEED A TRANSMITTING MEDIUM such as air.

( Up until 1905, physicists believed that such a medium was needed and called it "the aether" -a term still occasionally used today by journalists who like drama, and computer nutters who should know better - the "Ethernet". Einstein in 1905 showed conclusively that an ether simply is not necessary through his Special Relativity theory. Michelson and Morley managed very famously NOT to find the ether in the late C19th.)

The classical picture of a simple PLANE (polarized) electromagnetic wave. The direction of polarization is vertical in the diagram, the direction of the continuously changing E Field. E and B are at right angles to the direction of travel.

In the animation of earlier, the transmitting antenna is emitting vertically polarized waves as the electron motion is vertical. The receiving antenna must be vertically extended to detect this. ( It is for this reason that the polarized e/m waves behave apparently in the opposite fashion to a polarized wave in a rope which passes through a vertical slit. A sides of a vertical slit for e/m radiation acts as an antenna and REMOVES the radiation whereas a horizontal "slit" for e/m waves are not acting as antenna - the electrons cannot oscillate in response to the incoming wave, and the wave passes through in opposition to common sense.)

When an electron belonging to a molecule responds to e/m in the atmosphere, to ends up acting as a transmitter scattering radiation. The effect is more marked for blue frequencies than red. THE ATMOSPHERE SUBSEQUENTLY LOOKS BLUE!! ( This is called Rayleigh scattering.)

Different names for different frequencies -

Now we have sensors for virtually all frequencies. 150 years ago, we could only sense light directly through light causing chemical changes in the retina.

Then Heinrich Hertz found really low frequencies ( compared with light ) using a simple old Induction coil. This became the basis of the Radio Industry. Then Roentgen discovered really high frequency stuff - X rays, .............. Now we have VHF, UHF, microwaves, infrared, gamma rays, ultraviolet and we tend to use the lot.

Different names have been applied ONLY because different means of accelerating the electrons are needed for the different frequencies. Different means of detecting these waves are needed. THEY ARE ALL PART OF THE SAME FAMILY.