The Production and Propagation of Electromagnetic Waves – Learn
The Production of Electromagnetic Waves
An electromagnetic wave which propagates through space, is produced whenever an electric charge is oscillating (vibrating). Imagine a charged particle oscillating backward and forth. The moving charge would form an electric current that is oscillating in magnitude and direction. This will induce a magnetic field that oscillates in magnitude and direction proportional to the current. This changing magnetic field induces a changing electric field producing a changing magnetic field and so on. The oscillating fields radiate out through space at a speed of c and are self-perpetuating. The rate of the charged particles oscillation can be varied and this would change the characteristics of the electromagnetic wave that is produced. Charged particles can be made to oscillate in a radio transmitter antenna when the electrons are forced to oscillate in response to an electrical signal produced by a sound recording or someone speaking into the microphone.
The wavelength of the electromagnetic wave that is produced by a moving charge is determined by the acceleration of the electrons. The electromagnetic spectrum consists of a series of groups or ranges of wavelengths which are categorised based on how they interact with matter. When Maxwell determined his theory of electromagnetism, only the visible and infra-red parts of the EM spectrum were known to scientists. Maxwell predicted that there would be many other forms of EMR that would make up the EM spectrum. This led scientists on a race to find and categorise the other parts of the EM spectrum. All other parts of the spectrum were soon discovered and Heinrich Hertz was notably the first to produce and detect radio waves. Importantly, this formed the modern communications of the time.
The diagram below illustrates the different ranges that form the EM spectrum:
The Propagation of Electromagnetic Waves
We have addressed above how electromagnetic waves are self-perpetuating. This is a result of the oscillating charge producing a changing electric field which produces a changing magnetic field and so on. Electromagnetic waves travel most effectively through a vacuum, where they travel at a speed equal to c and do not interact with matter. Space is the closest we have to a natural vacuum and EMR will propagate through space efficiently at a speed of c.
When electromagnetic waves travel through other media such as air, water or glass, an electromagnetic wave travels slower than it would in the near vacuum of space. This is a result of the electromagnetic radiation interacting with atoms in the medium. Atoms will absorb the energy of the incident electromagnetic wave, setting electrons within the atoms in motion, oscillating at the same frequency as the electromagnetic wave until they re-emit a wave a short time later. This slight delay slows down the propagation and hence the speed of the wave. The amount that the wave is delayed depends on features of the medium such as the distance between the atoms and the properties of the atoms in the medium.