What is the Electromagnetic Spectrum?
by Vinnoja Thurairethinam
Most of us are used to considering situations that we could either visualize or had some way of getting a feel for what the physical situation is. When it comes to electromagnetic waves, we cannot see them or sense many of them. And the only way to really describe them is in terms of mathematics.
Physicists use the word “field” to describe a quantity that depends on position in space and time. For example, if we take the temperature of the air, the temperature reading will vary according to where and when we make our measurements. Electromagnetism was one of the first occasions when the term “field” was used. Electromagnetic field is now defined as a region of space produced by electrically charged objects that affects the behaviour of charged particles within it.
When it comes to electricity and magnetism, we are used to thinking of electric and magnetic fields as separate existences things. However, just like space and time, they are really just two different faces of the same coin.
The history of Maxwell’s equations
Electromagnetism is an example of how we do physics, and the ideas that go into, physics today. When physics was first being investigated as a separate subject in its own right in the 17th/18th century Newton was working on light and gravitation. What was he motivated by? He did experiments. He recorded the results of experiments and produced a set of equations that would describe his results.
Thermodynamics was developed by engineers in the 18th/19th century wanting to understand steam engines. So the laws of thermodynamics were again driven by trying to understand the results of experiments and finding equations that explain those results.
James Clerk Maxwell FRS FRSE (13 June 1831 – 5 November 1879) was a Scottish scientist in the field of mathematical physics.
This was how physics was studied until James Clerk Maxwell came on the scene. Things changed radically. Before his work there were some equations governing electricity and every experiment that had been done at the time could be explained by these equations for electricity. On the other hand, there were equations for magnetism explaining all of the experiments that had been seen for magnetism.
However, Maxwell felt that there shouldn’t be two separate theories of electricity and magnetism. He thought there should be a unified theory of electromagnetism. Therefore he tried to put the equations of electricity together with the equations of magnetism. However, when he tried to do so, they contradicted each other.
So Maxwell decided to change one of those equations slightly using Ampere’s Law.
André-Marie Ampère (20 January 1775 – 10 June 1836) was a French physicist and mathematician who was one of the founders of the science of classical electromagnetism, which he referred to as “electrodynamics”. The SI unit of measurement of electric current, the ampere, is named after him.
After Maxwell had modified Ampere’s Law all of the equations fitted together perfectly. The term that he had to add, which changed Ampere’s law, was so small that no experiment would be able to detect it. So the equations would still be able to explain the data that was collected. Thus Maxwell changed the equations not because he was trying to understand an experiment but because he was trying to unify two different theories.
The unexpected bonus of Maxwell’s equations was that they predicted that there was a new kind of wave, a wave of electricity and magnetism – an electromagnetic wave.
According to Maxwell’s theory of electromagnetism, when a charged particle, such as an electron, accelerates, it gives out electromagnetic waves.
Maxwell’s new wave travelled at 300,000,000 m/s – the same as the speed of visible light in a vacuum. This was too much of a coincidence. Thus Maxwell guessed that visible light was a wave of electricity and magnetism. Nobody, apart from Michael Faraday perhaps, had the slightest idea that there was a connection between light and electricity and magnetism. But there it was, written in Maxwell’s equations: light was an electromagnetic wave.
Michael Faraday, FRS (22 September 1791 – 25 August 1867) was an English scientist who contributed to the fields of electromagnetism and electrochemistry. His main discoveries include those of electromagnetic induction, diamagnetism and electrolysis.
Electromagnetic waves are made up of electric and magnetic fields which are perpendicular to each other and to the propagation direction of the wave. So if the propagation of the wave is along the x-axis, then the electric field is along the y-axis and the magnetic field is around the z-axis.
In the case of water waves, the thing that changes as the wave passes by is the level of the water, which goes up and down. In the case of electromagnetic waves, it is the strength of the magnetic and electric force fields which changes. However, the fields aren’t fixed variables. As they move forward, they are both rotating about the direction of propagation as an axis – i.e. the x-axis. Nonetheless, the angle between them will always be 90°.
They are also in phase with each other. They both become zero at exactly the same point and reach their maximum at the same point.
Electromagnetic waves are not made up of particles of matter but rather of photons (which are little quanta (or packets) of energy). Electromagnetic radiation is composed of rays of pure energy. They waves transmit this energy, which is partly carried by the electric field and partly by the magnetic field. The energy is usually measured in eV (electron volts).
So all electromagnetic waves travel at the speed of light, which is 3 x E8 m/s. All electromagnetic waves can also propagate through a vacuum, unlike for example sound waves, which need a medium to travel through.
Electromagnetic waves are continuous range of wavelengths and frequencies. This means there are no breaks between the different types of electromagnetic waves. According to the wave equation (v = λf), the product of wavelength and frequency of an electromagnetic wave must stay the same (3 x E8). Therefore, when the wavelength is small, the frequency will be high. They are inversely proportional to each other. Moreover, the higher the frequency of the wave, the more energy it carries.
When Maxwell’s theory became known, scientists did experiments that showed that there were different regions, or groups of wavelengths, in the electromagnetic spectrum. They started developing practical uses for each group, and soon each group was given different names, such as radio waves and x-rays.
If we want to get a full view of the sky, we make a telescope for each region of the EM spectrum and we combine those images. Those are the beautiful images of galaxies and nebulae that we often see.
Quantum Theory cannot hurt you
Faber and Faber Limited 2007
Pages 88 and 89
QED – The strange Theory of Light and Matter
Richard P. Feynman
Penguin Books 1985