An introduction to electromagnetic waves
Kowdham Srikumar 12G
My report is an introduction to the electromagnetic spectrum, what it is and who was responsible for identifying what an electromagnetic wave actually is.
James Clerk Maxwell FRS FRSE (13 June 1831 – 5 November 1879) was a Scottish mathematical physicist whose most prominent achievement was to formulate a set of equations that describe electricity, magnetism, and optics as manifestations of the same phenomenon, namely the electromagnetic field. Maxwell’s achievements concerning electromagnetism have been called the “second great unification in physics”, after the first one realised by Isaac Newton. He is mainly responsible for the model of an electromagnetic wave that we are taught in school.
Heinrich Hertz clarified and expanded Maxwell’s work.
Heinrich Rudolf Hertz (22 February 1857 – 1 January 1894) was a German physicist who clarified and expanded James Clerk Maxwell’s electromagnetic theory of light. Hertz is distinguished from Maxwell because he was the first to conclusively prove the existence of electromagnetic waves by engineering instruments to transmit and receive radio pulses using experimental procedures that ruled out all other known wireless phenomena. The scientific unit of frequency – cycles per second – was named the “hertz” in his honour.
Electromagnetic radiation (EM radiation or EMR) is a form of radiant energy, propagating through space via photon wave particles. In a vacuum, it propagates at a characteristic speed, the speed of light, normally in straight lines. EMR is emitted and absorbed by charged particles. As an electromagnetic wave, it has both electric and magnetic field components, which oscillate in a fixed relationship to one another, perpendicular to each other and perpendicular to the direction of energy and wave propagation.
The electromagnetic waves that compose electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. The diagram above shows a plane linearly polarized EMR wave propagating from left to right. The electric field is in a vertical plane and the magnetic field in a horizontal plane. The two types of fields in EMR waves are always in phase with each other with a fixed ratio of electric to magnetic field intensity.
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. It extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometres down to a fraction of the size of an atom. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous.
In physics, the Planck length, denoted ℓP, is a unit of length, equal to 1.616199(97) × E−35 metres. It is a base unit in the system of Planck units, developed by physicist Max Planck. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, Planck’s constant, and the gravitational constant.
Most parts of the electromagnetic spectrum are used in science for spectroscopic and other probing interactions, as ways to study and characterize matter. In addition, radiation from various parts of the spectrum has found many other uses for communications and manufacturing.
The different parts of the electromagnetic spectrum can be classified according to their characteristics, how they are produced and how they interact with matter. Some groupings in the electromagnetic spectrum include radio waves, microwaves, x-rays, and visible light.
Different effects determine the creation and detection for a range of wavelengths.
Electromagnetic waves are transverse waves, similar to water waves in the ocean or the waves seen on a guitar string. A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right angled) to the direction of energy transfer.
Electromagnetic waves like all the waves have:
The amplitude of electromagnetic waves relates to its intensity or brightness (as in the case of visible light).
With visible light, the brightness is usually measured in lumens. With other wavelengths the intensity of the radiation, which is power per unit area or watts per square meter is used. The square of the amplitude of a wave is the intensity.
The wavelengths of electromagnetic waves go from extremely long to extremely short and everything in between. The easiest way to measure the wavelengths is to measure the distance between neighbouring peaks on the wave.
The speed of electromagnetic waves in a vacuum is approximately 300,000 kilometres per second. When these waves pass through matter, they slow down slightly, according to their wavelength.
The frequency of any waveform equals the speed divided by the wavelength and is the number of waves passing a point per second. The units of measurement are in cycles per second or Hertz.
Electromagnetic waves, like all transverse waves can be polarised. Polarisation is a property of waves that can oscillate with more than one orientation. In an electromagnetic wave such as light, both the electric field and magnetic field are oscillating but in different directions; by convention the “polarisation” of light refers to the polarisation of the electric field. The oscillation of these fields may be in a single direction (linear polarization), or the field may rotate at the optical frequency (circular or elliptical polarization). In that case the direction of the fields’ rotation, and thus the specified polarization, may be either clockwise or counter clockwise; this is referred to as the wave’s chirality or handedness.
A polariser is an optical filter that transmits only one polarisation.
The above image shows that EM waves vibrate at all orientations, perpendicular to the direction of energy travel.
A polarising filter removes most of these to produce vertical or horizontal plane polarised wave, as seen below. Crossing two polarising filters can stop the wave completely as shown in the diagram below.
You can find out more about the different parts of the electromagnetic spectrum in the reports from my colleagues in year 12 physics.