The Electromagnetic Spectrum

The family of electromagnetic (EM) waves, the electromagnetic spectrum, permeates the cosmos. Much of the EM spectrum is invisible to our eyes but all parts – from radio waves to gamma rays – have been characterised, mostly in the 20th century CE. This article introduces the concept of waves as well as the nature of electric and magnetic fields, based on the observations of Hans Christian Ørsted and Michael Faraday.

Likewise, the intimate relationship between variable electric and magnetic fields as revealed by James Clerk Maxwell and Heinrich Hertz is examined. In addition, the wave particle “duality” of EM radiation, including the ideas of Albert Einstein and Max Planck, is considered. Throughout the article, special attention is given to space-related aspects of the EM radiation such as multi wavelength astronomy, which involves looking at the universe in different parts of the spectrum.

The Electromagnetic Spectrum’s history

For most of history, visible light was the only known part of the electromagnetic spectrum. The ancient Greeks recognised that light travelled in straight lines and studied some of its properties, including reflection and refraction. The study of light continued, and during the 16th and 17th centuries, conflicting theories regarded light as either a wave or a particle.

The first discovery of electromagnetic radiation other than visible light came in 1800, when William Herschel discovered infrared radiation. He was studying the temperature of different colours by moving a thermometer through light split by a prism. He noticed that the highest temperature was beyond red. He theorised that this temperature change was due to “calorific rays” that were a type of light ray that could not be seen.

The next year, Johann Ritter, working at the other end of the spectrum, noticed what he called “chemical rays” (invisible light rays that induced certain chemical reactions). They were later renamed ultraviolet radiation.

Electromagnetic radiation was first linked to electromagnetism in 1845, when Michael Faraday noticed that the polarisation of light travelling through a transparent material responded to a magnetic field. During the 1860s, James Clerk Maxwell developed four partial differential equations for the electromagnetic field. Two of these equations predicted the possibility and behaviour of waves in the field. Analysing the speed of these theoretical waves, James Clerk Maxwell realised that they must travel at a speed that was about the known speed of light. This startling coincidence in value led James Clerk Maxwell to make the inference that light itself is a type of electromagnetic wave.

James Clerk Maxwell’s equations predicted an infinite number of frequencies of electromagnetic waves, all travelling at the speed of light. This was the first indication of the existence of the entire electromagnetic spectrum.

The Electromagnetic Spectrum

Everything in the universe emits some kind of “light” (electromagnetic radiation or “EM” radiation, for short) but often it is not the kind of light that we are used to (our eyes can see just a small part of the electromagnetic spectrum, the so-called visible part). This visible part consists of the colours that we see in a rainbow. Each of these colours actually corresponds to a different energy and a different wavelength (for example, blue light has more energy and a shorter wavelength than red light). EM waves always carry energy and the energy is directly related to its frequency.

Beyond the red part of the visible spectrum lie types of EM radiation which are invisible to our eyes and have increasingly long wavelengths: infrared, microwave and radio waves. Beyond the blue part of the visible spectrum lie different types of radiation which are also invisible and have increasingly short wavelengths: ultraviolet, X-rays and gamma-rays.

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 1025 hertz, corresponding to wavelengths from thousands of kilometres down to a fraction of the size of an atomic nucleus.

This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelength) end.

The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. 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 (a unit of length that is the distance light travels in one unit of Planck time).

Gamma rays, X-rays, and high ultraviolet are classified as ionising radiation as their photons have enough energy to ionise atoms, causing chemical reactions. Exposure to these rays can be a health hazard, causing radiation sickness, DNA damage and cancer. Radiation of visible light wavelengths and lower are called non-ionising radiation as they cannot cause these effects.

In most of the frequency bands, a technique called spectroscopy can be used to physically separate waves of different frequencies, producing a spectrum showing the constituent frequencies. Spectroscopy is used to study the interactions of electromagnetic waves with matter.

The Physical Basis of Electromagnetic Waves

In the 1800s, James Clerk Maxwell and Heinrich Rudolf Hertz studied how EM waves are formed and how fast they travel. James Clerk Maxwell formulated elegant mathematical equations which showed that electromagnetic waves are formed when an electric field couples with a magnetic field. Magnetic and electric fields of an EM wave are perpendicular to each other and to the direction of the wave. The frequency of an EM wave is determined by the frequency of vibration (oscillation) of charges in the source.

The Wave-Particle Duality of light

A fundamental property of EM radiation is that it can interact with matter in a number of characteristic ways. Under certain conditions it behaves like a wave (reflection, refraction, polarisation, interference, and so on). However, Albert Einstein tried to explain the so-called photoelectric effect, whereby some metals emit electrons when exposed to light. He realised that this effect could not be explained if light existed exclusively as waves. He argued that if light also consisted of individual particles (later called photons), they could result in electrons being ejected from the metallic atoms.

In this way, Albert Einstein explained light’s dual personality: sometimes it behaves as a wave while at other times it behaves a particle. A perfect black body is a theoretical object that is a perfect absorber of the radiation that hits it. Hence, it is also a perfect emitter (of radiation). Max Planck explained the emission of a hot black body by assuming that the energy is carried by discrete “bundles” or “packets” (photons).