From enabling us to walk around and not bump into things to developing highly advanced directed energy weapons, the electromagnetic spectrum is vitally important to many aspects of our modern lives. But, life as we know it would also not be possible if electromagnetic radiation, notably visible light, did not exist.
For most of human history we have only known (but not fully understood) a very small portion of the spectrum - namely visible light and "heat" in the form of infrared light. But, since the scientific enlightenment our knowledge of the spectrum, and applications using it, have literally revolutionized the way we live and perceive the world and the cosmos around us.
Let's take a look at what exactly EM radiation is, and take a whistle-stop tour through the history of its discovery.
What is the electromagnetic spectrum?
Light is the phenomenon that allows us to see. However, human eyes can not perceive the entire range of wavelengths or frequencies that make up electromagnetic radiation (EM) — collectively termed the "electromagnetic spectrum," of which visible light is but a small portion.
Electromagnetic radiation is energy that travels and spreads out as it goes. For example. visible light that comes from a lamp in your house or the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum include microwaves, infrared light, ultraviolet light, X-rays, and gamma-rays.
Electromagnetic waves are characterized on the basis of their respective energy (E), frequencies (f), and wavelengths (λ). Frequency describes how many wave patterns or cycles pass by a particular point in a given time. Frequency is often measured in Hertz (Hz), where a wave with a frequency of 1 Hz will pass by at 1 cycle per second.
Wavelength is defined as the total distance that exists between the peak of one wave and the peak of the next. Wavelength and frequency are inversely related, or to put it another way, the larger the frequency, the smaller the wavelength – and vice versa. Frequency, wavelength, and energy govern the position of different types of energy in the electromagnetic spectrum.
When electromagnetic energy travels through space, it spreads out to form a broad spectrum of radiation, which involves all the different frequencies that exist between the short-range gamma rays and long-range radio waves. Each wave, with a different frequency than the others, forms its own separate frequency band within the spectrum, and these different bands collectively form the electromagnetic spectrum.
Frequency bands not only reveal the differences between the properties of various electromagnetic waves but also affect the ways that these waves interact with matter. The frequency value in the electromagnetic spectrum ranges from below one Hz to above 1019hertz, and the wavelengths can vary from around the size of an atomic nucleus to thousands of kilometers.
Most of the electromagnetic waves are not visible to the human eye, as human eyes can only perceive light waves that have wavelengths of between around 700 nanometers (nm), or 2.76 × 10−5 inches and 380 nm (1.5 × 10−5 inches). This part of the electromagnetic spectrum is, for this reason, commonly referred to as the visible light spectrum.
Electromagnetic radiation can also be defined in terms of a stream of mass-less packets of energy, called photons, traveling in a wave-like pattern at the speed of light.
The different types of radiation are defined by the amount of energy found in the photons. Radio waves have photons with low energies, microwave photons have a little more energy than radio waves, infrared photons have still more, then visible, ultraviolet, X-rays, and, the most energetic of all, gamma-rays.
The energy, wavelength, and frequency of different parts of the electromagnetic (EM) spectrum are given as:
There is an inverse relationship between frequency and wavelength, but the energy of an EM wave is positively affected by its frequency and amplitude. Therefore, light rays with higher frequency and shorter wavelengths have greater amounts of energy. Longer wavelengths and lower frequency result in lower energy.
EM waves with the highest frequencies, such as gamma, X-ray, and ultraviolet (UV) have the lowest wavelengths, whereas the long-range waves that fall in the radio, microwave and, infrared regions of the spectrum have the lowest energy and frequency values.
Among all the forms of EM radiation, gamma rays have the maximum frequency and, therefore, penetrating power. For this reason, such rays are used in radiotherapy and radio-oncology.
Radio waves, on the other hand, have the highest wavelength and are best suited for long-range communication devices and equipment (such as navigation systems, broadcasting setup, radio, wireless technology, etc).
Who discovered the electromagnetic spectrum?
In a sense, so to speak, we have known about the visible light and infrared parts of the EM spectrum since the earliest days of our species. But it wasn't really until the 1800s that we finally began to systematically attempt to study it in detail.
One of the most important pioneers in the field, astronomer William Herschel, published the results from a series of experiments he conducted in 1800 that led him to identify what is now known as infrared radiation. Herschel had been using telescopes to observe the Sun, and protecting his eyesight with dark glass filters. He noticed that some colors of filter seemed to allow through more of the light, while others transmitted more radiation that warmed things up.
As a result of these observations, Herschel set up an experiment where sunlight was passed through a slit and then through a prism, forming a spectrum on his table. Using thermometers, he measured the temperature at different points in the spectrum.
He found out that the highest temperature actually occurred in the empty region of the spectrum beyond red light. Herschel came to the conclusion that 'heat' and light are part of the same spectrum.
Later, a German chemist, Johann W. Ritter, was intrigued with Herschel’s findings. In 1801, he noticed that invisible light beyond the optical region of the electromagnetic spectrum darkened silver chloride. He used a prism to split sunlight and then measured the relative darkening of the silver chloride as a function of wavelength. He found that the region just beyond violet produced the most darkening, and so this region was eventually christened "ultraviolet".
At the same time, physicist Alessandro Volta reported the invention of a battery, which allowed experimenters to begin working with continuous direct current. Around 20 years later, Hans Christian Ørsted demonstrated a link between electricity and magnetism when he showed that a compass needle would move when brought close to a current-carrying wire. In the early 1830s, Michael Faraday demonstrated that drawing a magnet through a loop of wire could generate current.
Faraday suggested there was an invisible “electrotonic state” or field surrounding the magnet. He suggested that changes in this electrotonic state are what cause electromagnetic phenomena, and hypothesized that light itself was an electromagnetic wave. There was clearly a system at work, but it was not yet clearly understood.
In the 1850s, James Clerk Maxwell, an English scientist, set out to make mathematical sense of Faraday’s observations. In a series of papers over the next decade, he developed a scientific theory to explain electromagnetic waves. Focusing on mathematics, he described how electricity and magnetism are linked and how they move in concert to make an electromagnetic wave.
Maxwell's work was revolutionary, and allowed for the unification of the following laws:
- Gauss's Law: According to Gauss's Law, the net outward normal electric flux for any closed surface is directly proportional to the total electric field within that closed surface.
- Gauss's Law for Magnetism: The magnetic flux for a closed surface comes out to be zero because the inward flux value at the south pole is equal to the outward flux at the north pole.
- Faraday's Law: States that an electromotive force (EMF) induced by a change in magnetic flux depends on the change in flux at the time (t), and by the number of turns of coils.
- Ampere's Law: This relates the net magnetic field along a closed loop to the electric current passing through the loop. It states that the closed line integral of the magnetic field around a current-carrying conductor is equal to absolute permeability times the total current through the conductor.
Maxwell's equations described the behavior of electric and magnetic fields and their influence on other objects. In his analysis, Maxwell also concluded that EM waves must travel at what later turned out to be the speed of light, and finally, that light was an electromagnetic wave. Through his equations, Maxwell also described the possibility of numerous EM waves with different frequencies, and therefore, he mathematically predicted the presence of the electromagnetic spectrum.
However, there was no experimental evidence for Maxwell's theories. After Maxwell's death, physicists George Francis FitzGerald and Oliver Lodge worked to strengthen the link to light, but it was German researcher, Heinrich Hertz who, in 1888, published work that demonstrated the first detection of radio-frequency waves.
He also went on to verify that electromagnetic waves exhibit light-like behaviors of reflection, refraction, diffraction, and polarization. Hertz was also able to calculate the speed of these invisible waves, which was quite close to that now known for visible light. His work would eventually lead to the innovation of the radio, cellular networks, air traffic control systems, and many other important inventions.
In the years that followed, Wilhelm Roentgen discovered X-rays (also called the Roentgen rays) and Paul Villard discovered what would later be named gamma rays. Physicists Ernest Rutherford and Edward Andrade also studied gamma rays and revealed significant details about their wavelength and other properties. While studying radioactive decay, Rutherford distinguished gamma rays from alpha and beta rays due to their higher degree of penetration through matter.
What are some interesting facts about the electromagnetic spectrum?
1. EM radiation is either harmless or very dangerous to living things
Humans and every other living thing can be exposed to two main types of EM wave radiations. The first is non-ionizing or low wave radiations that come from things like cellphones, Bluetooth headsets, microwave ovens, etc. The other is ionizing radiation such as UV rays from the sun, gamma rays, x-rays, etc. Continuous exposure to high amounts of ionizing radiation may lead to cancer, insomnia, skin burn, blindness, and various other kinds of neurological or physiological disorders.
2. Thank you lucky stars you can only see visible light
If human eyes could perceive all the rays in the electromagnetic spectrum, then we would not be able to see anything but an overwhelming glow. The excess of light may turn things and objects unclear to our eyes, and in that case, it would be impossible for our brain to understand the information coming through our eyes.
3. Some animals can see other parts of the spectrum
There are various animals that can see different parts of the electromagnetic spectrum, bees, and hedgehogs, for example, can see some light in the UV part of the spectrum.
Various insects and animals such as mosquitos, snakes, and bullfrogs use portions of the infrared spectrum to hunt down their host or prey. Bats use high-frequency (>20KHz) ultrasonic waves to detect the presence of obstacles and prey.
4. Cats and dogs aren't actually colorblind
Previously, it was believed that cats and dogs are completely color blind - but that’s not actually true. Cats and dogs only have blue and green cones in their eyes, meaning they lack red cones, which are present in humans.
This means they have a much more muted perception of color than humans. Because cats and dogs are not sensitive to red light, they do have difficulty distinguishing some colors. For example, it is likely that dogs can distinguish red from blue but often confuse red and green, likely seeing both as shades of grey. Dogs can also notice different shades of blue and green, and cat eyes are well equipped to see blue and yellow shades.
5. Different parts of the spectrum have interesting properties
Microwaves are not interrupted by rain, fog, smoke, or clouds, and gamma rays have the ability to pass through the whole human body. The large Hubble telescope that is used by NASA and the European Space Agency to see distant stars and galaxies, works by interacting with UV rays and can also capture visible, and near-infrared wavelengths.
6. There is a reason red is chosen for stop signs
In the EM spectrum, the red color light has the lowest frequency and longest wavelength of visible light. This means it can be easily noticed by human eyes from a great distance.
This is why warning signals, traffic stoplights, tower lights, etc. are red in color. The color red is also commonly associated with danger in many cultures.
7. Here's why the sky is blue
The visible light that passes through the atmosphere is made up of all the colors of the rainbow. So why is the sky blue?
As the lightwaves enter our atmosphere, the visible light waves collide with nitrogen and oxygen molecules in the atmosphere and are scattered. The amount of scattering depends on the wavelength of the light.
The smaller the wavelength of the light, the more it is scattered. Blue and violet light has the shortest wavelengths, so are more scattered. Because the Sun emits a higher concentration of blue light waves and our eyes are more sensitive to blue light, the sky appears blue rather than violet.
8. The "Northern Lights" are very special indeed
A recent report suggests that the formation of auroras, like the famous Aurora Borealis, or Northern Lights, takes place when strong electromagnetic waves arise during a geomagnetic storm, through a phenomenon known as Alfven waves.
And that, EM-enthusiasts, is your lot for today.
For most of human history, while we could "see" and "feel" some forms of EM radiation, we did not understand the full EM spectrum. It took millennia of advancement for us to truly appreciate this amazing natural phenomenon. Without our knowledge of it today, our world would literally and figurately look very different.