The Physics of The Electromagnetic Spectrum

Table of Contents How does the electromagnetic spectrum work?Order of the electromagnetic wavesWho discovered the…

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 — collectively termed the “electromagnetic spectrum,” and which visible light is only a small part of.

Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and 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 are microwaves, infrared light, ultraviolet light, X-rays, and gamma-rays.

The electromagnetic waves are distinguished 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. 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.

How does the electromagnetic spectrum work?

Source: Steve Johnson/pexels

When electromagnetic energy travels through space, it spreads out to form a broad spectrum of light, 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 1025hertz, and the wavelengths can vary from around the size of an atomic nucleus to thousands of kilometers. 

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
The Visible Spectrum. Source: Britannica

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 740 nanometers (nm), or 2.9 × 10−5 inches and 380 nm (1.5 × 10−5 inches). This part of the electromagnetic spectrum is called the visible light spectrum.

Order of the electromagnetic waves

Electromagnetic radiation can also defined in terms of a stream of mass-less particles, called photons, traveling in a wave-like pattern at the speed of light. Each photon contains a certain amount of energy. The different types of radiation are defined by the 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.

Th energy, wavelength and frequency of different parts of the electromagnetic (EM) spectrum are given as:

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
Source: University of Rochester

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. 

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
Source: Chegg Study

Among all the light rays, gamma rays have the maximum frequency and penetrating power. These are used in radiotherapy and radio-oncology. Radio waves have the highest wavelength, therefore, they are best suited for wide-range communication devices and equipment (such as navigation systems, broadcasting setup, radio, wireless technology, etc).

Who discovered the electromagnetic spectrum?

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
William Herschel. Source: Wikimedia Commons

The history of the elucidation of the electromagnetic spectrum can be said to have begun in 1800. That year, astronomer William Herschel published a series of papers describing experiments that led him to identify what is now known as infra-red radiation. Herschel had been using telescopes to observe the Sun, protecting his eyesight with dark glass filters. He noticed that some filters 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.

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 ‘ultra’ violet.

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.

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
A TV remote uses infrared waves. Source: Karolina Grabowska/pexels

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. 

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
James Maxwell. Source: Wikimedia Commons

Maxwell’s work unified the following laws:

Gauss’s Law: According to Gauss 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: It states that an electromotive force (EMF) induced by a change in magnetic flux depends on the change in flux at time (t), and by the number of turns of coils.

Ampere Law: This relates the net magnetic field along a closed loop to the electric current passing through the loop. It states that 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.

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
Source: Victoria Web

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.

Interesting facts about the electromagnetic spectrum

Revealing The Cosmos: The Physics of The Electromagnetic Spectrum
Source: cottonbro/pexels

Both the visible and invisible parts of the electromagnetic spectrum have an undisputed significance because light rays not only affect humans but also bring various biological and chemical changes that happen in the natural world. 

  • The human body may experience two types of EM wave radiations, one is the non-ionizing or low wave radiations that comes from the use of cellphones, Bluetooth headsets, microwave ovens, etc. Second is the 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.
  • 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.
  • There are various animals that can see different parts of the electromagnetic spectrum, bees and hedgehogs can see light in the UV part of the spectrum, various insects and animals such as mosquitos, snakes, and bullfrogs use infrared vision to hunt down their host or prey. Bats use high-frequency (>20KHz) ultrasonic waves to detect the presence of obstacles and prey.
  • Previously, it was believed that cats and dogs are completely colorblind, but that’s not actually true. Cats and dogs only have blue and green cones in their eyes – 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. Dogs can also notice different shades of blue and green, and cat eyes are well equipped to see blue and yellow shades. 
  • 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 European Space Agency to see distant stars and galaxies, works by interacting with UV rays.
  • In the EM spectrum, the red color light has the lowest frequency and longest wavelength of visible light, so it can be easily noticed by human eyes from a great distance, and this is why warning signals, traffic ‘stop’ light, tower lights, etc. are red in color.
  • The visible light that passes through the atmosphere is made up of all the colours of the rainbow. So is the sky blue? As they 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 highest 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.
  • 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.

From our kitchen’s microwave ovens to the sun and large artificial satellites, EM waves are used in a wide number of inventions.