The Electromagnetic Spectrum

Light is everywhere. I know right! What a startling revelation! We cannot help but notice the light in front of us, from light bulbs and LED’s, to candles and campfires, to lightning and laser beams, to bioluminescence like glow worms and angler fish, to the Sun and all the other stars in the universe. If you were to direct a sound wave through an underwater bubble, then that bubble will collapse in on itself and produce light. There are still some questions as to why that happens, but it is at least another source of light!

An air bubble collapsing in on itself........
An air bubble collapsing in on itself……..

We cannot see all light though, and scientists like to refer the light we can’t see as radiation rather than light. The visible spectrum that we can see only makes up less than a millionth percent of the entire spectrum of radiation in the universe. We of course call this radiation the Electromagnetic Spectrum.

But before I get into too much detail, let us start by thinking about what exactly light is and how it travels about. Light is a wave, with many peaks and troughs that are all moving up and down in a straight line. Now, if you were to start at any point in the wave then follow the wave past a peak and past a trough until you are back at the same height of the point that you started at, then this is the length of one wave, and it is rather fittingly is called one wavelength. Each type of radiation in the electromagnetic spectrum has a different wavelength: some have small wavelengths meaning they produce a high number of waves over a short distance, whereas radiation with a large wavelength will produce very few waves over that same distance, if even an entire wave.

How to work out the wavelength....
How to work out the wavelength….

Let’s focus on the light that we can see to begin with. The visible spectrum has a wavelength ranging from 380 nanometres to 750 nanometres. A nanometre is 0.000000001 metres, so you can already tell that we are working on a microscopic scale. Now the reason the visible spectrum has such a differing range is because the white light that we can see is actually a combination of 7 colours. Red, Orange, Yellow, Green, Blue, Indigo, Violet, with red light having the 750nm wavelength, violet light having the shorter 380nm wavelength, and all the other colours having the wavelengths in between. A handy way to remember the order is Richard Of York Gave Battle In Vain. Admittedly I’ve never actually bothered to check whether there was a Richard of York who died in battle but it is a good way to remember nonetheless!

The visible spectrum
The visible spectrum

We can of course split white light into its colours by using a simple triangular prism. If you own a copy of Pink Floyds ‘Dark Side of the Moon’ then you will know exactly what I am talking about. Although scientists and philosophers had realised that light could be split into colours since the ancient Greeks, it was Isaac Newton who first discovered the use of prisms to disassemble and reassemble white light in the 17th Century, and picked out the 7 colours believing that it had a link to the 7 musical notes, the 7 days in a week, and the then 7 known objects in the Solar System.

I'll see you on the Dark Side of the Moon
I’ll see you on the Dark Side of the Moon

The visible spectrum was all everyone believed there was for the next two centuries, until in 1800 astronomer William Herschel was interested in the temperature of each colour of light. He decided to split natural sunlight onto a large table, and then placed a thermometer in each portion of light, with an extra thermometer just beyond the red as a control to measure the air temperature of the room. He went away for an hour or so, and when he came back and checked each thermometer he found that his theory that each colour had a different temperature was proved correct, with blue being the coolest and red being the hottest. However, when he checked the control thermometer for the room temperature he found that it was even hotter still than the red light. What he had in fact discovered was Infra-red light, which has a longer wavelength than red light and is invisible to our eyes.

William Herschel carrying out his experiement
William Herschel carrying out his experiement

We have developed a way to view Infra-red through cameras which can be used to decipher the temperature of objects. These are used in the fire service to check for any trapped occupants of burning buildings, and even are now being used to discover new stars in faraway galaxies, allowing us to see a different kind of image than with ordinary telescopes.

The temperatures Infra-red can show
The temperatures Infra-red can show
How Infra-red can show more detail of distance galaxies
How Infra-red can show more detail of distance galaxies

A year after Infra-red radiation was discovered, a German physicist known as Johann Wilhelm Ritter discovered that invisible rays just beyond the violet part of the visible spectrum darkened silver chloride soaked paper quicker than violet light itself. This was the discovery of Ultraviolet light, which is partly the reason we can get sunburnt on those hot summer days!

Scorpions look different when viewed under Ultraviolet
Scorpions look different when viewed under Ultraviolet

So now we have 3 parts of our Electromagnetic spectrum, and all of these are in the middle of our range with Ultraviolet having a wavelength of 100-400nm, Visible Light of 380nm-750nm, and Inra-red of 700nm-1mm. If we carry on to smaller wavelengths then we have X rays at 0.01nm-1nm and finally to Gamma rays at 1 picometre- 10 picometres (which is 0.000000000001m). And on the far side of Infra-red we have Microwaves (1mm-1cm) and Radio waves (1cm-1km)

The discovery of X rays is usually credited to Wilhelm Rontgen, a German physicist who systematically studied X rays in 1895, even though the effects of X rays had already been observed for the previous 20 years. We of course use X rays to look our bones to help see if there has been any chips, breaks or fractures.

x-rays

Gamma radiation was discovered in 1900, by a French chemist and physicist called Paul Villard, and is a result of the decay in atoms.

Microwaves can be bracketed in with Radiowaves, which were predicted James Clerk Maxwell in his equations in 1864. In 1888, they were first demonstrated by Heinrich Hertz when he built a spark gap radio transmitter using a horse trough, a wrought iron point spark and Leyden Jars, a device that stores static electricity.

So in order of wavelength size, from largest to smallest, the Electromagnetic Spectrum consists of Radio waves, Microwaves, Infra-red, Visible light, Ultraviolet,  X rays, Gamma rays.

The Electromagnetic Spectrum
The Electromagnetic Spectrum

I was taught another phrase at school to remember the order:

Rangers Madness Is Very Ugly Across Glasgow.

Being a Rangers fan who currently lives in Glasgow I’ve always taken offence to that!

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