What is electromagnetic radiation?
Astronomers talk about light, radiation, gamma rays, x-rays, microwaves, infrared, ultraviolet and radio waves. There are many terms but in fact they are different types of the same thing. But what is it?
Our eyes can only detect visible light waves. Until very recently, all we knew about radiation was what we could see with our own eyes - visible light. For a long time, philosophers pondered on what exactly visible light was. How fast could it travel? Did it have any weight? What exactly was it?
Five hundred years after the Muslim philosopher Ibn al-Haitham discovered the laws of refraction, Sir Isaac Newton investigated light and used a prism to demonstrate that white light could be refracted into its constituent colours - white light was a combination of all the colours of the visible light spectrum.
On further investigation, it was found that what made each colour distinct was that light travels in waves. Each colour had its own wavelength, the distance between the crests or troughs of adjacent waves that ranged from violet - the shortest wavelength of 400 nanometres - to red with the longest wavelength of 700 nanometres.
The other visible colours lie between these two extremes. Wavelengths could obviously be any length from just above zero up to near infinite length. The problem was that outside the very small range of 400 to 700 nanometres, we couldn't see it.
Sunburn
Two labels were created: the infrared (wavelengths more than 700 nanometres) and the ultraviolet (wavelengths below 400 nanometres). Scientists knew that infrared light and ultraviolet light did exist. Ultraviolet light, without us being able to see it, caused sunburn. Similarly infrared was discovered by William Herschel who found that light beyond visible red could make things hot.
Because using the word 'light' is confusing – in everyday language light is always visible - the name 'radiation' began to be used instead. Visible light became the name only of the radiation which lies between 400 and 700 nanometres.
Waves or particles
Nineteenth-century scientists did a lot of investigation into radiation. They found that it can be described in terms of a stream of photons, which are massless particles containing an amount of energy, travelling in a wave-like pattern and moving at the speed of light. The process which caused this to happen was electromagnetic – so it was then more-correctly called electromagnetic radiation. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons.
The electromagnetic spectrum is subdivided into seven regions according to wavelength. Each portion of the spectrum interacts with matter in a slightly different way and is given a different name. In space, a solid object releases electromagnetic radiation at many wavelengths but with peaks at a wavelength corresponding to the temperature of the object. Cooler objects release most of their energy at longer wavelengths and hotter objects at shorter wavelengths.