The electromagnetic spectrum gives information of the power radiated by a certain source for given frequencies. As much as it is challenging to understand and interprete a given spectrum, as much information can it reveal. The medium, electromagnetic waves propagate in is just space. This is different to other kinds of waves that require a “real” acting medium like sound waves. The electromagnetic spectrum has different frequencies, i.e. different energies. A lot of interesting thins happen at high energies but starting from the UV, the radiation may destabilize matter and be extremely harmful. However, if we closely study electromagnetic theory we may say that the usual energies we experience everyday are not really dangerous – if they would, we would not be there…
People like to give things a name. This is also true for electromagnetic radiation where you find different energy bands. The highes energies have gamma rays, followed by x-rays, ultra-violet radiation, visible light, infrared radiation, microwaves (hot!), andfinally, radio waves which may have very long wavelengths up to hundreds of meters. Radio waves with small energy cannot be seen by us. However, they are able to transmit signals from television, cell phones, radio, and so on. Large objects like stars and intergalactic gases may also emit radio waves. Microwaves are invisible either. They can be used to transmit data inside telephones, enable Doppler weather radar. In addition, they have very fundamental applications in astrophysics as scientists can study the Milky Way! See also the very nice wiki article on cosmic microwave background.
The third longest wavelengths have infrared waves. These are heat waves i.e. emitted from the sun. Also snakes use infrared waves to hunt down warm animals! (Ok, and of course Predators, if you have seen the movie :)) Humans use IR sensors for military tactics, hunting. And watch out if you have a camera in the cinema – you leave a clear IR footprint that can be used against you!
Now these were all different kinds of electromagnetic waves that we may not see directly. Of course the situation changes if we talk about the visible or optical wavelengths. There a a plethoria of different sources of visible light as the sun and other stars, light bulbs, fastly moving particles or very hot metals. This part of the spectrum is “our domain” and we all have a very good feeling for it, lockily! Here are most of the interesting phenomena happening and without the ability to see visible light, we would have been gone extinct millions of years ago! Now let’s come to the harmful parts of the spectrum. Ultra-violet radiation may burn our skin during the summer. But take care, we cannot see it, UV may even be strong with very thick clouds! UV radiation may also cause cancer but is luckily shielded by the ozone layer (save it!). Stars and other very hot objects (>10kK) may emit UV radiation.
Humans very much depend on the actual composition of the sun’s electromagnetic spectrum. Almost all of our energy we may use comes from the sun through radiation of different wavelengths. This source of energy is the main reason for our existance, for the beauty of nature and all of its diversity. The fascination of the electromagnetic spectrum was always there and will always be as long as humans are able to survive!