Electromagnetic Spectrum — The electromagnetic spectrum describes the various types of electromagnetic radiation based on their wavelengths.
Radio, representing wavelengths from a few feet to well over a mile, is at one end of the spectrum. Gamma ray radiation is at the other end: the wavelength of the harder types is so short, in the subatomic range, that we do not have instruments capable of directly measuring it.
While the above classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic radiation. For example some low energy gamma-rays actually have a longer wavelength than some high energy X-rays.
This is possible because “gamma-ray” is the name given to the photons generated from nuclear decay or other nuclear and subnuclear processes, whereas X-rays on the other hand are generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma-ray and x-ray is related to the radiation source rather than the radiation wavelength.
Generally, nuclear transitions are much more energetic than electronic transitions, so most gamma-rays are more energetic than x-rays. However, there are a few low-energy nuclear transitions (eg. the 14.4 keV nuclear transition of Fe-57) that produce gamma-rays that are less energetic than some of the higher energy X-rays.
Use of the radio frequency spectrum is regulated by governments. This is called frequency allocation.
Radio is at the weak end of the spectrum, with low energy and long wavelength. It’s used for transmission of data, via modulation. Television, mobile phones, wireless networking and amateur radio all use it.
Microwaves come next. They can cause entire molecules to resonate. This resonance causes water to move rapidly and enables the microwave oven to cook food.
Between 300 GHz and the mid-infrared, the absorption of electromagnetic radiation by molecular vibration in the Earth’s atmosphere is so great that the atmosphere is effectively opaque to electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window freqency ranges. However, there are certain wavelength ranges (“windows”) within the opaque range which allow partial transmission, and can be used for astronomy.
It should be noted that the average Microwave oven is, in close range, powerful enough to cause interference with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.
The next category is infra-red. This makes chemical bonds resonate. When a chemical bond resonates, the vibrations add internal energy to the molecule. The molecule becomes hot. The bulk substance becomes hot when its molecules’ bonds are all resonating. When you touch it, you feel its warmth or you lose the tip of your finger, depending on how violent the resonance is.
Visible radiation (light)
After infra-red comes visible light. This is the range in which the sun and stars similar to it emit most of their radiation. When this is scattered or reflected by an object, we can infer the existence of the object. I can see the light scattered from my room’s light by my keyboard, so my brain infers that the keyboard exists.
Next comes ultraviolet. This is radiation whose wavelength is shorter than the violet end of the visible spectrum. It was discovered to be useful for astronomy by a Mariner probe at Mercury, which detected UV that “had no right to be there”. The dying probe was turned over to the UV team full time.
The UV source turned out to be a star, but UV astronomy was born. Being very energetic, UV can break chemical bonds. Chlorine will not normally react with an alkane, but give it UV and it reacts quickly. This is because the UV breaks the bond holding chlorine atoms into molecules of Cl2. Lone atoms are extremely reactive and will react with the otherwise almost-inert alkanes. It also makes a mess of DNA, causing cell death at best and uncontrolled cell reproduction (cancer) at worst.
After UV come X-rays. Hard X-rays are of shorter wavelength than soft X-rays. X-rays are used for seeing through some things and not others, as well as for high-energy physics and astronomy. Black holes and neutron stars emit x-rays, which enable us to study them.
After hard X-rays come gamma rays. These are the most energetic photons, having no lower limit to their wavelength. They are useful to astronomers in the study of high-energy objects or regions and find a use with physicists thanks to their penetrative ability and their production from radioisotopes.