# Electromagnetic spectrum

The electromagnetic spectrum describes the various types of electromagnetic radiation; they can be described in terms of wavelength, frequency, or energy per photon. These are directly coupled (see also table and SI prefix):

• wavelength times frequency is speed of light: 300 Mm/s, i.e. 300 MmHz
• the energy of a photon is 4.1 feV per Hz, i.e. 4.1μeV/GHz
• wavelength times energy per photon is 1.24 μeVm

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, so-called hard radiation: the wavelength of the harder types is so short, in the subatomic range, that we do not have instruments capable of directly measuring it.

 Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultrahigh frequency VHF = Very high frequency HF = High frequency MF = Medium frequency LF = Low frequency VLF = Very low frequency VF = Voice frequency ELF = Extremely low frequency

## Classifications

While the 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. Radio Waves can be detected at the Ultra High Frequency (UHF), Very High Frequency (VHF), Shortwave (HF or high frequency), Medium Wave (AM), Longwave, Very Low Frequency (VLF), and Extreme Low Frequency (ELF) bandwidth.

### Microwaves

The extremely high frequency (EHF) of 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. Low intensity microwave radiation is used in Wi-Fi.

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.

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. A person can see the light scattered from his or her room's light by his or her keyboard, so his or her brain infers that the keyboard exists.

Since current computer screens only use 3 primary colours, only the red, green and blue actually consist of single colours in the image, the rest is composite. Not drawn to scale.

### Ultraviolet light

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.

### X-rays

After UV come X-rays. Hard X-rays are of shorter wavelengths 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.

### Gamma rays

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.

Note that there are no defined boundaries between the types of electromagnetic radiation. Some wavelengths have a mixture of the properties of two regions of the spectrum. For example, red light resembles infra-red radiation in that it can resonate some chemical bonds.