The Electro-Magnetic Spectrum

View the U.S. Full Spectrum Chart (Acrobat format - make sure to set zoom to 100%)

For data communications, we will look at the RF frequency bands, and the light bands for optical fiber systems.  But for a better understanding it helps to look at the full spectrum.  Use of the radio frequency spectrum is regulated by the FCC and this is called frequency allocation.  Here we summarize the full spectrum, from 3 Hz to Infinity.

NOTE:  special thanks to Wikipedia for much of this info

Definition of a Spectrum

A spectrum is a full sequence of frequencies which run from a specific minimum, and increase continuously to a specific maximum.  For example, the audio spectrum and the electro-magnetic spectrum.

Electromagnetic waves move at the speed of light through space.  Audio waves move at the speed of sound and require air to propagate (sound cannot propagate in a vacuum - i.e. outer space).  The two spectrums methods of propagation are completely different.

The two spectrums include the same lower frequencies.  However, audio cannot propagate through air beyond a specific maximum frequency.  Electromagnetic waves, on the other hand, have no upper limit on frequency.  

The properties of an electro-magnetic wave are EXTREMELY COMPLEX, and as such will not be covered here.  Suffice it to say, they carry electrical and magnetic waves, which alternate in intensity with a sine wave property, much the same way that AC current does. The full spectrum is broken up into several classifications, which are further divided into frequency bands.  

The Waves - Frequency vs Wavelength

Electromagnetic waves travel at the speed of light, and have a sinusoidal pattern - defined by both frequency and wavelength.  The frequency of the waves is inversely proportional to the wavelength, by a factor of the propagation speed (the speed of light) !!  Therefore:

freq = speed of light/wavelength = 3x10⁸/wavelength 

     wavelength = speed of light/frequency = 3x10⁸/freq

*** freq is in Hertz  and  wavelength is in Meters  

The Electro-Magnetic Spectrum

Basically, as you move from lower to higher frequencies - the lower portion of the spectrum is Radio frequencies, then you get to visible light frequencies, and then invisible ultra-high frequencies which are used for X-rays.

• Radio Waves - (3 Hz to 300 GHz) - divided into ten frequency ranges (10 freq bands)
• Microwaves - (1 GHz to 110 GHz) - fully contained within the RF spectrum
• Infrared radiation - (300 GHz (1 mm) to 400 THz (750 nm) - Near, Moderate, and Far bands
• Visible radiation (light) - (400 THz to 700 THz)
• Ultraviolet light - Near and Extreme bands (700 THz to 30 PHz - 380 to 10 nm)
• X-rays - Soft and Hard bands - (30PHzto 60EHz - 5pm to 10 nm)
• Gamma rays - radiation

Within these major classifications, there are a number of defined bands of frequencies.  Here we list them, with the hight frequencies at the top:

γ = Gamma rays
HX = Hard X-rays
SX = Soft X-Rays
EUV = Extreme ultraviolet
NUV = Near ultraviolet
Visible light
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

Radio Frequency, or RF Spectrum (3 Hz to 300 GHz)

Refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. Radio waves cover the low-frequency, long-wavelength end of the spectrum. It is 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.Such frequencies account for the following parts of the spectrum:

Note: above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

The ELF, SLF, ULF, and VLF bands overlap the AF (audio frequency) spectrum, which is approximately 20–20,000 Hz. However, sounds move at the speed of sound, rather than the speed of light.

Microwaves (1 to 110 GHz)

Microwaves occupy the upper portion of the RF range - the extremely high frequency (EHF).  Microwaves are typically absorbed by molecules that have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave radiation is used in Wi-Fi.

It should be noted that an average Microwave oven in active condition 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.

Infrared radiation (300 GHz to 400 THz)

The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:

• Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in the Earth's atmosphere absorbs so strongly in this range that in renders the atmosphere effectively opaque. However, there are certain wavelength ranges ("windows") within the opaque range which allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as "sub-millimeter" in astronomy, reserving far infra-red for wavelengths below 200 μm.

• Mid-infrared, from 30 THz (10 μm) to 120 THz (2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, that is, when the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region since the mid-infrared absorption spectrum of a compound is very specific for that compound.

• Near-infrared, from 120 THz (2.5 μm) to 400 THz (750 nm). Physical processes that are relevant for this range are similar to those for visible light.

Visible radiation (light)

After infrared comes visible light. This is the range in which the sun and stars similar to it emit most of their radiation. It is probably not a coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another.

The spectrum of visible light (400 to 700 THz)

Ultraviolet light (10 to 380 nm)

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.

Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of visible light, but longer than that of soft X-rays. The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelength of visible light. It is colloquially called black light, as it is invisible to the human eye.

UV itself can be subdivided into near UV (380-200 nm wavelength) and extreme or vacuum UV (200-10 nm). When considering the effects of UV radiation on human health and the environment, the range of UV wavelengths is often subdivided into UV-A (380-315 nm), UV-B (315-280 nm), and UV-C (280-10 nm). See 1 E-7 m for a list of objects of comparable sizes.

Ordinary glass is transparent to UV-A but is opaque to shorter wavelengths. Quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths.

The sun emits ultraviolet light in the UV-A, UV-B, and UV-C bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet light that reaches the Earth's surface is UV-A. (Some of the UV-C light is responsible for the generation of the ozone.)

X-rays (30PHz-60EHz  or  5pm-10 nm)

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.  An X-ray (German: Röntgenstrahlen) is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz to 60 EHz). X-rays are primarily used for diagnostic medical imaging and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous.

X-rays with a wavelength longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays. Hard X-rays reach a frequency that overlaps the range of long-wavelength (low energy) gamma rays

Gamma Rays (60 EHz to Infinity  or  10 nm to 0 nm)

After hard X-rays come gamma rays. These are the most energetic photons, having no lower limit to their wavelength, or no upper limit to their frequency. 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. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering.

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.

Gamma Rays are an energetic form of electromagnetic radiation produced by radioactivity or other nuclear or subatomic processes. Gamma rays are a form of ionizing radiation and are produced by nuclear transitions - while X-rays are produced by energy transitions due to accelerating electrons.

NOTE:  Because it is possible for some electron transitions to be of higher energy than nuclear transition, there is an overlap between low energy gamma rays and high energy X-rays.