## Electromagnetic Spectrum

#### Learning Objective

• Calculate frequency or photon energy, identify the three physical properties of electromagnetic waves

#### Key Points

• The electromagnetic spectrum includes common regimes such as ultraviolet, visible, microwave, and radio waves.
• Electromagnetic waves are typically described by any of the following three physical properties: frequency (f), wavelength (λ), or intensity (I). Light quanta are typically described by frequency (f), wavelength (λ), or photon energy (E). The spectrum can be ordered according to frequency or wavelength.
• Electromagnetic radiation interacts with matter in different ways in different parts of the spectrum. The types of interaction can range from electronic excitation to molecular vibration depending on the different types of radiation, such as ultraviolet, X-rays, microwaves, and infrared radiation.

#### Terms

• spectrumA range of colors representing light (electromagnetic radiation) of contiguous frequencies; hence electromagnetic spectrum, visible spectrum, ultraviolet spectrum, etc.
• photonThe quantum of light and other electromagnetic energy, regarded as a discrete particle having zero rest mass, no electric charge, and an indefinitely long lifetime.
• gamma rayElectromagnetic radiation of high frequency and therefore high energy per photon.

## Range of the Electromagnetic Spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The electromagnetic spectrum of an object has a different meaning: it is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length (1.616 x 10-35 m), although in principle the spectrum is infinite and continuous.

Most parts of the electromagnetic spectrum are used in science for spectroscopic and other probing interactions, as ways to study and characterize matter. In general, if the wavelength of electromagnetic radiation is of a similar size to that of a particular object (atom, electron, etc.), then it is possible to probe that object with that frequency of light. In addition, radiation from various parts of the spectrum has been found to have many other uses in communications and manufacturing.

## Energy of Photon

Electromagnetic waves are typically described by any of the following three physical properties: the frequency (f) (also sometimes represented by the Greek letter nu, ν), wavelength (λ), or photon energy (E). Frequencies observed in astronomy range from 2.4×1023 Hz (1 GeV gamma rays) down to the local plasma frequency of the ionized interstellar medium (~1 kHz). Wavelength is inversely proportional to wave frequency; hence, gamma rays have very short wavelengths that are a fraction of the size of atoms, whereas other wavelengths can be as long as the universe. Photon energy is directly proportional to the wave frequency, so gamma ray photons have the highest energy (around a billion electron volts), while radio wave photons have very low energy (around a femto-electron volt). These relations are illustrated by the following equations:

$f = \frac{c}{\lambda} or f= \frac{E}{h} or E= \frac{hc}{\lambda}$

c = 299,792,458 m/s is the speed of light in vacuum

h = 6.62606896(33)×10−34 J s = 4.13566733(10)×10−15 eV s = Planck’s constant.

Whenever electromagnetic waves exist in a medium with matter, their wavelength is decreased. Wavelengths of electromagnetic radiation, no matter what medium they are traveling through, are usually quoted in terms of the vacuum wavelength, although this is not always explicitly stated. Generally, electromagnetic radiation is classified by wavelength into radio wave, microwave, terahertz (or sub-millimeter) radiation, infrared, the visible region we perceive as light, ultraviolet, X-rays, and gamma rays. The behavior of electromagnetic radiation depends on its wavelength. When electromagnetic radiation interacts with single atoms and molecules, its behavior also depends on the amount of energy per quantum (photon) it carries.

## Interaction of Elecromagnetic Radiation with Matter

Electromagnetic radiation interacts with matter in different ways in different parts of the spectrum. The types of interaction can be so different that it seems justified to refer to different types of radiation. At the same time, there is a continuum containing all these different kinds of electromagnetic radiation. Thus, we refer to a spectrum, but divide it up based on the different interactions with matter. Below are the regions of the spectrum and their main interactions with matter:

• Radio: Collective oscillation of charge carriers in bulk material (plasma oscillation). An example would be the oscillation of the electrons in an antenna.
• Microwave through far infrared: Plasma oscillation, molecular rotation.
• Near infrared: Molecular vibration, plasma oscillation (in metals only).
• Visible: Molecular electron excitation (including pigment molecules found in the human retina), plasma oscillations (in metals only).
• Ultraviolet: Excitation of molecular and atomic valence electrons, including ejection of the electrons (photoelectric effect).
• X-rays: Excitation and ejection of core atomic electrons, Compton scattering (for low atomic numbers).
• Gamma rays: Energetic ejection of core electrons in heavy elements, Compton scattering (for all atomic numbers), excitation of atomic nuclei, including dissociation of nuclei.
• High-energy gamma rays: Creation of particle-antiparticle pairs. At very high energies, a single photon can create a shower of high-energy particles and antiparticles upon interaction with matter.

This classification goes in the increasing order of frequency and decreasing order of wavelength, which is characteristic of the type of radiation. While, in general, the classification scheme is accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as electric power, although the latter is, in the strict sense, not electromagnetic radiation at all.