Electromagnetic Spectrum Explorer

What the electromagnetic spectrum is

The electromagnetic spectrum is the whole range of electromagnetic radiation, ordered by frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. They are one kind of wave at different frequencies, not seven different things. All of them travel through a vacuum at the same speed, c = 299,792,458 m/s, so wavelength and frequency are locked together by c = λf, and each photon carries an energy E = hf = hc/λ.

Moving right along the explorer raises the frequency and the photon energy and shortens the wavelength, all at once. The range drawn runs from 1 kHz to 10²³ Hz, wavelengths from about 300 km down to 3 fm: twenty powers of ten on one axis. The strip at the top keeps all of it in view, and the lens below magnifies the 1.2 powers of ten around the marker, which is where the finer divisions, the colours of visible light and the named examples appear.

Reading the explorer

Drag or tap along either strip, type a value, or step through the bands with the buttons under the scene. A value can be typed in any unit: a length sets the wavelength, hertz the frequency, and electronvolts or joules the photon energy, and the other two follow. The readouts give:

• Band, one of the seven, from the edges in the table below.
• Sub-band, the finer name: the ITU’s numbered radio bands, the CIE’s UV-A, UV-B and UV-C or IR-A, IR-B and IR-C, or a colour inside the visible.
• Wavelength, frequency and photon energy, each in the unit that suits its size, so a radio wave reads in metres and a gamma ray in picometres.
• Per mole, the energy of a mole of photons, the figure to set against bond energies, since those are quoted per mole.

The wave drawn through the lens has its crests spaced in proportion to the wavelength, so it tightens towards the right as the wavelength shortens. Only that ratio is to scale: across the lens the spacing changes about sixteenfold. The panel under the scene says what produces the band, what it is used for, and which named examples fall inside it, and each example gives the source of its figure.

The seven bands and where their edges come from

Nothing physical happens at any of these edges. The spectrum is continuous, so every boundary is an agreement, and different fields agree on different ones.

• Microwaves are 300 MHz to 300 GHz, the definition the US Occupational Safety and Health Administration uses and exactly bands 9 to 11 of the ITU’s nomenclature in Recommendation ITU-R V.431-8. The ITU’s Radio Regulations count everything below 3000 GHz as radio, so microwaves are the top of the radio range rather than a separate kind of wave.
• Visible light is 380 to 780 nm. The International Commission on Illumination (CIE) says there are no exact limits, and puts the lower one between 360 and 400 nm and the upper between 760 and 830 nm. NASA’s educational pages use 380 to 700 nm.
• Infrared starts at 780 nm, where the CIE’s IR-A begins, and runs to 300 GHz, which is 0.9993 mm: the CIE’s 1 mm to within 0.07 percent.
• Ultraviolet, X-rays and gamma rays are divided at 10 nm and 10 pm by common convention, and other sources draw both lines elsewhere. The CIE’s UV-A, UV-B and UV-C cover 100 to 400 nm.

The same table names the region on the Wavelength and Frequency Calculator and the Photon Energy Calculator, so every page of this site names a wavelength from the same edges.

Worked example: a microwave oven

A microwave oven runs at 2.45 GHz. What are its wavelength and its photon energy?

• Wavelength: λ = c / f = 299792458 / 2.45e9 = 0.1224 m, which is 12.24 cm.
• Energy: E = hf = 6.62607015e-34 × 2.45e9 = 1.623e-24 J.
• In electronvolts: 1.623e-24 / 1.602176634e-19 = 1.013e-5 eV, which is 10.13 µeV.
• Per mole: E × N_A = 6.62607015e-34 × 2.45e9 × 6.02214076e23 = 0.9776 J/mol, which the explorer shows as 0.0009776 kJ/mol.

That photon is about 360,000 times too weak to break a carbon to carbon single bond, at around 350 kJ/mol, and about 2,500 times smaller than the thermal energy kT of a molecule at 20 °C. So a microwave oven cannot break bonds or ionise anything: each photon carries far too little. Its field oscillates 2.45 billion times a second, and the water molecules it keeps turning back and forth pass that energy on to the food as heat.

At the other end of the scale, the gamma ray from technetium-99m, the isotope gamma cameras image in nuclear medicine, carries 140.5 keV: a wavelength of 8.824 pm and a frequency of 33.98 EHz, about 14 billion times the energy of the oven’s photon. Type 2.45 GHz or 140.511 keV into the explorer to check the figures.

Named examples on the spectrum

Why X-rays and gamma rays overlap

In most of physics the two names describe where a photon comes from, not how short its wavelength is. X-rays come from electrons, whether dropping between the inner energy levels of an atom, braking sharply in a metal target or bending in the magnets of a synchrotron. Gamma rays come from atomic nuclei, and from particle processes such as a positron annihilating with an electron. Those sources overlap in energy.

The 14.41 keV gamma ray from an iron-57 nucleus, the line Mössbauer spectroscopy uses, has a wavelength of 86.02 pm. The 17.48 keV X-ray from a molybdenum target has more energy and a shorter wavelength, 70.93 pm. To draw one scale the explorer follows the common convention of calling everything shorter than 10 pm a gamma ray, and its Sub-band readout says “Named by its source” across both bands as a reminder.

Astronomy is the exception. Astronomers often cannot know how a photon was made, and they choose their detectors by its energy, so they name the two by energy. That is why the flashes that thunderstorms send into space are called gamma rays, although fast electrons produce them.

Why visible light is such a thin slice

Visible light runs from 380 to 780 nm, a factor of about 2.05, which is roughly a third of one power of ten out of the twenty drawn.

Its colours are computed rather than painted. Each wavelength’s CIE 1931 colour matching values, from the analytic fit Wyman, Sloan and Shirley published in 2013, are converted to the sRGB colours a screen can show with the matrix in the W3C’s CSS Color specification. Every pure spectral colour lies outside what sRGB can show, so each is drawn as the nearest sRGB colour of the same hue, with some white added. Beyond 420 nm at one end and 650 nm at the other the hue of spectral light barely changes, so the strip holds those hues and darkens towards 380 and 780 nm instead, to show vision fading out rather than stopping. The Double-Slit Simulator shows how interference measures these wavelengths.

Ionising and non-ionising radiation

With ordinary light, whether a photon can knock an electron out of an atom depends on the photon’s energy and the atom, not on how many photons arrive. A hydrogen atom needs 13.6 eV, a wavelength of 91.2 nm or less, and a water molecule 12.6 eV, 98.2 nm or less. UV-A and UV-B photons carry 3.1 to 4.4 eV, so they can ionise neither. They harm skin chemically instead: DNA absorbs UV-B, and two neighbouring thymine or cytosine bases on the same strand bond together. X-rays and gamma rays, from 124 eV upwards, ionise the atoms that absorb them.

Brightness changes none of this. A brighter source sends more photons, each with the same energy, which is why a dim ultraviolet lamp can cause sunburn that a bright red lamp cannot, and why the Photon Energy Calculator needs only a wavelength.

What this explorer leaves out

• Brightness. It describes a single photon. How many photons there are, which is what intensity measures, never enters E = hf.
• Materials. Every wavelength here is a vacuum wavelength. In glass or water light slows by the refractive index and its wavelength shortens in step, while its frequency, photon energy and band stay the same. The Wavelength and Frequency Calculator takes any wave speed, and the Refractive Index Table lists the indices.
• The atmosphere. It does not show which bands reach the ground. NASA notes that the atmosphere absorbs almost all of the Sun’s UV-C and about 95 percent of its UV-B.
• Exact edges. The edges are the conventions set out above. Other sources place them elsewhere, especially between ultraviolet, X-rays and gamma rays.
• Exact colours. The strip is the nearest a screen can show, not the colours themselves, and its fading ends are a display choice.
• Anything outside 1 kHz to 10²³ Hz. The 50 Hz of mains electricity and the gigaelectronvolt gamma rays that space telescopes record are electromagnetic too, beyond the ends drawn here.

Common mistakes

• Picturing infrared as red and ultraviolet as violet. Infrared is not red light and ultraviolet is not violet light. The names only say which side of the visible band they sit on, which is why both are drawn in grey.
• Thinking a radio wave is a sound wave. A radio carries sound as a signal, but the wave itself is electromagnetic and travels at c, while sound is a pressure wave that travels at about 343 m/s in air.
• Treating microwaves as something other than radio. They are the top of the radio range, split off because the technology changes, not the physics.
• Thinking every gamma ray carries more energy than every X-ray. Outside astronomy the names follow the source, and the energies overlap: the iron-57 gamma ray above is weaker than the molybdenum X-ray.
• Calling all ultraviolet ionising. Only the shortest wavelengths are. Sunburn comes from UV-B, which cannot ionise.
• Expecting round numbers to agree in every unit. 300 GHz is 0.9993 mm, not 1 mm, because c is 2.998 × 10⁸ m/s rather than 3 × 10⁸. A table that quotes both has rounded one of them.