Strongest line
656.3 nm
01 · Instruments
Set the axis yourself
Every quantity on this page is one number seen three ways: a length, a rate and a packet of energy. Drag, type or tap here first; further down, the page walks through the same axis band by band.
Logarithmic scrubber21 decades
Wavelength λ
550 nm
Visible · green
Wavelength
550 nm
Frequency
545 THz
Photon energy
2.25 eV
Band
Visible
Same size as
a mitochondrion
Reaches the ground
Yes - optical window
Every wave at oncesound and light on one frequency axis
02 · Scale
What each wavelength is the size of
Wavelength is a length. Matching it against ordinary objects explains most of what a band can and cannot do.
03 · The premise
There is only one spectrum. The names are ours.
A radio wave and a gamma ray are the same object. Both are a self-propagating disturbance in the electromagnetic field, both travel at exactly 299 792 458 m/s in vacuum, and both deliver energy in discrete photons. The only difference between them is how fast the field oscillates - and that single number spans more than twenty powers of ten.
The boundaries you are about to scroll through are not features of nature. They are drawn around how we make the radiation and how we catch it. X-rays and gamma rays overlap completely in energy; what separates them by convention is origin. Terahertz radiation is simultaneously the top of the microwave band and the bottom of the far infrared, and it got its own name mainly because for forty years nobody had a decent source for it.
The band you observe is not always the band that was emitted. The cosmic microwave background left its source as visible and near-infrared light at about 3000 K; 13.8 billion years of cosmological redshift stretched it by a factor of roughly 1100, and it arrives as microwaves at 2.725 K.
c = λf
Wavelength and frequency are locked together by the speed of light. Halve one and you double the other - which is why a single axis is enough to navigate the whole spectrum.
c = 299 792 458 m/sE = hf = hc/λ
Photon energy scales with frequency, not with brightness. A dim ultraviolet lamp delivers more energy per photon than the most powerful radio transmitter ever built.
hc = 1239.84 eV·nmE ≳ 10 eV
Around 10 eV is a useful reference scale for single-photon ionisation of many atoms and molecules, and it falls in the upper ultraviolet. There is no universal ionisation threshold for all matter: the energy required depends on the species and its environment, and ranges from roughly 3.9 eV for caesium to 24.6 eV for helium.
λmax·T = 2898 μm·K
Wien's displacement law. Every warm object radiates, and its peak wavelength is fixed purely by temperature. You at 305 K peak near 9.5 μm; the Sun's photosphere at 5772 K peaks at 502 nm - in the middle of the band your eyes evolved to use.
θ ≈ 1.22 λ / D
For a conventional far-field optical system, diffraction places a fundamental limit on angular resolution for a given wavelength and aperture. The diffraction limit. No conventional far-field instrument resolves detail much finer than this, whatever its quality. It is the reason radio telescopes are kilometres wide and lithography keeps chasing shorter wavelengths.
04 · Before light, sound
The wave you have already met
Sound is not on the electromagnetic spectrum. It is a pressure wave in matter, it travels at 343 m/s in air rather than c, and it cannot cross a vacuum at all. It is here because it is the one wave whose spectrum you can check against your own body, and because the words below - infrasound, audible, ultrasound - are the exact same kind of naming convention you are about to meet eight more times.
Acoustic spectrum0.1 Hz to 10 MHz
Frequency
440 Hz
The scrolling wave starts at 100 km, a 3 kHz radio wave at the bottom of the VLF band. The radio band itself reaches down to 3 Hz, and the ELF, SLF and ULF waves below the start of the scroll are listed in the radio table.
13 · Atmosphere
What the air lets through
Two broad windows plus a handful of narrow infrared gaps are the entire reason ground-based astronomy exists - and the reason every X-ray telescope has to be launched.
Atmospheric opacityfull vertical column
Transparent - reaches sea level
Absorbed - high altitude or orbit only
Vertical axis: fraction absorbed by a full atmospheric column.
14 · Adjacent, and often confused
Alpha and beta are not on this spectrum
The journey ends at gamma, and gamma is usually spoken of in the same breath as alpha and beta. Only one of the three belongs here. The other two are matter - pieces of atom thrown out at speed - and the difference is not academic: it decides what stops them, and what they do to you.
Deflection in a magnetic fieldB into the page
Ernest Rutherford named alpha and beta in 1899, purely by how far they travelled through matter, and added gamma in 1903. Send them through a magnetic field and the classification explains itself. Alpha carries charge +2 and the mass of 7294 electrons, so it bends gently. Beta carries charge −1 and almost no mass, so it bends hard, and the other way. Gamma does not bend at all - no charge, no rest mass. That is what you expect of a photon, and the reason gamma is the only one of the three that is electromagnetic radiation.
Penetrationwhat stops what
Charged particles have a range: they lose energy continuously and then simply stop. Photons and neutrons do not - they are attenuated exponentially, so no thickness guarantees that nothing gets through, only a half-value layer. Note the last lane: lead is excellent against gamma and poor against neutrons, which are slowed by hydrogen. There, a water tank beats a lead wall.
| Type | What it is | Charge | Rest mass | Typical energy | Stopped by | On this spectrum |
|---|
Two different words
Ionising radiation covers all of these - anything able to strip an electron from an atom, whether by Coulomb force (alpha, beta) or by the photoelectric effect, Compton scattering and pair production (gamma). Electromagnetic radiation means the field alone, and covers only gamma. Cosmic rays, mostly protons, are particles too. A neutrino is a particle that barely interacts with anything at all.
15 · From the electron's side
What the photon actually does when it arrives
Bands are a human filing system. An electron does not know it has been hit by "ultraviolet" - it knows only how much energy arrived, and what that amount is enough to do. Walk up the energy ladder and the response changes in discrete, nameable steps.
Interaction chamberE = 4 μeV · λ = 310 mm
Nothing quantum happens
16 · Atoms write the lines
Why elements have colours
An electron bound to an atom cannot take any energy it likes - only the exact differences between the atom's own levels. Every element therefore emits and absorbs a fixed set of wavelengths, unique as a barcode. This is how we know what stars are made of without going there.
Emission spectrumHydrogen
Hydrogen energy levelsone atom, three bands
Hydrogen is a single proton and a single electron, and it still writes lines into three different bands. Every transition that ends on the ground state (n = 1) releases at least 10.2 eV and lands in the ultraviolet - the Lyman series. Transitions ending on n = 2 release 1.9 to 3.4 eV and land mostly in visible light, running into the near ultraviolet toward the series limit at 364.6 nm - the Balmer series, the red Hα at 656.3 nm that colours most emission nebulae. Transitions ending on n = 3 are infrared.
Nothing about the atom changed. What changed is where the transition ends, and therefore how much energy is left over.
En = −13.598 eV / n² - the Rydberg formula for hydrogen, with the Rydberg energy of 13.606 eV reduced slightly because the proton is not infinitely heavy. Every line in the picture is a difference between two values of that one expression, and the series limit at n → ∞ is the ionisation energy.
Run this backwards and you get the Sun. A continuous spectrum leaves the photosphere; cooler gas above it removes exactly the wavelengths its atoms would have emitted, leaving the dark Fraunhofer lines. Select ☉ above to see them.
17 · Converter
Photon calculator
Enter any one of the three; the other two follow exactly from c = λf and E = hc/λ.
Visible
Non-ionising
18 · The same room, nine ways
What a scene looks like in each band
One room, one moment, rendered by the physics of each band rather than by taste. Note especially what happens at the two ends: at long wavelengths diffraction erases all detail, and at very short ones there are simply not enough photons to form a picture.
Imaging benchVisible · 550 nm
19 · Wave behaviour
Six things waves do, at every wavelength
These are not properties of light in particular. They follow from being a wave at all (polarisation needs a transverse wave, so sound in air is the one exception), which is why the same five behaviours turn up in an antenna, a laser cavity, an MRI scanner and a pair of sunglasses.
20 · Optics
Six things light does when it meets matter
These happen where light meets matter, and most of them come down to the refractive index being different on the two sides: its real part sets the phase speed, its imaginary part sets absorption. Open any card for the full derivation and a live model.
21 · What a wave actually is
Two fields, at right angles, sustaining each other
The single sine used everywhere on this page is shorthand. An electromagnetic wave is a changing electric field generating a magnetic field, which generates an electric field, and so on, each perpendicular to the other and to the direction of travel. No material is waving. There is no medium.
Field labdrag to rotate
Polarisation
0°
Maxwell's four equations say that a changing electric field produces a magnetic field and a changing magnetic field produces an electric one. Put those together and the pair can sustain each other while travelling, with no charges and no matter anywhere nearby. That is light.
In vacuum, Maxwell's equations give electromagnetic waves a propagation speed determined by the electromagnetic constants, consistent with the defined value of c. The speed falls out of two constants measured in a laboratory with batteries and magnets: c = 1/√(ε0μ0). When Maxwell computed it in 1862 the number matched the measured speed of light closely enough that he wrote light must be an electromagnetic disturbance. Historically this is a little problematic in modern SI because c is exact by definition, while ε₀ and μ₀ are no longer both exact constants in the old pre-2019 sense.
The direction the electric field points is the wave's polarisation. Sunlight arrives unpolarised, a jumble of every angle. Scattering and reflection sort them, which is what a polarising filter then exploits.
Energy flows along the Poynting vector S = E × B / μ0, which points exactly along the axis of travel. Its magnitude is the intensity, and it is what a detector integrates.
Nineteenth-century physics assumed a medium, the luminiferous aether, because every other known wave needed one. Michelson and Morley failed to find it in 1887, and that null result became one of the key experimental supports for special relativity.
22 · The other kind of wave
Sound, and why the numbers look familiar
Audible sound spans 20 Hz to 20 kHz - numerically the same range as the ELF-to-VLF radio bands. That coincidence is the reason you can plug a VLF antenna into a speaker and simply listen to the ionosphere.
Not the same physics
Sound is a pressure wave in matter: it needs air, travels at about 343 m/s, and stops dead in vacuum. Light is a disturbance in the electromagnetic field: it needs nothing, and travels 874 000 times faster. A 1 kHz sound wave is 34 cm long; a 1 kHz radio wave is 300 km long. The frequencies match, the waves do not.
Tone generator440 Hz · 78 cm in air
Audio starts on your first click
Natural radioVLF, heard directly
These are synthesised here, but the real signals need no conversion at all: a lightning stroke's radio emission at 3 kHz is already at a frequency your ear can process, so a wire and an amplifier are a complete receiver.
Sonify the spectrum -
Take the wavelength currently selected on the scrubber in section 01 at the top of the page, halve its frequency until it falls inside hearing, and play the result. Red light comes out about 40 octaves below itself. This is a transposition, not a sound light makes.
-
23 · Applied
Which industries live in which band
Almost every sector uses more of the spectrum than it realises. The same hospital runs radio, infrared, ultraviolet, X-ray and gamma equipment in adjacent rooms, chosen band by band for what the photon energy is able to do.
24 · How the spectrum was found
Two centuries of widening the window
Nobody set out to discover a spectrum. Each band was found by somebody looking for something else, and each one arrived with an instrument nobody had needed before.
25 · Temperature
Every warm thing glows, and its temperature picks the colour
A black body radiates across the whole spectrum at once, but the peak sits at one wavelength fixed only by temperature. Slide the dial and watch the curve walk from microwaves to ultraviolet.
Planck curveVisible
Temperature
5772 K
Peak wavelength
502 nm
Apparent colour
λmax = b / T
Wien's displacement law, with b = 2.8978 × 10−3 m·K. Double the temperature and the peak halves. Total power goes as T4, which is why a small rise in temperature is such a large rise in output.
26 · Dose
What ionising actually costs, in numbers
Wavelength tells you whether radiation can ionise. Absorbed and effective dose are key quantities used to characterise exposure, but the relationship between dose and biological effect also depends on radiation type, the tissues exposed, dose distribution, dose rate and other factors. The scale below is logarithmic; each step to the right is ten times more.
Effective doselogarithmic · each gridline is ten times more
None of these numbers are advice.
27 · Check
Nine questions that catch the usual misconceptions
Each one targets a place where intuition reliably goes wrong. Your score is kept on this device.
Best score
- / 9
Bands read
0 / 8
28 · Reference
The constants this page calculates with
Physical constants are numerical inputs, not Spectrum Atlas content. c, h, e and k are exact by definition; b, c₂, σ and the eV forms of h and hc follow exactly from them and are shown rounded; the rest are CODATA 2022 measured values. Every name links to its article.