01 · One wave — many faces
Maxwell unified electricity, magnetism and light in 1865.
When James Clerk Maxwell wrote down his four equations in the 1860s, he discovered that an oscillating electric and magnetic field can break free and travel through empty space as a wave — at a speed that matched exactly the already measured speed of light. The conclusion was astonishing: light is an electromagnetic wave. It later turned out that radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays are exactly the same kind of wave. The only thing that separates them is the wavelength.
Two simple equations tie the whole spectrum together. The first concerns the geometry of the wave, the second quantum physics — where the energy sits in the photon itself:
Both say the same thing: short wavelength means high frequency and high energy. That is why a gamma photon carries more than a million million times as much energy as a radio photon — even though both are the same kind of wave. A practical rule of thumb for photons in and around the visible:
02 · Explore the spectrum
Drag the bar — or use the arrow keys — and watch the numbers and the wave change.
03 · The seven regions
From everyday radio signals to the most violent radiation in the universe. Tap a card to jump there in the explorer.
04 · The narrow strip we can see
All visible light fills only a hair-thin slice of the spectrum: about 380–750 nm.
It is a thought-provoking fact: everything our eyes have ever seen — sunsets, rainbows, the starry sky in my eVscope2 — lies in a strip narrower than an octave. The frequency barely doubles from red to violet. Colour is wavelength: long-wave light is seen as red, short-wave as violet.
Just beyond each end lie the neighbours: infrared (the heat radiation we feel but do not see) on the red side, and ultraviolet (what gives sunburn) on the violet side. Many animals see further in both directions than we do — bees, for example, see into the UV.
05 · Three worked examples
The equations applied to reality.
06 · How the spectrum was discovered
Bit by bit, over a century.
- 1800 William Herschel measures the temperature just beyond the red end of the rainbow and discovers infrared — the first invisible radiation.
- 1801 Johann Wilhelm Ritter finds that something beyond the violet end blackens silver salts: ultraviolet.
- 1865 James Clerk Maxwell predicts electromagnetic waves theoretically and calculates their speed to be the speed of light.
- 1887 Heinrich Hertz produces and detects radio waves in the laboratory and confirms Maxwell.
- 1895 Wilhelm Röntgen discovers X-rays and photographs the bones in his wife's hand.
- 1900 Max Planck introduces energy quanta, and Einstein (1905) gives light the photon — the basis of E = hf.
07 · Seeing the universe at every wavelength
The same object tells different stories in different regions.
The atmospheric window
The Earth's atmosphere is transparent only in two broad windows: the visible (with a little near-infrared) and the radio region. That is why my eVscope2 at Marshøj can capture galaxies in visible and near-IR light from the Earth's surface — but ultraviolet, X-rays and gamma rays from space are absorbed by the air and need space telescopes such as Hubble, Chandra and Fermi.
Each wavelength reveals something new: cool dust shines in the infrared (James Webb sees through dust clouds), hot gas around black holes glows in X-rays, and the cosmic microwave background — the oldest light in the universe, cooled to 2.7 K — lies in the microwave region. Multi-wavelength astronomy is the art of putting all these pictures together into one.