It looks like a coincidence and it is not. Energy and wavelength are tied by E = hc/λ, and hc — Planck's constant times the speed of light — happens to come out as 1239.84 eV·nm. Both of those constants have been exact by definition since the SI redefinition in 2019, so 1240 is not a measurement that might improve. It is a rounding, and it is good to one part in 1,300.
That is why a 620 nm red photon is 2 eV, why silicon with its 1.1 eV band gap stops responding past about 1,100 nm, and why anyone working in optics can convert between the two in their head.
It is vacuum wavelength only. Put the same photon in glass and its wavelength shortens by the refractive index — about a third in a typical crown glass — while its energy does not change at all. Feed the glass wavelength into 1240/λ and you get an energy the photon never had. Frequency is the safe quantity to carry across a boundary, because frequency is what stays fixed.
And it is per photon, not per beam. A 1 mW green laser and a 1 mW radio transmitter deliver the same power; the laser does it with about 3 × 1015 photons a second and the transmitter with roughly 1025. Nothing about the total tells you what a single photon can do when it arrives, and for anything to do with breaking bonds it is the single photon that matters.
The spectrum is one continuous thing. Nothing changes at 700 nm except that your eye stops responding, and the lines drawn on every textbook chart are a mixture of human physiology, engineering practice and historical accident.
Visible is defined by the observer, not the light — the usual 380–700 nm is a convention, and real eyes keep a little sensitivity past both ends. X-ray and gamma actually overlap, and the distinction between them is not wavelength at all: gamma rays are the ones emitted by an atomic nucleus, X-rays the ones from electrons. A 100 keV photon is an X-ray or a gamma ray depending entirely on where it came from, which is why this tool labels that region by origin as well as by number.
The one boundary that is physical is ionisation, and even that is a threshold rather than a line — it depends on what is being ionised.
The chart above draws the spectrum as though all of it were available. From the bottom of the atmosphere, almost none of it is. Air is a filter with two clear gaps in it — the optical window and the radio window — and outside those, a column of ordinary sea-level air is as good as a wall.
The optical window is about 300 nm to 1.1 µm, and the visible band sits inside it. That is not a coincidence in either direction. The Sun's output peaks near 500 nm, the atmosphere happens to be clear there, and eyes evolved in the overlap — there would be no advantage in being sensitive to light that never arrives. Ozone closes the window's short end hard: at 250 nm the transmission is effectively zero, which is the only reason surface life is possible at all.
The radio window runs from roughly 1 cm to about 10 m, and it is far wider in ratio terms — three decades against less than one. It is bounded at the short end by water vapour and oxygen and at the long end by the ionosphere, which reflects rather than absorbs: below about 10 MHz the ionosphere bounces signals back, which keeps shortwave broadcasting working around the curve of the Earth and keeps low-frequency radio astronomy impossible from the ground.
Between them, in the thermal infrared, is a third gap at 8–13 µm. A surface at 288 K radiates with its peak near 10 µm, right in the middle of it, and that gap is how the planet loses heat to space. Carbon dioxide's band at 15 µm sits on the long edge of it, water vapour's at 6.3 µm on the short edge, and adding absorber narrows the gap from both sides. The greenhouse effect is not a metaphor about glass; it is this window closing.
Point the scrubber at 550 nm, the middle of the visible: about 80% of it gets through, and you can observe from a hillside.
Now move to 6.3 µm, a water-vapour band. Transmission collapses to a couple of per cent. There is no exposure time long enough to fix that — the signal is not faint, it is absent — and this is the whole argument for putting an infrared telescope in space rather than on a mountain.
Move again to 21 cm, the hydrogen line. Transmission is back near 98%, which is why the map of neutral hydrogen in our galaxy was drawn from the ground with equipment that would fit in a car park — and why the largest telescopes ever built are radio dishes rather than mirrors.
Because they are the ones that arrive. The Sun's output peaks near 500 nm and the atmosphere is clear from roughly 300 nm to 1.1 µm; sensitivity outside that would cost energy and return nothing. The visible band is the overlap between the star we orbit and the air we live under, not a property of light.
Because "transmitted" is not the same as "undeflected". Rayleigh scattering by air molecules goes as 1/λ4, so blue is scattered about ten times more strongly than red — it still reaches you, just from every direction rather than from the Sun. At sunset the path through the air is far longer, so more of the blue is scattered out before it arrives.
From orbit, entirely. Every X-ray and gamma-ray telescope ever operated has been above the atmosphere, because the absorption is not partial — it is total, and it is total within the first few tens of kilometres. The upside is that the same wall is what stops the sterilising part of the Sun's output from reaching the ground.
No — that is one of the most repeated errors in physics. Water's strong rotational resonances are far higher in frequency; 2.45 GHz sits on the broad shoulder of that absorption precisely because it is weak enough to penetrate several centimetres into food rather than cooking the surface. The frequency was chosen for that penetration depth and because the band was set aside for industrial use.
Radio and gamma differ only in wavelength, by a factor of about ten thousand million million. There is no change of substance anywhere along the axis, which is why one equation, E = hc/λ, runs from end to end without a special case.
Send light into glass and its speed drops, so its wavelength shortens, but its frequency, and therefore its energy, do not change. Tracking a photon across a boundary, carry the frequency and derive the rest.
You are radiating right now, peaking at about 9.3 µm in the far infrared. A thermal camera is not detecting heat in any special sense: it is looking at a part of the spectrum your eye has no receptors for. See a person’s curve in the Black Body tool →
A powerful beam of low-energy photons and a weak beam of high-energy ones are not interchangeable. Breaking a chemical bond needs one photon that can do it alone: why UV burns you and a far more powerful radio transmitter does not.
Wien’s law: the peak wavelength is 2.898 mm·K divided by the temperature. Double the temperature and the peak halves: why a cooling ember runs red and a welding arc blue-white. Watch the colour change in the Black Body tool →
Green light at 550 nm is 2.25 eV. E = hc/λ, and hc is 1239.84 eV·nm; since the 2019 SI redefinition h and c are exact by definition, so this is not a measurement that might be revised. 1240 is just a rounding, good to about one part in 7,800.
The 380–700 nm window is not a property of light. It is where the Sun is brightest at the ground and where our photoreceptors evolved to respond; the spectrum itself has no feature there at all.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.