What the electromagnetic spectrum covers
Light, radio waves, X-rays and gamma rays are all the same thing: electromagnetic radiation. They differ only in wavelength and frequency. The table below uses vacuum wavelength, because light travels fastest in a vacuum. The frequency column comes from f = c / lambda, where c = 299,792,458 m/s. The energy column comes from E = hc / lambda, using hc = 1239.84 eV nm.
These boundaries are conventions, not physical walls. A 1 mm wave sits at the edge of microwave and infrared, and different references draw the line in slightly different places. The physics does not change at a boundary. Only the label changes.
Visible light in nanometres
Visible light is the narrow slice your eye can detect. It runs from about 380 nm to 750 nm. One nanometre is one billionth of a metre. The colours below are the conventional ranges used by this site.
A green laser pointer at 532 nm sits in the green band. Its frequency is c / 532 nm = 563.5 THz. Its photon energy is 1239.84 / 532 = 2.33 eV. That single number tells you the light can trigger some chemical reactions but not ionize air.
The formula lambda = c / f
Wavelength is how long one wave cycle is. Frequency is how many cycles pass each second. They are tied together by the speed of light. If you know the frequency, divide c by it to get the wavelength. If you know the wavelength, divide c by it to get the frequency.
Worked example. An FM radio station broadcasts at 100 MHz. That is 100,000,000 cycles per second. lambda = 299,792,458 / 100,000,000 = 2.998 m. So the wavelength is about 3 m. A half-wave dipole for that station would be about 1.5 m long in free space, and a bit shorter in real wire.
In a medium other than vacuum, light slows down. The new speed is c / n, where n is the refractive index. Water has n = 1.333, crown glass about 1.52, diamond 2.417. The frequency stays the same when light enters glass or water. The wavelength gets shorter. That is why a straw looks bent in a glass of water: the light changes speed and direction, but its colour (frequency) does not change.
For sound, the same formula works with the speed of sound. At 20 C, sound travels at 343 m/s in air. A musical note A4 at 440 Hz has a wavelength of 343 / 440 = 0.78 m.
How to look up a wavelength
Type any wavelength into the lookup box. The tool returns the band and, if the value falls in the visible range, the colour. It uses the boundaries in the first table. If your value lands within 1 nm of a boundary, treat the answer as approximate. The band name is a label, not a measurement.
For photon energy in eV from wavelength in nm, use E = 1239.84 / lambda. For example, 1 nm gives 1239.84 eV, about 1.24 keV. That is a soft X-ray. For frequency from wavelength, use f = c / lambda. For wavelength from frequency, use lambda = c / f.
If you need the de Broglie wavelength of a particle, that is a different formula: lambda = h / (m v). It applies to matter, not light. The de Broglie wavelength calculator handles that case. For antenna lengths, the antenna length calculator applies a velocity factor. For photon energy directly, use the photon energy calculator. Every formula on this site is listed at how we calculate.