2.5 eV to nm: Wavelength, Frequency and Band of the Light

A map of light, from gamma rays to FM radio

A green laser pointer at 532 nm vibrates 563.52 THz times a second, and every photon it sends out carries 2.331 eV. Give this calculator any one of those three numbers, in whatever unit you have it, and it returns the other two, the band of the spectrum the light belongs to, and a name for its color when there is one. The same box works for a 2.45 GHz microwave oven, which comes back as 12.24 cm, and for a 1 MeV gamma ray, which comes back as 1.24 pm.

380 to 750 nm

what a typical human eye responds to

555 nm

peak of daylight sensitivity, a yellowish green

1239.84

eV times nm, so energy in eV is 1239.84 / wavelength in nm

Three quantities that are really one

Light in a vacuum always travels at 299,792,458 m/s. That speed is fixed by definition, so wavelength and frequency cannot move independently: multiply them and you get the speed of light back, every time. Halve the wavelength and the frequency doubles. This is why a spectrometer can report nanometers while a radio engineer reports megahertz and both are describing the same thing.

The third quantity comes from quantum physics. Light is delivered in packets, photons, and each one carries an energy of E = h × f, where h = 6.62607015 × 10⁻³⁴ J·s is the Planck constant. Since the redefinition of the SI units, both h and the charge of an electron are exact numbers, which makes the shortcut in the tile above exact to the digits shown: a photon's energy in electronvolts is 1239.84 divided by its wavelength in nanometers. Red light at 650 nm gives 1.907 eV; violet at 405 nm gives 3.061 eV.

Color is the odd one out. It is not a physical property of the wave but a label the eye and brain put on a slice of wavelengths, and the edges of those slices are conventions rather than laws. The calculator uses the table from Wikipedia's "Visible spectrum" article, the ultraviolet bands of the ISO 21348 standard and the infrared bands of the CIE, and it says so wherever two of those conventions overlap.

Using the calculator in four moves

You know the - pick wavelength, frequency or photon energy, whichever number is printed on the laser, the lamp or the exercise.
Unit - wavelengths in pm, angstroms, nm, µm, mm, cm or m; frequencies from Hz up to PHz; energies in meV, eV, keV or MeV.
The value - type it as it appears, with a decimal point if it has one, such as 632.99.
Read the result - the headline names the band and the color, the tiles give frequency, photon energy in eV and joules and the wavenumber, and the table shows your band between its neighbors.

All nineteen bands with their edges converted

Every frequency and energy below is calculated from the wavelength edge in the second column, not copied from a chart, so the three columns always agree with each other and with the result. The lower edge belongs to the band, the upper edge to the next one. The result shows the five rows around your value; this is the whole ladder.

Band	Wavelength	Frequency	Photon energy	Convention
Gamma / hard X-ray	below 10.00 pm	above 29.98 EHz	above 123.984 keV	overlap, told apart by origin
X-rays	10.00 pm to 10.00 nm	29.98 PHz to 29.98 EHz	123.984 eV to 123.984 keV	older wavelength split
Extreme UV	10.00 nm to 100.00 nm	3.00 PHz to 29.98 PHz	12.398 eV to 123.984 eV	ISO 21348 runs it to 121 nm
UVC	100.00 nm to 280.00 nm	1.07 PHz to 3.00 PHz	4.428 eV to 12.398 eV	ISO 21348
UVB	280.00 nm to 315.00 nm	951.72 THz to 1.07 PHz	3.936 eV to 4.428 eV	ISO 21348
UVA	315.00 nm to 380.00 nm	788.93 THz to 951.72 THz	3.263 eV to 3.936 eV	ISO 21348 UVA continues to 400 nm
Violet	380.00 nm to 450.00 nm	666.21 THz to 788.93 THz	2.755 eV to 3.263 eV	visible spectrum table
Blue	450.00 nm to 485.00 nm	618.13 THz to 666.21 THz	2.556 eV to 2.755 eV	visible spectrum table
Cyan	485.00 nm to 500.00 nm	599.58 THz to 618.13 THz	2.480 eV to 2.556 eV	visible spectrum table
Green	500.00 nm to 565.00 nm	530.61 THz to 599.58 THz	2.194 eV to 2.480 eV	visible spectrum table
Yellow	565.00 nm to 590.00 nm	508.12 THz to 530.61 THz	2.101 eV to 2.194 eV	visible spectrum table
Orange	590.00 nm to 625.00 nm	479.67 THz to 508.12 THz	1.984 eV to 2.101 eV	visible spectrum table
Red	625.00 nm to 750.00 nm	399.72 THz to 479.67 THz	1.653 eV to 1.984 eV	visible spectrum table
Deep red, edge of vision	750.00 nm to 780.00 nm	384.35 THz to 399.72 THz	1.590 eV to 1.653 eV	gap between the two conventions
IR-A (near infrared)	780.00 nm to 1.40 µm	214.14 THz to 384.35 THz	885.601 meV to 1.590 eV	CIE
IR-B	1.40 µm to 3.00 µm	99.93 THz to 214.14 THz	413.281 meV to 885.601 meV	CIE
IR-C	3.00 µm to 1.00 mm	299.79 GHz to 99.93 THz	1.240 meV to 413.281 meV	CIE
Microwaves	1.00 mm to 1.00 m	299.79 MHz to 299.79 GHz	1.240 µeV to 1.240 meV	usual 300 MHz to 300 GHz
Radio	1.00 m and up	below 299.79 MHz	below 1.240 µeV	everything longer

The microwave row reads 299.79 rather than 300 because the edge is set at exactly 1 m and 1 mm; round figures in hertz and round figures in meters cannot both be exact, and a calculator that shows its arithmetic has to pick one.

Lines you can check against a real source

Each wavelength here comes from a published reference, and the rest of the row is what the calculator returns for it. Anything with a laser, a lamp or a spectral line in it is a good way to confirm the tool before trusting it with your own number.

Source	Input	Frequency	Photon energy	Verdict
Violet laser pointer	405 nm	740.23 THz	3.061 eV	violet
Blue laser pointer	473 nm	633.81 THz	2.621 eV	blue
Hydrogen F line (H-beta)	486.134 nm	616.69 THz	2.550 eV	cyan, 1.13 nm past the blue edge
Green laser pointer	532 nm	563.52 THz	2.331 eV	green
Sodium D2 line	588.995 nm	508.99 THz	2.105 eV	yellow
Sodium D1 line	589.592 nm	508.47 THz	2.103 eV	yellow, 0.41 nm short of orange
Helium-neon laser, vacuum	632.991 nm	473.61 THz	1.959 eV	red
Red laser pointer	650 nm	461.22 THz	1.907 eV	red
Hydrogen C line (H-alpha)	656.281 nm	456.81 THz	1.889 eV	red
Mercury germicidal lamp	253.7 nm	1.18 PHz	4.887 eV	UVC, invisible
Microwave oven	2.45 GHz	12.24 cm wavelength	10.132 µeV	microwaves, invisible
FM radio station	100 MHz	3.00 m wavelength	413.567 neV	radio, invisible

Line wavelengths from Wikipedia's Fraunhofer lines table, pointer wavelengths from its laser pointer article, the lamp's 253.7 nm peak from its ultraviolet article. Pointers vary by a few nanometers between makers, so treat those rows as typical rather than exact.

Things the spectrum does that a chart hides

Colors are unevenly wide. Cyan gets 15 nm of the table, red gets 125 nm. Measured in energy the gap shrinks but does not close: red spans 0.331 eV and cyan 0.077 eV. The edges are names people agreed on, so a narrow band is not a rarer kind of light, just a shorter stretch that got its own word.

One photon of green light outweighs millions of radio photons. At 532 nm a photon carries 2.331 eV; at 100 MHz it carries 413.567 neV, about 5.64 million times less. A radio transmitter still delivers plenty of power, it just does so with an enormous number of very weak packets.

The same light can be two things at once. Between 380 and 400 nm the visible spectrum table calls it violet while ISO 21348 still counts it as UVA. Type 390 nm and the result names both. At the other end, 750 to 780 nm is past the usual limit of the eye but short of where CIE starts infrared, so the calculator calls it deep red at the edge of vision rather than forcing a side.

Gamma rays and X-rays are not separated by wavelength. Older texts drew a line near 10 pm, and astrophysics still uses 100 keV, which is 12.40 pm. Modern physics names them by where they come from: X-rays from electrons, gamma rays from the nucleus. A 1 MeV photon from a nucleus is gamma, and artificial sources can make X-rays of the same energy, which is why the result names both.

A laser's wavelength depends on the air it is measured in. The helium-neon line is 632.991 nm in a vacuum and 632.816 nm in air. Frequency does not change when light enters air, so the vacuum figure is the one that gives the right 473.61 THz; typing the air figure returns 473.74 THz, about 0.03 % high.

Reading the verdict on your number

The headline changes wording by band, and each wording means something specific.

The result says	What it means
"is green" or another color	Between 380 and 750 nm. A color swatch appears, marked as a screen approximation, with the distance from the 555 nm sensitivity peak.
"also inside UVA"	380 to 400 nm, violet under one convention and ultraviolet under the other.
"is deep red" at the edge of vision	750 to 780 nm, past the typical limit of the eye and short of infrared as CIE defines it.
"invisible" with an ultraviolet band	Below 10 eV it is not ionizing by the US FCC threshold; from 123.98 nm down it is.
"invisible" with infrared, microwaves or radio	Longer than any color. The wavenumber tile is the one infrared spectroscopy uses.

The swatch is drawn with a widely used approximation of spectral colors in RGB. A screen builds every color from three primaries and cannot reproduce the saturation of a single wavelength, so the square is a guide to the hue and not a color measurement.

Wavelength and color, quickly answered

What color is 500 nm light?

Green, right at the lower edge of the green band, which runs from 500 to 565 nm. Anything from 485 up to just under 500 nm is cyan, so a source quoted as "about 500 nm" can look blue-green.

How do I convert wavelength to frequency by hand?

Divide the speed of light by the wavelength in meters. For 550 nm: 299,792,458 / 0.00000055 = 5.4508 × 10¹⁴ Hz, which is 545.08 THz. A shortcut for visible light: 299,792.458 / nm gives THz directly.

Is 400 nm ultraviolet or visible?

Both, depending on the convention. At exactly 400 nm the result says violet without the UVA remark, because ISO 21348 ends UVA there. At 399.95 nm, which is what 3.1 eV converts to, it adds "also inside UVA".

Why does 666.21 THz come out blue when the table says 666.21 THz is the violet edge?

It converts to 449.9969 nm, printed as 450.00 nm. The band is decided on the wavelength as printed, so the headline never reads "450.00 nm is violet" next to a table where 450 nm starts blue.

What wavelength is a 2 eV photon?

619.92 nm, orange, at 483.60 THz. With the shortcut: 1239.84 / 2 = 619.92.

At what wavelength does light become ionizing?

There is no single physical line. The US FCC uses 10 eV, which the calculator converts to 123.98 nm, in the far ultraviolet. Other bodies use higher thresholds, up to the 33 eV needed to ionize a water molecule; Wikipedia notes that only extreme ultraviolet counts as ionizing under every definition.

Can people see beyond 380 and 750 nm?

Under ideal conditions, yes. The Wikipedia visible spectrum article gives 310 nm and 1100 nm as the extended limits of human perception, with very bright sources. The 380 to 750 nm range is what a typical eye responds to in ordinary light.

What is the wavenumber tile for?

It counts waves per centimeter, 1 / wavelength in cm, the unit infrared and Raman spectra are plotted in. 550 nm is 18,181.82 cm⁻¹; very large or very small values switch to scientific notation.

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What wavelength is a 2.5 eV photon?

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