How do I convert wavelength to frequency?

Divide the speed of light by the wavelength. In vacuum that is 299,792,458 metres per second divided by the wavelength in metres, so 500 nanometres gives about 600 terahertz.

What is the photon energy of a given wavelength?

It is Planck's constant times the frequency, or equivalently hc divided by the wavelength. A handy shortcut is that energy in electron volts is about 1240 divided by the wavelength in nanometres, so 500 nm is roughly 2.48 eV.

Does wavelength or frequency change when light enters glass?

The wavelength changes and the frequency does not. Light slows to c divided by the refractive index, and since frequency is set by the source, the wavelength must shorten to match. That is why frequency is the quantity that survives a fibre unchanged.

What is a wavenumber and why do spectroscopists use it?

It is the number of wave cycles per centimetre, the reciprocal of the wavelength in centimetres. Spectroscopy favours it because it is directly proportional to photon energy, so an infrared spectrum in wavenumbers has a linear energy axis.

Are the values I convert sent anywhere?

No. Every conversion is arithmetic on defined physical constants running in your browser, with no network request.

Wavelength & Frequency Converter

Convert between wavelength, frequency and photon energy, with the spectrum band, wavenumber and refractive index handled.

The Wavelength & Frequency Converter runs entirely in your browser. The values you enter are used only for the conversion on your device.

Open the Speed Calculator

About Wavelength & Frequency Converter

Wavelength & Frequency Converter treats wavelength, frequency and photon energy as three views of the same wave, related by c = λf and E = hf. Enter any one of them, in any unit from picometres to kilometres and from hertz to petahertz, and the other two follow along with the spectroscopic wavenumber. The result is labelled with its region of the electromagnetic spectrum — radio, microwave, infrared, visible, ultraviolet, X-ray or gamma — and visible wavelengths also get an approximate colour name. A refractive index setting covers the case that catches people out: in glass or water light slows down, so the wavelength shortens while the frequency stays exactly the same.

Features

Converts between wavelength, frequency and photon energy in both directions
Seven wavelength units, six frequency units and four energy units
Photon energy reported in both joules and electron volts
Spectroscopic wavenumber in reciprocal centimetres
Names the spectrum band and the approximate colour for visible light
Refractive index setting for air, water, glass and optical fibre
Presets for common lasers, WiFi, FM radio and fibre wavelengths
Scientific notation for the very large and very small values involved

How to use the Wavelength & Frequency Converter

Choose which quantity you are starting from
Enter the value and pick its unit
Set the medium if the light is not travelling through vacuum
Read the equivalent frequency, wavelength, photon energy and wavenumber

Example

Input

500 nm in vacuum

Output

599.6 THz · 2.480 eV · 20,000 cm⁻¹ · visible light, green

Green light near the middle of the visible band, where the eye is most sensitive.

Common errors & troubleshooting

The frequency comes out as an enormous number. — That is correct for light. Visible wavelengths sit near 600 terahertz — six hundred million million cycles per second — which is why optical work quotes wavelength instead.
The wavelength changes when you pick a medium but the frequency also changes. — Check which quantity you entered. If you enter a wavelength measured inside the medium, the corresponding wave has a different frequency than the same wavelength in vacuum. Entering the frequency instead keeps it fixed and shortens the wavelength, which is the usual physical case.
The photon energy in joules looks like zero. — It is around 10⁻¹⁹ joules for visible light, which is why electron volts exist as a unit. The eV figure next to it is the one to read.
A visible wavelength shows no colour name. — Colour names are only applied between about 380 and 750 nanometres. Outside that the light is not visible, so no colour applies.

Frequently asked questions

How do I convert wavelength to frequency?: Divide the speed of light by the wavelength. In vacuum that is 299,792,458 metres per second divided by the wavelength in metres, so 500 nanometres gives about 600 terahertz.
What is the photon energy of a given wavelength?: It is Planck's constant times the frequency, or equivalently hc divided by the wavelength. A handy shortcut is that energy in electron volts is about 1240 divided by the wavelength in nanometres, so 500 nm is roughly 2.48 eV.
Does wavelength or frequency change when light enters glass?: The wavelength changes and the frequency does not. Light slows to c divided by the refractive index, and since frequency is set by the source, the wavelength must shorten to match. That is why frequency is the quantity that survives a fibre unchanged.
What is a wavenumber and why do spectroscopists use it?: It is the number of wave cycles per centimetre, the reciprocal of the wavelength in centimetres. Spectroscopy favours it because it is directly proportional to photon energy, so an infrared spectrum in wavenumbers has a linear energy axis.
Are the values I convert sent anywhere?: No. Every conversion is arithmetic on defined physical constants running in your browser, with no network request.

Related tools

Speed Calculator — Solve speed, distance or time and convert between mph, km/h and m/s.
Electricity Cost Calculator — Estimate the energy use and running cost of an appliance.
Ohm’s Law Calculator — Solve voltage, current, resistance and power from any two values.
Scientific Notation Converter — Convert numbers to and from scientific, e-notation and engineering notation.
Ideal Gas Law Calculator — Solve PV = nRT for pressure, volume, moles or temperature in any common unit.
Resistor Color Code Calculator — Convert resistor color bands to resistance and back, with tolerance.