What is the relationship between frequency and wavelength of light?

Frequency and wavelength are inversely proportional: f = c/λ. Higher frequency means shorter wavelength and higher energy. Visible light frequencies range from about 430 THz (red) to 790 THz (violet).

Does light frequency change in different media?

No. When light enters a medium like glass or water, its speed and wavelength decrease but its frequency stays the same. This is why dispersion occurs — different frequencies (colors) are refracted by different amounts.

How do I convert wavelength from nm to frequency in THz?

Use f(THz) = 299,792.458 / λ(nm). For example, 500 nm corresponds to 299,792.458 / 500 ≈ 599.6 THz.

What is a photon's energy in electron-volts?

Photon energy in eV equals E = 1240 / λ(nm). For a 620 nm red photon, E = 1240/620 = 2.0 eV. This shortcut works because hc ≈ 1240 eV·nm.

What is wave number and why is it used?

Wave number (k = 1/λ) measures the number of wave cycles per unit distance. Spectroscopists prefer cm⁻¹ because it is directly proportional to energy and frequency, making spectral comparisons intuitive.

How many photons per second does a 1 mW laser emit?

It depends on wavelength. For a 532 nm green laser at 1 mW: E_photon = hc/λ ≈ 3.73 × 10⁻¹⁹ J. Photons/s = 0.001 / 3.73 × 10⁻¹⁹ ≈ 2.68 × 10¹⁵ photons per second.

Frequency of Light Calculator

Calculate light frequency from wavelength using f = c/λ. Visible spectrum color mapping, photon energy in eV, wave number, and electromagnetic spectrum reference table.

Wavelength

Unit

Refractive Index of Medium1 for vacuum, 1.33 for water, 1.5 for glass

Number of Photons

Power (for photon rate)

Frequency

5.4508e+14 Hz

545.0772 THz

Photon Energy

3.6117e-19 J

2.2543 eV

Wave Number

1.8182e+6 m⁻¹

18,181.82 cm⁻¹

Speed in Medium

299,792,458 m/s

100.00% of c

Wavelength in Vacuum

550.00 nm

5.5000e-7 m

Total Energy (N photons)

3.6117e-19 J

For 1 photon(s)

Photons per Second

2.7688e+18 /s

At 1 W power

Spectrum Region: Green

Visible Spectrum Reference

Color	Wavelength Range	Frequency Range
Violet	380–450 nm	666.2–788.9 THz
Blue	450–495 nm	605.6–666.2 THz
Green	495–570 nm	526.0–605.6 THz
Yellow	570–590 nm	508.1–526.0 THz
Orange	590–620 nm	483.5–508.1 THz
Red	620–750 nm	399.7–483.5 THz

Electromagnetic Spectrum Bands

Band	Wavelength	Frequency	Energy (eV)
Radio	> 1 m	< 300 MHz	< 1.24 μeV
Microwave	1 mm – 1 m	300 MHz – 300 GHz	1.24 μeV – 1.24 meV
Infrared	700 nm – 1 mm	300 GHz – 430 THz	1.24 meV – 1.77 eV
Visible	380 – 700 nm	430 – 790 THz	1.77 – 3.26 eV
Ultraviolet	10 – 380 nm	790 THz – 30 PHz	3.26 – 124 eV
X-ray	0.01 – 10 nm	30 PHz – 30 EHz	124 eV – 124 keV
Gamma	< 0.01 nm	> 30 EHz	> 124 keV

Planning notes, formulas, and examples

About the Frequency of Light Calculator

The frequency of light is one of the most fundamental quantities in physics, connecting wave behavior to photon energy through quantum mechanics. Every color you see corresponds to a specific frequency of electromagnetic radiation — red light oscillates at roughly 430 THz while violet reaches nearly 790 THz.

This calculator converts any wavelength of electromagnetic radiation into its corresponding frequency using the core relationship f = c/λ, where c is the speed of light in vacuum (299,792,458 m/s). It supports wavelengths from radio waves to gamma rays, and accounts for different media through the refractive index input.

Beyond frequency, the tool computes photon energy in both joules and electron-volts (eV), the wave number used in spectroscopy, and the number of photons emitted per second at a given power level. A color-coded visible spectrum map shows where your wavelength falls. Whether you are a physics student studying optics, an engineer designing laser systems, or a spectroscopist analyzing emission lines, this calculator eliminates tedious unit conversions and delivers all key quantities together.

When This Page Helps

Converting between wavelength and frequency involves large exponents and easy-to-confuse unit prefixes (nm, μm, THz, GHz). A single misplaced decimal can throw your answer off by orders of magnitude. This calculator handles all unit conversions automatically, computes derived quantities like photon energy and wave number in one step, and visually maps wavelengths to the visible spectrum for quick identification. It is invaluable for optics labs, spectroscopy analysis, and physics coursework.

How to Use the Inputs

Enter the wavelength of the electromagnetic radiation in the Wavelength field.
Select the appropriate unit (nm, μm, mm, cm, or m) from the Unit dropdown.
Optionally change the refractive index to calculate frequency in a medium other than vacuum (e.g., 1.33 for water).
View the computed frequency, photon energy (J and eV), wave number, and speed in the output cards.
Check the visible spectrum panel to see which color your wavelength corresponds to.
Use preset buttons for common wavelengths (red, green, blue, UV, IR, microwave).
Enter power to compute photons per second for laser or LED applications.

Formula used

Frequency of Light:
  f = c / λ  (in vacuum)
  f = v / λ  where v = c / n (in a medium)

Photon Energy:
  E = hf = hc / λ

Wave Number:
  k = 1 / λ

Where:
  c = 299,792,458 m/s (speed of light)
  h = 6.626 × 10⁻³⁴ J·s (Planck constant)
  n = refractive index of medium
  λ = wavelength in meters

Example Calculation

Result: 5.451 × 10¹⁴ Hz (545.1 THz)

A 550 nm wavelength (green light) in vacuum has frequency f = 299,792,458 / (550 × 10⁻⁹) ≈ 5.451 × 10¹⁴ Hz or about 545.1 THz. Its photon energy is approximately 2.25 eV.

Tips & Best Practices

Visible light spans roughly 380–750 nm; wavelengths outside this range are invisible to the human eye.
Photon energy in eV is convenient for comparing with atomic energy levels and semiconductor band gaps.
The refractive index slows light but does not change its frequency — only the wavelength in the medium changes.
Spectroscopists often use wave number (cm⁻¹) instead of frequency; divide the wave number in m⁻¹ by 100 for cm⁻¹.
For multi-photon processes, multiply single-photon energy by the number of photons involved.
Laser wavelengths are usually specified in vacuum wavelength even when the beam travels through air (n ≈ 1.000293).

Understanding the Electromagnetic Spectrum

The electromagnetic spectrum spans an enormous range of frequencies, from radio waves with frequencies below 300 MHz to gamma rays exceeding 30 exahertz. Visible light occupies only a narrow band between roughly 430 and 790 THz, yet this tiny slice powers photosynthesis, human vision, and optical communications. Understanding where a given wavelength falls on the spectrum matters for applications ranging from medical imaging (X-rays) to telecommunications (microwaves and infrared).

Photon Energy and Quantum Mechanics

Einstein demonstrated that light consists of quantized packets of energy called photons, where each photon carries energy E = hf. This relationship bridges wave optics and quantum mechanics — the color of light is not just a wave property but determines the energy delivered per photon. This principle underpins the photoelectric effect, laser design, photovoltaic cells, and spectroscopic analysis of atoms and molecules.

Practical Applications in Optics and Engineering

Fiber-optic communications use infrared wavelengths (1310 nm and 1550 nm) where glass fiber has minimum attenuation. LED and laser design requires matching photon energy to semiconductor band gaps. Medical applications use specific UV wavelengths for sterilization and phototherapy. In all these cases, converting between wavelength, frequency, and energy is essential for system design and performance optimization.

Sources & Methodology

Last updated: March 7, 2026

Frequently Asked Questions

Frequency and wavelength are inversely proportional: f = c/λ. Higher frequency means shorter wavelength and higher energy. Visible light frequencies range from about 430 THz (red) to 790 THz (violet).