Photon Energy Calculator
Calculate light energy from wavelength or frequency with Planck's equation and convert results to joules, electron volts, and kcal/mol.
About photon energy
A photon is a discrete quantum of electromagnetic radiation. Its energy depends directly on frequency and inversely on wavelength. Planck's relation states that photon energy equals Planck's constant multiplied by frequency. Because frequency equals the speed of light divided by wavelength in vacuum, the same relation can be written as Planck's constant times the speed of light divided by wavelength. This calculator accepts either wavelength in nanometers or frequency in terahertz and evaluates both equivalent forms with defined physical constants. Short wavelengths correspond to high frequencies and energetic photons. Gamma rays and X-rays occupy the high-energy, short-wavelength end of the electromagnetic spectrum, while radio waves have much lower photon energies. Visible light covers only a narrow range. A violet photon near 400 nanometers carries more energy than a red photon near 700 nanometers even though both travel at the same vacuum speed. Color therefore reflects frequency and photon energy, not a different propagation speed in vacuum. Joules are the SI unit of energy, but a single photon's energy is usually a very small decimal value in joules. Electron volts offer a convenient scale for atomic, optical, and semiconductor work. One electron volt is the energy gained by one elementary charge moving through a potential difference of one volt. The calculator divides joules by the exact elementary-charge conversion to report electron volts. Spectroscopists, photochemists, and materials researchers commonly use this unit to compare photons with electronic energy levels and band gaps. Chemistry often describes energy per mole rather than per particle. Multiplying one-photon energy by Avogadro's constant gives joules per mole, which the calculator converts to kilocalories per mole. This makes it easier to compare absorbed photon energy with bond dissociation energies and reaction energetics. A calculated photon energy does not by itself prove that a reaction will occur: absorption probability, allowed transitions, competing relaxation paths, quantum yield, and the molecular environment remain important. The calculation assumes propagation in vacuum and uses the vacuum wavelength. Light in a material has a shorter local wavelength because its phase velocity changes, while its frequency remains fixed at an interface. For a wavelength measured inside a medium, refractive index must be considered before using a vacuum-energy relationship. The calculator uses exact SI values for Planck's constant, the speed of light, elementary charge, and Avogadro's constant, then rounds only the displayed results. It is suitable for optics, spectroscopy, astronomy, photochemistry, and quantum-physics exercises.
Photon energy examples
| Light | Approximate photon energy |
|---|---|
| 400 nm violet light | 3.10 eV per photon |
| 550 nm green light | 2.25 eV per photon |
| 700 nm red light | 1.77 eV per photon |
| 300 THz infrared light | 1.24 eV per photon |
How to calculate photon energy
- Choose wavelength in nanometers or frequency in terahertz as the known quantity.
- Enter one positive value; the alternate input is cleared automatically.
- Select Calculate photon energy to apply Planck's equation.
- Read wavelength, frequency, joules, electron volts, and molar energy from the result.
Frequently asked questions
How is photon energy calculated from wavelength?
Multiply Planck's constant by the speed of light and divide by vacuum wavelength. The wavelength must be converted to meters when SI constants are used.
Why do shorter wavelengths have higher energy?
Frequency increases as wavelength decreases because their product is the speed of light. Photon energy is proportional to frequency, so it rises as wavelength becomes shorter.
What is an electron volt?
An electron volt is a small energy unit convenient for individual particles and atomic transitions. It equals exactly 1.602176634 times ten to the minus nineteenth joule.
Why report energy in kcal/mol?
Chemists commonly compare energies for a mole of particles or molecules. The molar conversion helps relate photon absorption to familiar bond and reaction energies.
Does refractive index change photon energy?
A photon's frequency remains constant when it crosses a stationary material boundary, so its energy remains constant. Its wavelength and phase velocity change inside the material.