Photon Energy Calculator
Light Doesn't Arrive Continuously — It Arrives in Packets
Every photon carries a discrete amount of energy determined entirely by its frequency (or equivalently, its wavelength) — not by how bright the light source is. This was one of the pivotal discoveries of early quantum theory: doubling a light source's intensity sends more photons, but each individual photon's energy stays fixed by its color. This calculator computes that per-photon energy from either frequency or wavelength.
The Formulas
From wavelength: E = h × c / λ
h is Planck's constant, 6.626×10&supminus;³&sup4; J·s, and c is the speed of light, 3×10&sup8; m/s. Results are given in both joules and electronvolts (eV), since joules are inconveniently tiny at the scale of a single photon.
Why Photon Energy, Not Just Brightness, Matters
- Photoelectric effect and solar cells — a photon needs enough individual energy to free an electron from a material; more dim light of the wrong color does nothing, which is exactly what stumped classical physics before quantum theory.
- Medical and industrial imaging — X-ray and gamma-ray photon energies determine how deeply radiation penetrates tissue or material, directly shaping imaging and shielding requirements.
- LED and laser design — the color of light a semiconductor emits is set by its bandgap energy, which corresponds to a specific photon energy.
- Spectroscopy — identifying elements and compounds by the specific photon energies (wavelengths) they absorb or emit is a cornerstone of chemistry and astronomy.
Photon Energy Across the Spectrum
| Light | Wavelength | Photon Energy |
|---|---|---|
| Microwave | 1 cm | 1.988×10&supminus;²³ J (0.0001 eV) |
| Red light | 700 nm | 2.840×10&supminus;¹&sup9; J (1.7724 eV) |
| Green light | 500 nm | 3.976×10&supminus;¹&sup9; J (2.4814 eV) |
| Violet light | 400 nm | 4.970×10&supminus;¹&sup9; J (3.1017 eV) |
| Soft X-ray | 10 nm | 1.988×10&supminus;¹&sup7; J (124.0687 eV) |
Shorter wavelength means higher photon energy — a 10 nm X-ray photon carries about 70 times the energy of a 700 nm red-light photon.
How to Use This Calculator
- Choose a mode: From Frequency or From Wavelength.
- For the frequency mode, enter Frequency in Hz.
- For the wavelength mode, enter Wavelength in meters.
- Select Calculate to get photon energy in both joules and electronvolts, along with the worked formula.
Related Calculations
To relate a photon's frequency and wavelength directly, use the Wavelength Calculator, or see how total radiated power scales with temperature using the Blackbody Radiation Calculator.