How to use this calculator
- Enter or choose start from.
- The energy per photon updates as you type. Use Copy result or Share to keep it.
Method
E = h × f = h × c ÷ λ; E(eV) = E(J) ÷ e; E per mole = E × N_A
- h
- Planck constant, 6.62607015 × 10⁻³⁴ J·s (exact)
- c
- speed of light, 299,792,458 m/s (exact)
- e
- elementary charge, 1.602176634 × 10⁻¹⁹ C (exact)
- N_A
- Avogadro constant, 6.02214076 × 10²³ mol⁻¹ (exact)
- λ
- vacuum wavelength
- f
- frequency
Worked example
- hc = 6.62607015 × 10⁻³⁴ × 299,792,458 = 1.98645 × 10⁻²⁵ J·m.
- E = 1.98645 × 10⁻²⁵ ÷ (500 × 10⁻⁹ m) = 3.97289 × 10⁻¹⁹ J.
- E = 3.97289 × 10⁻¹⁹ ÷ 1.602176634 × 10⁻¹⁹ = 2.4797 eV.
A 500 nm (green) photon carries about 2.48 eV, or 3.97 × 10⁻¹⁹ J.
What the result means
A handy shortcut is E (eV) ≈ 1239.84 ÷ λ (nm). Compare the result with a material’s band gap or work function to see whether light of that colour can excite electrons or eject them (the photoelectric effect).
Assumptions and limits
- Wavelength is the vacuum wavelength. All four constants are the exact defining values of the 2019 SI.
- In a medium the wavelength is shorter but the photon energy is unchanged; enter the vacuum wavelength or the frequency.
Questions
Why is energy per mole useful?
It puts photon energies on the same scale as chemical bond energies (typically 150–1,000 kJ/mol), showing which light can break which bonds.
What is 1 eV in wavelength?
About 1,239.84 nm, in the near infrared.