Using the model
Symbols, assumptions and limitations
h is the Planck constant, exactly 6.62607015 × 10⁻³⁴ J·s. c is the speed of light in vacuum, exactly 299792458 m/s. ν is frequency, λ₀ is vacuum wavelength, n is the refractive index of the selected medium, v is phase speed in that medium, and λmedium is wavelength in the medium. When light crosses a stationary boundary, frequency ν and photon energy E remain constant; phase speed and wavelength change. Listed refractive indices are approximate sodium-D values at 20 °C, with dry air specified at 1 atm.
Worked examples
See the method in practice
Visible light
A 500 nm photon has a frequency of about 5.996 × 10¹⁴ Hz.
Photon energy
The same 500 nm photon carries about 3.973 × 10⁻¹⁹ J, or 2.480 eV.
Questions
Frequently asked
Does light slow down in matter?
Its phase speed becomes c/n, and its wavelength becomes λ₀/n. Frequency and photon energy remain unchanged at the boundary.
Are refractive indices constant?
No. They depend on wavelength, temperature, material composition and, especially for gases, pressure. The list provides approximate reference values for sodium-D light at 20 °C.
Is photon energy proportional to wavelength?
It is inversely proportional: shorter wavelength means higher frequency and energy.
Sources & review
Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.
NIST CODATA fundamental constantsWritten by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology