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Astronomy & optics

Photon energy, frequency and photon rate

Calculate a photon’s energy and frequency from its vacuum wavelength, and the photon rate of monochromatic radiation.

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01

Your inputs

The starting values are an example.

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02

Your result

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Your result will include the relevant values and units.

Understand the calculation

Formula and example

How it is calculated

E = h × c/λ; f = c/λ; photons per second = power/E

Worked example

500 nm corresponds to approximately 2.4797 eV per photon. At 1 mW of monochromatic power, the rate is approximately 2.517 × 10¹⁵ photons per second.

Things to check

  • The SI values used for c, h and elementary charge are defined exactly.
  • Use the vacuum wavelength; the wavelength in a medium can differ.

Sources and references