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Energy of a photon

Light can be described as a stream of photons. Each photon has an energy that depends only on the frequency, or on the wavelength. A short wavelength means a high frequency and more energy per photon. That is why ultraviolet light is more energetic than red light, even if a red lamp can emit many more photons. Planck's constant and the speed of light enter when the energy is calculated in joules.

E=hcλE = \frac{hc}{\lambda}photon energy

Symbols

hhPlanck's constant, 6.63·10⁻³⁴Js
ccspeed of light, 3.00·10⁸m/s
λ\lambdawavelengthm

Example

A photon has wavelength 600 nm, that is 600⋅10−9600 \cdot 10^{-9} m.

E=6.63⋅10−34⋅3.00⋅108600⋅10−9=3.315⋅10−19E = \dfrac{6.63 \cdot 10^{-34} \cdot 3.00 \cdot 10^{8}}{600 \cdot 10^{-9}} = 3.315 \cdot 10^{-19} J.

The wavelength must be in meters. 1 nm is 10−910^{-9} m.
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