Planck's constant, symbolized *h*, relates the energy in one quantum (photon) of electromagnetic radiation to the frequency of that radiation. In the International System of units (SI), the constant is equal to approximately 6.626176 x 10^{-34} joule-seconds. In the centimeter-gram-second (cgs) or small-unit metric system, it is equal to approximately 6.626176 x 10^{-27} erg-seconds.

The energy *E* contained in a photon, which represents the smallest possible 'packet' of energy in an electromagnetic wave, is directly proportional to the frequency *f* according to the following equation:

*E* = *hf*

If *E* is given in joules and *f* is given in hertz (the unit measure of frequency), then:

*E* = (6.626176 x 10^{-34}) *f*

and conversely:

*f* = *E* / (6.626176 x 10^{-34})

Also see electromagnetic field, energy, frequency, SI, and Table of Physical Units and Constants.

*This was last updated in*September 2005

*Posted by:*Margaret Rouse

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