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Planck's constant

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.

See a Khan Academy video, Introduction to light:

This was last updated in April 2015

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