What is the Planck's Constant?

by Alexander FufaeV

The Planck's constant $ h $ is a physical constant that appears in equations whenever the equation describes quantum phenomena. For example, it appears in the light quantum hypothesis:

Energy of a photon via Planck's constant

$$ \begin{align} W_{\text p} ~=~ h \, \class{violet}{f} \end{align} $$

In this equation Planck's constant connects the energy $ W_{\text p} $ of a photon (light particle) with the light frequency $ f $. From this equation you can also read the unit of the Planck's constant, if you rearrange the equation for $h$: Js (Joule seconds).

You can measure the value of the Planck's constant, for example, using the photoelectric effect. It has the value:

Value of the Planck constant

$$ \begin{align} h ~=~ 6.626 \, 070 \, 15 ~\cdot~ 10^{-34} \, \mathrm{Js} \end{align} $$

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In the photoelectric effect there is a linear relation between kinetic energy of the released electrons $ W_{\text{kin}} $ and photon frequency $ f $. The Planck's constant $ h $ is the slope of the straight line.