# What is the Planck's Constant?

Level 2 (without higher mathematics)
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:
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).