# Plank's Constant

In the year 1900 Max Planck wrote down an equation, E = hv, where E is the energy of a light wave, v is its frequency, and h is a constant which we now call Planck's constant. Planck's equation was the beginning of quantum theory. It said that energy and {22} frequency are the same thing measured in different units. Energy is measured in ergs and frequency in cycles. Planck's constant gives you the rate of exchange for converting frequency into energy, namely, 6 × 10–27 ergs per cycle. But in the year 1900 this made no physical sense. Even Planck himself did not understand it. He knew only that his equation gave the right answers to certain questions in the theory of radiation. But what does it mean to say that energy and frequency are the same thing? What this means only began to become clear twenty-five years later, when Planck's equation was built into the theory which we now call quantum mechanics.

## Notes:

Folksonomies: physics quantum physics equations equations

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Concepts:
Energy (0.978653): freebase | dbpedia
Quantum mechanics (0.940580): dbpedia | freebase | opencyc
Photon (0.776645): dbpedia | freebase
Physics (0.770478): dbpedia | freebase | opencyc
Light (0.609967): dbpedia | freebase | opencyc
Planck constant (0.571399): dbpedia | freebase | yago
Electromagnetic radiation (0.569347): dbpedia | freebase | opencyc
Maxwell\'s equations (0.557259): dbpedia | freebase | yago  Infinite in All Directions
Books, Brochures, and Chapters>Book:  Dyson , Freeman J. (2004-07-22), Infinite in All Directions, Harper Perennial, Retrieved on 2012-04-25