First recognized in 1900 by Max Planck, it was originally the proportionality constant between the minimal increment of energy, E, of a hypothetical electrically charged oscillator in a cavity that contained black body radiation, and the frequency, f, of its associated electromagnetic wave. In 1905 the value E, the minimal energy increment of a hypothetical oscillator, was theoretically associated by Einstein with a "quantum" or minimal element of the energy of the electromagnetic wave itself. The light quantum behaved in some respects as an electrically neutral particle, as opposed to an electromagnetic wave. It was eventually called the photon. 

Classical statistical mechanics requires the existence of h (but does not define its value). Eventually, following upon Planck's discovery, it was recognized that physical action cannot take on an arbitrary value. Instead, it must be some multiple of a very small quantity, the "quantum of action", now called the Planck constant. Classical physics cannot explain this fact. In many cases, such as for monochromatic light or for atoms, this quantum of action also implies that only certain energy levels are allowed, and values in between are forbidden.
Who originally associated the minimal energy increment of a hypothetical oscillator with a "quantum" or minimal element of the energy of the electromagnetic wave itself? When did he do this? What letter is the value called? How did the light value behave? As opposed to what? What was it later called? Who was energy E originally recognized by? What type of action can't take an arbitrary value? What must it be instead? What is that also known as? Is there another name for it? What is it? Is this fact widely understood in physics? What does this imply about energy levels? What about values in between?
Einstein In 1905 E as an electrically neutral particle an electromagnetic wave the photon Max Planck a physical one some multiple of a very small quantity the "quantum of action" yes the Planck constant no only certain ones are allowed they're forbidden