Can a body emit less or more than a quantum of energy??
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Now, before the quantum theory was put forward, there was no notion of natural units of radiant energy: it was believed that we could have any amount of energy, as small as we pleased, radiated by a hot body or a luminous atom. It could, however, be shown mathematically that, if this were true, we should expect a hot body to radiate nearly all its energy in the violet and ultraviolet end of the spectrum, which we know to be against the facts of observation.
The problem was solved in the first year of the present century, when Planck showed that, to get the right result, it was necessary to make a revolutionary hypothesis: to suppose that radiant energy was sent out in packets, as it were - in units or atoms of energy, just as matter existed in atomic units. We cannot have less than an atom of lead, say; any minute piece of lead must consist of a whole number of atoms. We cannot have an electric charge of less than an electron. In the same way, we cannot have less than a unit - or quantum, as it is called - of radiant energy, and any body that sends out or absorbs radiation must deal with one quantum or a whole number of quanta.
The little parcel of light of one particular frequency in which radiant energy is delivered is sometimes called a 'light dart', a very expressive term, but is more generally known as a photon. The photon is simply a quantum of radiant energy, the only object of sometimes using the new term being that 'quantum' is a more inclusive term, which can be applied to other things as well as light - for instance, to the vibration of whole atoms and molecules.