Introduction
If a light source ('the atom') is in the excited state with energy, it may spontaneously decay to a lower lying level (e.g., the ground state) with energy, releasing the difference in energy between the two states as a photon. The photon will have angular frequency and energy (=, where is the Planck constant and is the frequency):
where is the reduced Planck constant. The phase of the photon in spontaneous emission is random as is the direction in which the photon propagates. This is not true for stimulated emission. An energy level diagram illustrating the process of spontaneous emission is shown below:
If the number of light sources in the excited state is given by, the rate at which decays is:
where is the rate of spontaneous emission. In the rate-equation is a proportionality constant for this particular transition in this particular light source. The constant is referred to as the Einstein A coefficient, and has units . The above equation can be solved to give:
where is the initial number of light sources in the excited state, is the time and is the radiative decay rate of the transition. The number of excited states thus decays exponentially with time, similar to radioactive decay. After one lifetime, the number of excited states decays to 36.8% of its original value (-time). The radiative decay rate is inversely proportional to the lifetime :
Read more about this topic: Spontaneous Emission
Famous quotes containing the word introduction:
“Such is oftenest the young man’s introduction to the forest, and the most original part of himself. He goes thither at first as a hunter and fisher, until at last, if he has the seeds of a better life in him, he distinguishes his proper objects, as a poet or naturalist it may be, and leaves the gun and fish-pole behind. The mass of men are still and always young in this respect.”
—Henry David Thoreau (1817–1862)
“The role of the stepmother is the most difficult of all, because you can’t ever just be. You’re constantly being tested—by the children, the neighbors, your husband, the relatives, old friends who knew the children’s parents in their first marriage, and by yourself.”
—Anonymous Stepparent. Making It as a Stepparent, by Claire Berman, introduction (1980, repr. 1986)
“For better or worse, stepparenting is self-conscious parenting. You’re damned if you do, and damned if you don’t.”
—Anonymous Parent. Making It as a Stepparent, by Claire Berman, introduction (1980, repr. 1986)