4.1 INTRODUCTION
For many thousands of years, humans relied on the sun as their only source of light. The ability to create fire, and its by-product, light, led to a profound change in the way humans managed their time. Today, there are many options for creating light.
Light can be produced through the rapid change of state of an electron from a state of relatively high energy to a ground state of lower energy. The energy of the electron has to leave the atom somehow, and it is often emitted in the form of a photon of light. This light emission takes place in some optical materials. A series of controlled rapid transitions ...
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