The Atom as a Source of Light
We note that according to Maxwell’s theory of electromagnetism, light is nothing but a visible electromagnetic (EM) wave that has a frequency in the narrow range from about 4 × 1014-Hz to about 7 × 1014-Hz. The corresponding range of wavelengths is from about 4000-Å to about 7000-Å. Furthermore, EM waves are produced and emitted by accelerating electric charge. There are two interesting questions that immediately confront us: (1) We accept the premise that animal eyes evolved so as to be sensitive to sunlight. Still, what characteristics of animal eyes make them sensitive to this particular narrow range of frequencies? (2) What are the physical characteristics of the sun that cause sunlight to be concentrated in a particular range of frequencies? The answer to the first question is connected with the fact that the eye uses conglomerates of atoms, that is, molecules, as detectors of EM waves. This chapter therefore focuses on the atom as a source and receiver of EM waves. The answer to the second question has to do with the fact that the sun is a body that is in equilibrium, with a surface temperature of about 5,800 degrees above absolute zero, that is 5,800 Kelvin [=(5800+273)∘C=6, 073∘C]. That there is a connection between temperature and frequency must be surprising to the beginning student of physics; as we will see later on this chapter, the connection stems from their common link with energy.