Why is the sky blue?
equal but opposite electric charge. When the atom is in equilibrium, the nucleus sits in the centre of the electron cloud. 201 Since they are oppositely charged, the electron cloud and the nucleus attraction each other and this force is analogous to a spring’s restoring force. The centre of the electron cloud and the nucleus can be conceived as being connected by a spring. 202 Though this might sound counterintuitive, this model will be justified later by assuming the charge density of the electron cloud is constant. An electric field incident on an atom in equilibrium exerts forces on the nucleus and the electron cloud respectively. This force tries to move them apart, tugging the imaginary spring. When a spring with two masses attached on either ends is tugged, it stretches and contracts periodically. In other words, it oscillates. During the oscillation of the electron cloud and nucleus the charges in the atom become asymmetrically distributed and an oscillating electric dipole is formed. Light being a wave carried in electromagnetic fields would naturally create oscillating electric dipoles in atoms. 203 Since energy is conserved, light must impart of its energy to create oscillating dipoles in the air molecules, doing work. But accelerating charges disturb and create waves in electromagnetic fields in almost every directions around them, just like a hand plucking the centre of a spider’s web, so the work done of light on the molecules is converted back into light energy without any irretrievable lost. 204 The waves created have the same frequency as the frequency of incident light since they must be equal to the frequency of the oscillating dipoles.
201 Lipson, S. G., and H. Lipson. ‘13.1 The Classical dispersion theory.’ Optical Physics. London: Cambridge U.P., 1969. 469-70. Print. 202 Lipson, S. G., and H. Lipson. ‘13.1.1 The classical atom.’ Optical Physics. London: Cambridge U.P., 1969. 469- 70. Print. 203 Feynman, Richard P., Robert B. Leighton, and Matthew L. Sands. ‘2-2 Physics before 1920.’ The Feynman Lectures on Physics. Definitive ed. Vol. 1. San Francisco: Pearson/Addison-Wesley, 2006. 2-3-2-5. Print. 204 Jenkins, Francis A., and Harvey Elliott White. ‘22.12 Theory of Scattering.’Fundamentals of Optics. 4th ed. Auckland: McGraw-Hill, 1981. 470-71. Print.
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