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Unformatted text preview: Section 9 – Infra-Red, Ultra-Violet and Visible Spectroscopy 109 Section 9: Infra-Red, Ultra-Violet and Visible Spectroscopy 9.1 Fundamentals of Spectroscopy 9.1.1 The Electromagnetic Spectrum • Light has historically been the most enigmatic of natural phenomena. Even the very limited part of the electromagnetic spectrum we can see, so-called “white light”, was demonstrated to consist of a range of colors – a spectrum – only in the 17 th century. • Light was believed to travel in waves. Wave mechanics, at the time this was realized, required that a wave propagate in some sort of medium. In the absence of a visible medium, thinkers of the day postulated that light propagated through a medium known as “the ether”. No properties of this medium were ever measured. • By the late 19 th century, Maxwell had developed a mathematical formalism that described light in terms of oscillating (wave-like), perpendicular electric and magnetic fields. At the same time, it was realized that the electromagnetic spectrum extended beyond the range of what humans could see. • At the end of the 19 th century, Maxwell’s equations for electromagnetic fields provided a working description of light. Physics at this time was considered a solved problem – the universe ran like a clockwork machine. • At the start of the 20 th century, Planck and Einstein had described the photoelectric effect – an experiment in which light appeared to act as a particle. Thus, light was observed to act as a wave or a particle, depending on the nature of the experiment conducted. • This observation spawned a revolution in physics: ○ Not only light, but also matter had both particle and wave characteristics. The energies of light and matter were also found to have only certain discrete values: they are quantized systems. ○ The quantum particle for light, the photon, has zero rest mass, but can be diverted by gravitational fields. ○ The speed of a photon in vacuum is a constant, regardless of experimental frames. The speed of a photon in vacuum also represents the current theoretical speed limit for information, energy or particle exchange in the universe. ○ The total amount of energy available to a particle ( ie . the energy of its complete annihilation) is proportional to its mass. The proportionality constant is the square of the speed of light in a vacuum. Section 9 – Infra-Red, Ultra-Violet and Visible Spectroscopy 110 • These observations, which accounted for a large number of Nobel prizes awarded in the first half of the 20 th century, presented scientists with an apparent conundrum. Light (and matter) sometimes behaves as if they are waves, sometimes as if they are particles. Which is the true picture? Neither....
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