Wavelike Properties of Light

When light interacts with matter, several processes take place, sometimes simultaneously. It can be absorbed, which results in a decrease of the primary intensity, or it can be transmitted without attenuation. The propagation velocity v of light depends on the optical density of the medium. It is related to the absolute (vacuum) velocity of light c by 

When light passes through a condensed phase consisting of oriented electrical dipoles (polarizer), or when it is reflected from a dipolar interface it becomes polarized (Fig. 9.2). It means that orthogonally randomly oriented E, M vectors are now 

A typical optical sensor consists of modules found in conventional spectroscopy, for example, source, monochromator, cuvet, detector, and so on. The big difference is that light travels between these modules through optical fibers or waveguides. As it travels, the beam of light interacts with interfaces between media of different optical densities and therefore of different refractive indices. The laws that govern its behavior at these boundaries are the most important aspects of the operation of optical sensors (Okamoto, 2000). 

The geometrical relationships for refracted light are given by Snell s law (Fig. 9.7). 

For the air/water interface at 25°C, the critical angle c is 43.75°. For values of 0i 0c, all the light is reflected to the optically dense phase. This condition is called total internal reflection. It is the key condition of optical wave guiding and thus the most fundamental condition of optical sensing. 

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Optical reflection is best understood in terms of the wavelike properties of light. The electric field vector associated with the wave oscillates in a plane as the wave propagates (Figure 17.1.6), and the intensity of the light is proportional to the square of the electric... 

The concept of dual character of light is particularly relevant to the discussion of optical sensors. In this introductory section, the basic quantized (corpuscular) aspects of light as they relate to optical sensors are reviewed first, followed by a brief review of physics of optical waveguides and optical fibers which rely on wavelike (continuous) properties of light. Detailed information can be found in analytical (e.g., Skoog et al., 1998) and specialized textbooks (e.g., Hollas, 2004). 

We have evidence for the wavelike nature of light. We also know that a beam of light photons behaves like a stream of tiny packets of energy called photons. So what is light exactly Is it a particle Is it a wave Scientists have agreed to explain the properties of electromagnetic radiation by using both wave and particle properties. Neither explanation is ideal, but currently these are our best models. 

It is the wavelike properties of the electrons that cause two atomic orbitals to form two molecular orbitals. Two atomic orbitals can combine in an additive (constractive) manner, just as two light waves or two sound waves can reinforce each other (Figure 1.3a). The constructive combination of two s atomic orbitals is called a a (sigma) bonding molecular orbital. 

According to the quantum theory, light is also quantised. The absorption or emission of light occurs by the transfer of energy as photons. These photons have both wavelike and particle-like properties and each photon has a specific energy, E, given by Planck s law ... 

Quantum mechanics began with a daring hypothesis by Louis de Broglie (he was a student at the time) if light has a dualistic wave/particle nature, why not matter His reasoning led to the prediction that a particle of mass m and velocity V would exhibit wavelike properties with wavelength... 

The answers to the above questions, not all of which need he presented here, were formulated between 1925 and 1926, in the revolution of modern quantum theory, which shook the foundations of physics and philosophy. Remarkably, the central theme of quantum theory was the nature of light, and what came to be called the wave-particle duality. But other broader implications of the new theory existed, and the first inkling of this was given in 1924 by Louis de Broglie (Fig. 3.26) in his doctoral dissertation. He postulated that particles may also possess wavelike properties and that these wavelike properties would manifest themselves only in phenomena occurring on an atomic scale, as dictated by Planck s constant. He also postulated that the wavelength of these matter waves, for a given particle such as an electron or proton, would be inversely proportional to the particle s momentum p, which is a product of its mass m and speed... 

In 1924, Louis de Broglie (1892-1987), studying at the Sorhonne, published his doctoral thesis in which he proposed an essential s)mimetry in nature Just as electromagnetic radiation (e.g., visible light. X-rays), commonly analyzed as waves, exhibits particle-like properties, matter may exhibit wavelike properties. The wavelength of such de Broglie... 

De Broglie reasoned that if light waves could behave like a stream of particles (photons), then perhaps particles such as electrons could possess wavelike properties. To quantify this connection, de Broglie began with the expression (from Einstein s theory of special relativity) for the momentum (jp) of the photon ... 

At about the same time, Schrodinger developed what came to be known as wave mechanics. Already in 1924, the French physicist Prince Louis De Broglie had suggested an analogy to Albert Einstein s earlier discovery that light waves have a particulate nature as well as their expected wave nature. De Broghe made the association run in the opposite sense. Why not suppose that particles such as electrons could likewise display wavelike properties The test for this idea would be to demonstrate experimentally that electrons produce diffraction and interference effects just hke classical waves, such as waves on the surface of water. ... 

Soon afterward, other phenomena such as Compton scattering. X-ray production, pair creation and annihilation could be interpreted successfully using a photon picture of light. Light still retains its wavelike properties as it travels through space. It assumes its photon or particle-like behavior only when it interacts with matter in a detector or at a target. 

Wave-particle duality means that matter has wavelike properties (as shown by the de Broglie wavelength and electron diffraction] and energy has particlelike properties (as shown by photons of light having momentum). These properties are observable only on the atomic scale, and because of them, we can never simultaneously know the position and speed of an electron in an atom (uncertainty principle). (Section 7.3)... 

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