Light particulate nature

If we think in terms of the particulate nature of light (wave-particle duality), the number of particles of light or other electi omagnetic radiation (photons) in a unit of frequency space constitutes a number density. The blackbody radiation curve in Fig. 1-1, a plot of radiation energy density p on the vertical axis as a function of frequency v on the horizontal axis, is essentially a plot of the number densities of light particles in small intervals of frequency space. 

The sunblocks zinc oxide, titanium dioxide, and iron oxide are inorganic chemicals that are not absorbed into the skin. These substances consist of opaque particles that reflect both visible and ultraviolet light. In addition, zinc oxide blocks virtually the entire UVA and UVB spectrum and thus offers overall protection. The particulate nature of these sunblocks enhances their effectiveness at reflecting sunlight. The smaller the particle size, the greater the surface area available for reflection, and the more effective the sun protection offered by the formulation. 

Figure 12.5 The photoelectric effect. Light with photons of energy hc/k approaches from the left, strikes the atoms in the metal, and ejects a photoelectron with a kinetic energy equal to the photon energy minus the work function of the metal. This demonstrates the particulate nature of light.

Photon Another term for a single quantum of light, a name chosen to emphasize the particulate nature of light. 

It is only for X-rays of short wave length that AX is a measurable amount in other words only for such radiation is the mass of the light quantum not too small compared with the rest mass of the electron m0. To the wave nature of the radiation a particulate nature is thus added in the complementary theory and vice versa, according to equation (4). 

The photoelectric effect was explained by Albert Einstein in 1905 using the principles of quantum physics developed by Max Planck. Einstein claimed that light was quantized— that is, it appeared in bundles of energy. While these bundles traveled in waves, certain reactions (like the photoelectric effect) revealed their particulate nature. This theory was further supported in 1923 by... 

Figure 41. Absorption (A) and circular dichroism (B) spectra of poly-L-glutamic acid, pH 3.85, as a function of particle size, which is achieved by sonication. As particle size increases, the ellipticity and absorption extrema are progressively dampened there is increased light scattering (curve b and c of A), and there is a marked red shifting of the CD extrema and crossover. These are distortions in the spectra due to the particulate nature of the optically active system. Adapted, with permission, from [110].

The linear term (b/xj in Eq. (16) is due to the particulate nature of light and electrons and is usually regarded as the dominant source of noise. Often it is. However, even when care is taken to assure a stable instrument and specimen, c is often about 0.01, so that the quadratic term is the largest for many data points. Repeated measurements of a nominally constant signal, for several different signal strengths, can provide estimates of a, b, and c for use in weighting. 

The Wave Nature of Light The Particulate Nature of Light... 

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. ... 

In the 1800s the wave nature of light and the particulate nature of matter dominated physical explanations of the physical world. Newton s and Maxwell s equations were thought to capture all reality, conferring to it an inescapable, deterministic character. Then, at the end of the nineteenth century numerous experiments were conducted that were not reconciled with these prevailing classical mechanical notions. 

Airborne particulates include dust, fume and aerosols. Many such particles are invisible to the naked eye under normal lighting but are rendered visible, by reflection, when illuminated with a strong beam of light. This is the Tyndall effect and use of a dust lamp provides a simple technique for the rapid assessment of whether a dust is present, its flow pattern, leak sources, the effects of ventilation, etc. More sophisticated approaches are needed for quantitative data. Whether personal, spot or static sampling is adopted will depend upon the nature of the information required. 

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