The Particulate Nature of Light

The movement of the photoelectrons causes a current to flow, and allows the 1 light intensity to be monitored. 

How can this photoelectric effect be understood The simplest explanation is that energy from the light is transferred to the electrons in the metal. If an electron picks up enough energy, it can break free from the surface of the metal. The more energy given to the electron, the faster it will travel after it leaves the metal. 

Despite elaborate efforts to make these observations of the photoelectric effect confiarm to existing wave concepts, consistent explanations of light based solely on waves ultimately failed. The only way to explain all of the experimental results was 

Recall that in Chapter 3 (p. 82) we used hvto indicate that light is required in a photochemical 

Here E represents the photon energy, and V is the frequency of the light. The h term is a constant, called Planck s constant, after German scientist Max Planck. Its value is 

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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. 

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. 

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... 

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. 

Electromagnetic radiation, which was previously thought to exhibit only wave properties, seems to show certain characteristics of particulate matter as well. This phenomenon, illustrated in Fig. 12.6, is sometimes referred to as the dual nature of light. 

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].

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. 

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). 

Dual nature of light the statement that light exhibits both wave and particulate properties. (12.2)... 

In a famous comment in Opticks, a book about the nature of light and colors, Newton proposed to find in specific attractions the explanations for all the reactions studied in chemistry. The principle of attraction could enlighten chemistry, Newton thought, when chemical qualities themselves were treated as special instances of universal forces. In the early eighteenth century, the question of particulate attraction became the special focus of two of Newton s... 

Particulate matter may physically become a factor in leaf physiological function if deposition is heavy enough to block light (critical to the photosynthetic machinery of the leaf) or if stomata become occluded by the particles. In the latter case both uptake of carbon dioxide for photosynthesis and water loss by the plant could be reduced. Particulate matter also has a potential to chemically alter the micro-environment of the leaf surface. Trace concentrations of heavy metals associated with deposited particles, as well as strong acid anions, may significantly modify the chemical nature of water drops or films of water in contact with plant surfaces. 

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