The Nature of Light

Unless otherwise noted, all art on this page is Cengage Learning 2014. 

The opening to a cavity inside a hoiiow object is a good approximation of a biack body The hoie acts as a perfect absorber. 

FIGURE 9.11 A good approximation of a blackbody is made by constructing a cavity with a very small hole in it. Defined as a perfect absorber or emitter of radiation, blackbodies do not absorb or emit radiation equally at all wavelengths. This diagram shows a blackbody s ability to absorb all radiation. Light that enters the small hole of the blackbody reflects off the inside surfaces, but has a very small chance of escaping the cavity before it is absorbed. 

When scientists began measuring the intensity or power density of light given off as a function of wavelength 7(A) at various temperatures, they made some interesting observations  

The total power per unit area, in units of watts per square meter (W/m ), given off by a blackbody at any temperature is proportional to the fourth power of the absolute temperature  

The speed of the wave, the distance traveled per unit time (in units of meters per second), is the product of its frequency (cycles per second) and its wavelength (meters per cycle)  

In a vacuum, all types of electromagnetic radiation travel at 2.99792458X10 m/s (3.00x10 m/s to three significant figures), which is a physical constant called the speed of light (c)  

As Equation 7.1 shows, the product of v and X is a constant. Thus, the individual terms have a reciprocal relationship to each other radiation with a high frequency has a short wavelength, and vice versa. 

For the radio signal Combining steps to ealeulate the frequency. 

Comment The radio station here is broadcasting at 92.3X10 s , or 92.3 million Hz (92.3 MHz), about midway in the FM range. 

The speed of light is an exact number and usually does not limit the number of significant figures in a calculated result in most calculations, however, the speed of light is rounded to three significant figures c - 3.00 x 10 m.  

Frequency is expressed as cycles per second, or simply redprocal seconds (s ), which is also known as hertz (Hz). 

Usually light is taken to mean electromagnetic radiation in the visible, near ultraviolet, and near infrared spectral range (Fig. 1.1). 

In the wave model, electromagnetic radiation is characterized by a wavelength, 2, a frequency, v, and a velocity, c. The three quantities are related by the relationship 2v = c. The value of c is constant (2.998 x 10 m s in vacuum), whereas 2 (and v) may cover a wide range of values. The SI units for 2 and v are the meter (m) and the hertz (Hz), respectively. In some cases, the wavenumber v 

In the quantum model a beam of radiation is regarded as a stream of photons, or quanta. A photon has a specific energy, E, related to the frequency of the radiation, V, by (1.1), 

The interaction of light with molecular systems is generally an interaction between one molecule and one photon. It can be written in the very general form (1-2), 

The energy associated with a certain frequency of light is related by the equation  

In developing the quantum mechanical model of the atom, it was found that the electrons can have only certain distinct quantities of energy associated with them, and that in order for the atom to change its energy it has to absorb or emit a certain amount of energy. The energy that is emitted or absorbed is really the difference in the two energy states and can be calculated by  

All electromagnetic radiation travels at about the same speed in a vacuum, 3 0 x 108 m/s. This constant is called the speed of light (c). The product of the frequency and the wavelength is the speed of light  

Let s apply some of the relationships. What wavelength of radiation has photons of energy 7.83 x 10 19 J  

This answer could have been calculated more quickly by combining the original two equations to give  

In the early 17th century, the nature of light was not well understood. Nowadajrs, we know that light is comprised of photons , which are quantized waves having some of the properties of particles. However, this concept was not made clear until the advent of quantiun electrod5mamics in the 1930 s, following Einstein s concept of relativity in the 1920 s. 

The concept of photons with wave properties has its roots in the study of optics and optical phenomena. Until the middle of the 17th century, light was generally thought to consist of a stream of some sort of particles or 

1) Fresnel and Thomas Young (1815) on interference and diffraction respectively 

2) Maxwell in 1873 who postulated that an oscillating electrical circuit should radiate electromagnetic waves 

3) Heinrich Hertz in 1887 who used an oscillating circuit of small dimensions to produce electromagnetic waves which had all of the properties of light waves 

Rutherford s experiment demonstrated that the total positive charge in an atom is localized in a very small region of space (the nucleus). The majority of a particles simply passed through the gold foil, indicating that they did not come near a nucleus. In other words, most of the atom is empty space. The diffuse cloud of electrons (which has a size on the order of 10 8cm) did not exert enough force on the a particles to deflect them. The plum pudding model simply did not explain the observations from the experiment with a particles. 

Atoms consist of electrons and protons in equal numbers and, in all cases except the hydrogen atom, some number of neutrons. Electrons and protons have equal but opposite charges, but greatly different masses. The mass of a proton is 1.67 X 10 24 grams. In atoms that have many electrons, the electrons are not all held with the same energy later we will discuss the shell stmcture of electrons in atoms. At this point, we see that the early experiments in atomic physics have provided a general view of the structures of atoms. 

This series of spectral lines for hydrogen became known as Balmer s series, and the wavelengths of these four spectral lines were found to obey the relationship 

Eventually, other series of lines were found in other regions of the electromagnetic spectrum. The Lyman series was observed in the ultraviolet region, whereas the Paschen, Brackett, and Pfund series were observed in the infrared region of the spectrum. All of these lines were observed as they were emitted from excited atoms, so together they constitute the emission spectrum or line spectrum of hydrogen atoms. 

The importance of the idea that energy is quantized is impossible to overstate. It applies to all types of energies related to atoms and molecules. It forms the basis of the various experimental techniques for studying the structure of atoms and molecules. The energy levels may be electronic, vibrational, or rotational depending on the type of experiment conducted. 

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In terms of their optical properties, all solids fall into one of two classes. Either they are transparent to light (here we are restricting the term "light" to visible radiation) or they are opaque. In the latter case, all of the radiation may be reflected. However, most solids reflect some wavelengths and absorb others. This is the condition that we call "color". If all visible wavelengths are absorbed, the solid is said to be "black" while reflectance of all visible wavelengths results in a "white" solid. We intend to show how "color" is measured but first must define the nature of "light". 

In this chapter we give a brief review of some of the basic concepts of quantum mechanics with emphasis on salient points of this theory relevant to the central theme of the book. We focus particularly on the electron density because it is the basis of the theory of atoms in molecules (AIM), which is discussed in Chapter 6. The Pauli exclusion principle is also given special attention in view of its role in the VSEPR and LCP models (Chapters 4 and 5). We first revisit the perhaps most characteristic feature of quantum mechanics, which differentiates it from classical mechanics its probabilistic character. For that purpose we go back to the origins of quantum mechanics, a theory that has its roots in attempts to explain the nature of light and its interactions with atoms and molecules. References to more complete and more advanced treatments of quantum mechanics are given at the end of the chapter. 

The nature of light absorption in a crystal is of no significance for theory. What is important here is that this absorption be photoelectrically active, i.e., results in a change of the concentration of free carriers in a crystal. This process may take the form either of the so-called intrinsic absorption accompanied by the transition of an electron from the valency to the conduction band, or of the so-called impurity absorption caused by an electronic transition between the energy band and the impurity local level. 

The Nature of Light What is a Photon Chandra Roychoudhuri, A.F. Kracklauer and Kathy Creath... 

Minnaert. M., 1954. The Nature of Light and Colour in the Open Air, Dover, New York. 

As discussed for several decades by a number of authors, the nature of light and photon physics is related not only to the propagation of plane wavefronts but also to axisymmetric wavepackets, the concepts of a rest mass, a magnetic field in the direction of propagation, and an associated angular momentum (spin). 

Einstein s idea started a truly revolutionary development in physics quantum mechanics, It opened up wide new horizons and clarified many outstanding problems in our view of the structure of matter, Quantum mechanics is based on the idea of wave-particle duality. Einstein first applied this idea to the nature of light, but it was... 

The nature of Light is to flow continually and we have agreed to call rays those effluxions of the sun mixed with Ether. Yet, we must not confound Light with the ray, or with the splendour and brightness. Light is the cause brightness the effect. 

This website provides an overview of photosynthesis, including leaf structure, the nature of light, chlorophyll and accessory pigments, chloroplast, and stages of photosynthesis. Several links to other photosynthesis resources are also provided. 

To appreciate properly how electro-optic ceramics function, it is first necessary to consider the nature of light and its interaction with dielectrics. 

Isaac Newton studied the nature of light, the laws of gravity, and the laws of motion around 1700. The SI unit of force is named after him. 

We can say, however, that whatever the nature of light may be, that of electrons and protons is similar. 

Light can be narrowly defined as the visible portion of the electromagnetic spectrum. A broader definition would include infrared, ultraviolet, and x-ray wavelengths, which are not visible to the eye. The nature of light has been the subject of controversy for thousands of years. Even today, while scientists know how light behaves, they do not always know why light behaves as it does. 

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