PHYSICS

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••,, "" . . ~: . , .. - . ......, 40 }! '"......•; , - .- ..-';": . ',~ t 'j :: ;:. - . < HII a Fig. 21.14. Diffraction pattern obtained from a helical structure of DNA cross-wise pattern of the DNA X-ray diffraction picture (fig. 21.14) was due to a repeating helical structure. 21.3.9. Polarization JIll b ~, ", '.~ Ull -- ", '~ Ell ,......, .. , .. Fig. 21.15. Representation of the vector jj; of: a unpolarized light; b linearly polarized light; c circularly polarized light; d elliptically polarized light. The light ray is travelling at right angles to the plane of the paper 152 Light is electromagnetic radiation consisting of electric (jj;) and magnetic (H) vectors vibrating at right angles to each other and to the direction of propagation..Conventional polarization is associated with the electric vector. An ordinary beam of light is produced by a large number of waves emitted by the atoms or molecules of the light source; the direction of vibration of the jj; c \ "', d , , .... , \, , , I I I, vector changes about every 10- 8 seconds. A wave of unpolarized light traveling at right angles to a plane perpendicular to the direction of propagation is depicted in fig. 21.15, a. A wave is said to be linearly polarized if jj; vibrates in the same direction at all times at a particular point (fig. 21.15, b). If the tip of the vector jj; rotates in a circle with time, the wave is said to be circularly polarized (fig. 21.15, c). If the ti p of the vector jj; moves in an elli pse, the wave is said to be elliptically polarized (fig. 21.15, d). There are some special dichroic crystals (e.g., tourmaline) and Polaroid films (polyvinyl alcohol) that will pass light vibrating in only one direction. Such a crystal is called the polarizer. If the light is plane polarized after passing through the crystal, it will be stopped by an analyzer placed at right angles to the polarizer. Optical activity is the ability of certain materials to rotate the plane of polarization of transmitted light. The angle (cp) through which the light is rotated by the material depends on the length (l) of the sample and on the concentration (C) if the substance is in solution: Source of light II Polarizer 21.3.10. Optical Activity qJ = aCI where a is the specific optical activity of the substance. a lj I - -1 1/7 Optical cell Fig. 21.16. Polarymeter Analyizer (21.13) ----+1 Registered system 153
A polarimeter is a device that uses optical activity to measure the angle of rotation of polarized light. It consists of an elongated optical cell mounted between two polarizing lenses, one of which is fixed (polarizer) and the other being free to rotate (analyzer). The unpolarized light that is passing through the polarizer is polarized; the analyser intercepts this polarised light beam with its transmission axis at an angle qJ to the axis of the polyrizer. This angle qJ is measured by the registering system (fig. 21.16). 21.4. QUANTUM OPTICS 21.4.1. Photoelectric Effect In the latter part of the 19 th century, experiments showed that when light is incident on certain metallic surfaces, electrons are emitted from the surfaces. This phenomenon is known as the photoelectric effect could not be understood within the framework of classical physics. Fig. 21.17 is a schematic diagram of an apparatus in which the photoelectric effect can occur. The evacuated glass tube contains a metal plate (the cathode), connected to the negative terminal of a battery. Another metal plate (the anode) is connected to the positive terminal of the battery. When the tube is kept in the dark, there is no current in the circuit. However, when monochromatic light of the appropriate wavelength strikes the shines on cathode, a current is generated that can be measured. Fig. 21.17. Circuit diagram for observing the photoelectric effect: A - anode; C - cathode The first successful explanation of the photoelectric effect was given by Einstein in 1905. He proposed that light could not only became in waves, but also come in packets of energy, called photons. As a photon hits an electron, it transfers a certain amount of energy. If the energy is sufficient, the electron is ejected, striking the surface of the metal where it can be detected. According to Einstein, the law of conservation of energy during the photoelectric effect is: mV 2 hv = A +-- (21.14) 2 where hv is the energy of the photon (here h is Plank's constant, v is frequency), A is called the work function of the metal, and m;2 the kinetic energy of the electron. The work function represents the minimum energy required for the photoelectric effect to occur: hv . = A min (21.15) or: c h-=A Am"' (21.16) where A is called the cutoff wavelength. max Example. A sodium surface is illuminated with light of wavelength 300 nm. The work function for sodium is 2.46 eV (I eV = 1.602.10- 191). Find the kinetic energy of the ejected photoelectrons. Solution. The energy of the illuminating photon is: 34 8 hv =he (6.626.10- J ·s)(3·10 m 'S-I) =6.626 .10- 19 J =4.14 eV. A. 300·1O- 9m 21.4.2. Laser The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser action can be understood by considering the main processes: pumping, spontaneous emission, stimulated emission, and absorption. Pumping. This is a process necessary for laser action by which the particles (atoms or molecules) of the active medium are elevated to an excited electronic state by means of electrical discharge, passage of an electric current, or exposure to an intense radiant source. Spontaneous Emission. Particles in an excited electronic state may lose all or part of their elevated energy by spontaneous 154 155
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Yuriy I. Posudin PHYSICS WITH FUNDA
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PHYSICS

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