Electromagnetic waves from dipole

On metal surfaces, two additional selection mles apply. The first is that only vibrations perpendicular to the surface are HREELS active. This mle follows from two phenomena unique at metal surfaces " (i) Electromagnetic waves polarized perpendicularly to the plane of incidence (parallel to the plane of the surface) undergo a 180° phase shift upon reflection. That is, at the metal surface, the out-of-phase electric-field vectors of the incident and reflected waves cancel each other as a result, no field exists that can couple with dipoles that oscillate parallel to the surface, (ii) The dynamic dipole moment generated by an oscillator that vibrates in the surface-parallel direction is cancelled by that of its image dipole (Figure 1) hence, there the net dynamic dipole moment is zero. On the other hand, if the real dipole is oriented perpendicularly to the surface, its dynamic dipole moment is reinforced by that of its image dipole. This selection mle is the same as that for infrared reflection-absorption spectroscopy (1RAS).°... 

Here a is the polarizability of the molecule. An oscillating dipole emits electromagnetic waves in all directions with electric field proportional to the acceleration of charges (d p/dt ) and decaying reciprocally with the distance r from the molecule. For a detector located in the horizontal yz plane at distance r = /y + from the origin (see Fig. 1.23) the scattered wave has electric field... 

C. Antenna. The antenna is used to make a transition from a guided wave (from the transmission line) to a radiated electromagnetic wave. The design of the antenna is influenced by many factors such as size, frequency, and electrical impedance. Antennas are normally of two types - omnidirectional and directional. The omnidirectional antennas are element type antennas such as monopoles or dipoles. The directional are horn-type antennas, parabolic dish type antennas such as a satellite communications antenna (SATCOM), or a phased-array antenna which can emit many beams at once. The characteristics of the antenna are a very important aspect of hazard evaluation. 

From Maxwell s theory of electromagnetic waves it follows that the relative permittivity of a material is equal to the square of its refractive index measured at the same frequency. Refractive index given by Table 1.2 is measured at the frequency of the D line of sodium. Thus it gives the proportion of (electronic) polarizability still effective at very high frequencies (optical frequencies) compared with polarizability at very low frequencies given by the dielectric constant. It can be seen from Table 1.2 that the dielectric constant is equal to the square of the refractive index for apolar molecules whereas for polar molecules the difference is mainly because of the permanent dipole. In the following discussion the Clausius-Mossoti equation will be used to define supplementary terms justifying the difference between the dielectric constant and the square of the refractive index (Eq. (29) The Debye model). 

From Maxwell s equations, the fundamental difference between static near fields (electromagnetic fields) and radiation (electromagnetic waves) is demonstrated. In Section 6.2, we treated the dipole. Consider that the dipole moment varies as a sine function with time. Now if the frequency is very high, the time delay will be noticeable if we are at a distance much longer than the wavelength X = c/f from the dipole. 

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