![]() Finally, an analysis based on Maxwell's equations for an arbitrarily polarized light beam propagating in an arbitrary direction is given using the partial -wave approach. The effects of finite optical and acoustic beamwidths and variation of acoustic frequency are considered in terms of momentum conservation. The diffraction process is described as a single as well as a multiple three -particle interaction. Next, a particle picture of diffraction in terms of photons and phonons is given. Owing to the finite width of the sound beam, the Bragg condition is relaxed, and the effect can be used to control the direction and phase of the diffracted beam or to determine the angular distribution of the acoustic power. In the case of Bragg diffraction, the energy is exchanged sinusoidally between the diffracted and undiffracted beams. It is found to be directly proportional to the square of the acoustic wavelength and inversely proportional to the optical wavelength. The transition length thus separates the region of multiple -order (Raman -Nath) diffraction from the region of single -order (Bragg) diffraction. Constructive interference is obtained, however, provided the light is incident at the Bragg angle, in which case the diffracted beam appears to be reflected from the acoustic wavefronts. A transition length (width of sound beam) is defined, above which all diffraction effects disappear due to destructive interference. Assuming that the sound column modulates only the phase of the incident light in both time and space, the frequencies, wavevectors, and intensities of the diffracted waves are obtained for normal incidence. But that's not "really" what happened.Abstract Diffraction of light by a sinusoidal sound wave is discussed in detail. Presto, at our macroscopic level, it looks like it started moving left. So no no one is stopping the left-moving side of A. Ahhh, but B just hit the wall, literally. So the left moving bit of spot A was being cancelled out by the right moving bit of spot B. Now before it got to the slit there was another spot to it's left that was doing the same thing. the left side of it is moving left and the right side is moving right (its spherical). So what happens at the slit? Well consider the spot right on the left edge of the opening. When you sum it all up, all the "sideways bits" sum to zero, and the only leftover terms are the ones from the original disturbance, moving outward. ![]() So wait, if the microscopic "things" are moving spherically, then why did you have a linear wavefront to start with? Because over an extended front, every bit of the wave that's going, say, right, has another bit somewhere else going left. ![]() At the microscopic level that's not what's happening, at that level its moving in all directions all the time. You see a linear wave, but that's because you're macroscopic. Think about it differently according to Huygens a wave front is the mathematical addition of an infinity of spherical waves. So then, why does this linear thing suddenly move sideways when it goes through a hole? ![]() You're thinking of the sound as "a wave", in this particular case you're looking at it where its roughly linear. It's ultimately the same reason why light refracts, Huygens's Principle. ![]()
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