InterviewSolution
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InterviewSolution
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Figure illustrates an arrangement employed in observations of diffraction of light by ultrasound. A plate light wave with wavelength `lambda = 0.55 mum` passes through the water-filled tank `T` in which a standing ultrasonic wave is sustained at a frequency `v = 4.7MHz`. As a result of diffraction of light by the optically inhomogemeous periodic structure a diffraction spectrum can be observed in the foacl plane of the objective `O` with focal length `f = 35 cm`. The separation between neighbouring maxima is `Deltax = 0.60mm`. Find the propegation velocity of ultrasonic oscillations in water. |
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Answer» When standing ulta sonic waves are sustained in the tank it behaves like a grating whose grating element is `d=(v)/(v) =` wavelength of the ultrasonic `v=` velocity of ultrasonic. Thus for maxima `(v)/(v)sin theta_(m) = m lambda` On the other hand `f tan theta_(m) = m Deltax` Assuming `theta_(m)` ti be small (beacuse `lambda lt lt (v)/(v))` we get `Deltax = (f tan theta_(m))/(m) = (f tan theta_(m))/((v)/(v lambda) sin theta_(m)) = (lambda v f)/(v)` or `v =(lambda v f)/(Deltax)` Putting the values `lambda = 0.55mum, v = 4.7MHz` `f = 0.35m `and `Deltax = 0.60 xx 10^(-3)m` we easily get `v=1.51km//sec`. |
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