What acousto-optic figure of merit is needed to get 50% of an input light beam to be redirected into a diffracted beam, if the acoustic intensity is 5 W/cm²? Use the following parameters: The input light has wavelength 600 nm, the index of refraction of the medium is 1.8, the acousto-optic cell size is 1 mm, the speed of sound in the medium is 5 × 10⁵ cm/s, and the density is 5 g/cm³. What value does this imply for the photoelastic constant?