Diffraction of Light by Ultrasonics at Oblique Incidence

Abstract

IT is well known that the first simplified theory of the diffraction of light by ultrasonics, which Sir C. V. Raman and N. S. Nagendra Nath¹ put forward in 1935, explains many experimental facts in a very striking way. The main point of the theory is to neglect the bending of the light-rays in the sound field altogether and to attribute the observed effects wholly to the local variations of the optical length in the sound-wave. If a plane light-wave falls normally on a sound-wave, the incident wave-front becomes a periodic corrugated wave-front. At oblique incidence it follows from this theory² that the corrugation vanishes for certain angles of incidence Φ_n which satisfy the equation in which λ^* denotes the wave-length of the sound and L the breadth of the sound field ; consequently there should be no diffraction spectra visible when the light is incident in any of these directions.