An optical effect of an ultrasonic field discovered by R. Lucas and P. Biquard, namely the broadening of a slit image, can be computed in a simple way which is based on a method by Wiener. It is useful in the frequency range below 1 Mc/sec for the point by point study of the pressure distribution in a stationary sound wave. Photographic records of this distribution were made. A simple method of measuring the wavelength is described.