Light propagation parallel to the helical axis is studied in a cholesteric medium distorted by the application of a magnetic field normal to the helical axis. This distortion introduces new components in the Fourier spectrum of the eigen waves. The amplitudes of these new components is calculated. It is shown that the most important effect results in a variation of the ellipticity of the components of De Vries waves (circular at zero-field). An experimental study using a cholesteric prism gives results in good agreement with the theoretical predictions.
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