In PDLC (Polymer Dispersed Liquid Crystal), both polymenr and LC (Liquid crystal) have slightly different refractive index values. When electricity is not applied, directions of the LC droplets has no preferred orientation. In this case, the difference between refractive indices of polymenr and LC results in the scattering of light. Therefore, the film becomes opaque.

On the other hand, when electricity is applied to the film, it tends to reorient LC droplets in such a manner that the direction becomes parallel to the electric field. In this case, the light passes through without being scattered and the film becomes transparent.