A sheet of cellophane is placed between two crossed Polaroids to demonstrate the effect of birefringence. Cellophane, a birefringent material, causes the polarized light passing through it to split into two components that travel at different speeds. When these components exit the cellophane and pass through the second Polaroid, they interfere, producing vivid interference colors. The colors change as the cellophane or Polaroids are rotated, illustrating the relationship between birefringence, polarization, and interference.