Ans. When monochromatic light passes through a glass prism, it splits into its constituent colours. However the same doesn't happen when light passes through a flat window pane. This is because of the peculiar structure of the glass prism. In a glass prism, the opposite sides are not parallel to each other. When light strikes the first surface, it undergoes refraction since glass is denser than air. The amount of bending of light depends on its wavelength. Because the opposite faces of a prism are not parallel, each wavelength of light emerges with a different bend and not parallelly. Whereas in the case of the windowpane, both the surfaces are parallel to each other and hence dispersion of light is not observed.