White is not a separate color with its own wavelength. Some processes can separate light into its colors. Triangular slab of glass. Light gets bent as it goes from air to glass to air. A rainbow comes out.
White sunlight is split into a rainbow of colors.
If your at the right place relative to the sun and raindrops in the sky, you see a rainbow. A grating can also separate colors but works on a different principle. Show students a grating. Like a microscopic picket-fence. Light gets spread and scattered in interesting ways by this picket fence.