White light is all the colors mixed together. When all the colors arrive at once, we see white. 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.