A spectrum is the band of distinct colours we obtain when the white light is split by a prism. We can recombine the component of white light by passing them through a prism placed upside down near the given prism. When we pass white light through two identical prisms held side by side with their refracting edges in opposite directions; the first prism disperses white light but the second prism recombines them. Thus light emerging from the second prism is white.
