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White created by light on a TV screen, for example, or through a prism, does not have its own specific wavelength. Rather, it is the sum of all the colours. Think of a rainbow: you can't see the colours of sunlight except when raindrops refract the light rays and reveal the whole spectrum. Because of this, scientists…

• Why does the writer say that white is not scientifically a colour?
• What material does the writer say was used before recorded history?