What is a spectrum?

A spectrum is a band of seven colors obtained after white light is split. This can be understood by the fact that if a white light (sunlight) is passed through a prism and allowed to fall on a white screen, the seven split colors can be seen. The arrangement of the colors are in the order: violet, indigo, blue, green, yellow, orange and red. This arranged group of colors is called a spectrum. A rainbow is an example of a natural spectrum

This also may help you remember the order of the colours in the spectrum-

R(red) O(orange) Y(YELLOW) G.(green) B(blue) I(indigo) V(violet)

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you can also remember it as:

(R)ead (O)ur (Y)ellow (G)rafic (B)ecause (I)t's (V)ictorious