On passing through a prism different parts of white light produce a series of coloured bands called ……… It spread from $7.5 \times {10}^{14}\mathrm{Hz} \left(\mathrm{violet}\right) \mathrm{to} 4.0 \times {10}^{14} \mathrm{Hz} \left(\mathrm{red}\right) \mathrm{zones}.$This spectrum is called ……… 

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