[MHGinTN]

'Purple' is a subjective designation used to identify a grouping of electromagnetic waves in the spectrum of electgromagnetic frequency; the 'oscillation limits' determine the position on the scale of electromagnetic frequency (I think! ... Physicists, Faraday, longshadow, and others will hopefully correct me here.)

[garbanzo]

Actually structures in our eyes called cones which are sensitive to various wavelengths of electromagnetic energy send signals to our brains which then process and interpret these signals as color.