Let $ I_a, I_b\: and \: I_c$ be the intensities and $ f_a,f_b\: and \: f_c$ be the frequencies for the curves a,b and c respectively

$\begin {array} {1 1} (a)\;f_a=f_b\: and \: I_a \neq I_b & \quad (b)\;f_a=f_c\: and \: I_a = I_c \\ (c)\;f_a=f_b\: and \: I_a = I_b & \quad (d)\;f_b=f_c\: and \: I_b = I_c \end {array}$