JEE Main · 2021 · 16 Mar 2021 · Shift 2 · Waves & Sound

In a given medium every colour travels at the same speed $c$, and $c=\nu \lambda$ ties the frequency to the wavelength, so one of the two cannot change while the other stays fixed. Red and blue are different colours of the visible spectrum, and the frequency of light is what the source fixes and what the eye registers as colour, so their frequencies differ, red near $4.3\times 10^{14}\mathrm{Hz}$ and blue near $6.7\times 10^{14}\mathrm{Hz}$. The relation $\lambda =c/\nu$ then makes their wavelengths differ as well, about $700\mathrm{nm}$ for red against about $450\mathrm{nm}$ for blue. Red light and blue light thus have different frequencies and different wavelengths.