Equation 23-8 says that the wavelength is shorter in a medium with a higher index of refraction, where the propagation speed is slower. For example, red light that has a wavelength \(\lambda_{\mathrm{vacuum}} = 750\) nm in vacuum has a wavelength in water (\(n\) = 1.33) equal to \(\lambda = \lambda_{\mathrm{vacuum}} / n = (750 \mathrm{nm}) / (1.33) = 564\) nm. The frequency of this light is the same in both media: \(f = c / \lambda_{\mathrm{vacuum}} = (3.00 \times 10^8 \mathrm{m}/\mathrm{s}) / (750 \times 10^{-9} \mathrm{m}) = 4.00 \times 10^{14}\) Hz in vacuum and \(f = v/\lambda = c / (n\lambda) = (3.00 \times 10^8 \mathrm{m}/\mathrm{s}) / ((1.33)(564 \times 10^{-9} \mathrm{m}))= 4.00 \times 10^{14}\) Hz in water.