Different kinds of electromagnetic waves have different frequencies and wavelengths. In Section 13-3 we learned that for a mechanical wave, the frequency \(f\) and wavelength \(\lambda\) are related to the propagation speed of the wave \(v_{\mathrm{p}}\) by \(v_{\mathrm{p}} = f\lambda\) (Equation 13-2). The same relationship holds for electromagnetic waves in a vacuum with \(v_{\mathrm{p}}\) related by \(c\):
Equation 22-2 tells us that the product of frequency \(f\) and wavelength \(\lambda\) has the same value, \(c\), for all electromagnetic waves in a vacuum. The longer the wavelength, the lower the frequency; the shorter the wavelength, the higher the frequency.