Because a photon has less energy after a collision than it had initially, \(E_{\mathrm{f}}\) is less than \(E_{\mathrm{i}}\), and the final wavelength \(\lambda_{\mathrm{f}}\) is greater than the initial wavelength \(\lambda_{\mathrm{i}}\). In other words, as the nergy of a photon decreases, its wavelength increases.