The IR Region Just below red in the visible region Wavelengths usually 2.5-25 um More common units are wavenumbers or cm-1, the reciprocal of the wavelength in centimeters Wavenumbers are proportional to frequency and energy Chapter 12Chapter 12 6 The IR Region • Just below red in the visible region. • Wavelengths usually 2.5-25 mm. • More common units are wavenumbers, or cm-1 , the reciprocal of the wavelength in centimeters. • Wavenumbers are proportional to frequency and energy. =>