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 12 6Chapter 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. =>