where formula_26 is the wavelength in the second (external) medium. Hence we can plot in units of "λ"2, as a function of the angle of incidence, for various values of formula_27 (Fig.12). As "θ" decreases towards the critical angle, the denominator approaches zero, so that increases without limit — as is to be expected, because as soon as "θ" is "less" than critical, uniform plane waves are permitted in the external medium. As "θ" approaches 90° (grazing incidence), approaches a minimum