Eq. (11) indicates that the amplitude of the evanescent wave falls off by a factor e as the coordinate y (measured from the interface) increases by the distance ‍ d = 1 / κ κ , commonly called the "penetration depth" of the evanescent wave. Taking reciprocals of the first equation of (12), we find that the penetration depth is d = λ λ 0 2 π π n 1 2 sin 2 ⁡ ⁡ θ θ i − − n 2 2 , where λ 0 is the wavelength in vacuum, i.e. 2 π π / k 0 . Dividing the numerator and denominator by n 2 yields d = λ λ 2 2 π π ( n 1 / n 2 ) 2 sin 2 ⁡ ⁡ θ θ i − − 1 , where λ λ 2 = λ λ 0 / n 2 is the wavelength in the second (external) medium. Hence we can plot d in units of λ 2 , as a function of the angle of incidence, for various values of n 1 / n 2 (Fig. 12). As θ i decreases towards the critical angle, the denominator approaches zero, so that d increases without limit – as is to be expected, because as soon as θ i is less than critical, uniform plane waves are permitted in the external medium. As θ i approaches 90° (grazing incidence), d approaches a minimum d min = λ λ 2 2 π π ( n 1 / n 2 ) 2 − − 1 . For incidence from water to air, or common glass to air, d min is not much different from ⁠ λ 2 / 2 π ⁠. But d is larger at smaller angles of incidence (Fig. 12), and the amplitude may still be significant at distances of several times d ; for example, because e is just greater than 0.01, the evanescent wave amplitude within a distance 4.6 ‍ d of the interface is at least 1% of its value at the interface. Hence, speaking loosely, we tend to say that the evanescent wave amplitude is significant within "a few wavelengths" of the interface.