Critical incident angle

The Evanescent wave is the X-ray field that propagates into the solid surface in a reflection geometry near the critical incidence angle. It is the exponentially damped transmitted beam. 

If M]>M2, then, if 6 2 = 90°, sin 6 1 =M2/mj. The incident angle at this moment is called the critical angle, designated by 9. If the incident angle is larger than the critical angle (6 1 >0c), then the incident wave will be totally reflected back to Medium 1. This is known as total reflection. 

However, at grazing incidence angles near the critical angle the reflected beam becomes important, and eventually becomes dominant. This is an important effect and the two-beam case is seriously in error for angles of incidence close to the critical angle. Fortunately, the reflected beam decreases in intensity as the fourth power of the scattering angle, and is weak compared with the diffracted... 

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