Critical angle in total reflection

The critical angle can be calculated according to the following equation:

E= Photons energy in keV
Z= Atomic Number of the reflector
A= Atomic mass in g/mol
ρ= density in g/cm3

The values of α_crit laid between ~0.04° for Plexiglas at 35 keV, to ~0.55° for Gold at 8.4 keV (W-Lα). For Fused Quartz α_crit is 0.10° at 17.44 keV (Mo-Kα).

From the above equation two important conclusions can be made:

• A given energy photons are reflected only below a certain glancing angle called Critical Angle.
  
  
• A reflector set on a particular angle will reflect only certain photons out of a polychromatic beam, namely, those the energy of which fulfills the Total Reflection condition.