Very high reflectance mirrors can be fabricated using multi-layer
quarter-wave stacks of form g(HL)mHa. {here g and a represent glass
and air}.

(a)  Evaluate the characteristic matrix for such a 3-layer stack (m=1)

(b)  Determine the reflectance for the stack for nH=2.32(ZnS) and
nL=1.38(MgF2).  Assume normal incidence and ng=1.5 and na=1.0.

if you have adobe look at this                        
http://www.oriel.com/netcat/VolumeIII/pdfs/mirrtech.pdf