function Reff = M2Reff(type, M, a)
% Reff = M2Reff('type', M, a)

n = 1;

R = -2*n/M(2,1);

ca = cos(a*pi/180);

switch lower(type)
  case 't'
    Reff = R/ca;
  case 's'
    Reff = R*ca;
  otherwise
    error('unknown type')
end