  The cosmic microwave background (CMB) carries information from the last
scattering surface that puts constraints on the multitude of proposed
cosmological models and the gravitation theories they are based on. One class
of such theories is $f(R)$ gravity, which has become an interesting endeavour
to correct for the degeneracies of the concordance model.
  We presents a description of CMB anisotropies generated by tensor
perturbations in $f(R)$ theories of gravity. The temperature and the $E$-mode
polarisation power spectra in the special case of $f(R)=R^n$ are computed using
a modified version of CAMB package.
