  Using Wenzel-Kramers-Brillouin (WKB) approximation the scalar and tensor
power spectra are obtained. Scale invariant spectra are obtained and the
spectral indices come very close to the observed data from WMAP and Planck
experiments. The advantage of this method is that, it is valid even when
slow-roll approximation fails. Constraints on the tensor to scalar ratio r is
also studied with the WKB Approximation. We use the Power law inflation as the
base model as it allows comparison with exact results.
