A definition of fractional Fourier transform as the generalization of ordinary Fourier transform is given at the beginning. Then due to optical reasons the fractional transform of a so-called chirp functions is considered in both theory and practical simulations. Because of a quadratic phase factor which is common in the definition of the transform and some optical concepts, a comparison between these concepts such as Fresnel diffraction, spherical wave, thin lens and free space propagation and the transform has been done. Finally an optical setup for performing the fractional transform is introduced.