Complex sentence: In this case, the function is the scattering amplitude and through the use of the optical theorem the imaginary part of the scattering amplitude is related to the total cross section which is a physically measurable quantity.
Split into simpler sentences: In this case, the function is the scattering amplitude. Through the use of the optical theorem the imaginary part of the scattering amplitude is then related to the total cross section, which is a physically measurable quantity.