In most of the Spectroscopic contexts, the Electro Magnetic Radiation interacting with matter has to be mathematically dealt with by describing the sinusoidally varying radiation in terms of the exponential of the imaginary argument. This is the euler equation or expression as one may find in the embedded Flash movie made from the Microsoft powerpoint presentation file.
Particulrly in the context of Nuclear Magnetic Resonance, the current day pulsed NMR methods require setting of different RF pulses with well synchronised phase relationships and associating them with change of the coordinate axis along which the radiation causes perturbation. Typically using phase sensitive detection techniques, experiment in which "Phase Cycling" features require a clear perception of the Laboratory frame and rotating frames of references to design pulse sequences to compenaste for instrumental artefacts or to regulate and specify the required spin system evolutions and select out the specified responses of spin systems.
And, this slide-show feature made available to intenet viewing by an embedded flash file conversion is meant for the beginners.