This chapter begins with a broad history of the notion of electromagnetic spectra and the devices used to explore spectral properties using the phenomena of “diffraction” and “dispersion”. It discusses these properties in the recent context of quantum field theory and electromagnetic “wave-particle duality”. In Sect. 25.1.1 we derived the relativistic Doppler shift in Minkowski spacetime. In the absence of a viable quantum theory of gravitation, we illustrate in Sect. 36.1 how wave-particle duality may be implemented in the Einstein-Maxwell paradigm. We then bring together all the assumptions that are embodied in this paradigm and discuss the gravitational redshift associated with the gravitational field outside the event horizon of a Schwarzschild spacetime metric. In particular, affinely-parametrised timelike geodesic worldlines offer models for massive “test particles” and “clocks” are defined to exhibit the proper time of such test particles as defined in Sect. 13.2 . Null geodesics are taken to model classical “light signals” where “light” is an synonym for any wave in the electromagnetic spectrum associated with the Maxwell paradigm. In Sect. 36.2 we give expressions for the general frequency ratio for observers in arbitrary spacetimes and mention some of the schemes that have been devised to implement this formula in the context of space astrometry and cosmology.