Ray Optics, Fermat's Principle, and Applications to General Relativity

All kind ofinformationfrom distant celestialbodies comestous intheform of radiation. Inmost the of electromagnetic cases thisradiation propagation be can as a reasonable in described, terms of This is approximation, rays. truenot inthe but also inthe radio only ofthe electro opticalrange range Forthis the of reason laws areoffundamental magneticspectrum. optics ray for importance and astronomy, astrophysics, cosmology. to According arethe of general relativity, light light likegeodesics a rays Lorentzianmetric whichthe is described. how by spacetimegeometry This, is true as as the under the ever, only long light are rays freelypropagating influence ofthe fieldwhich is coded in the only gravitational spacetime ge If a is in an medium ometry. light ray influenced, addition, by optical (e. g. , then it will not follow ofthe by a a plasma), light like geodesic spacetime metric. It is true that for radiation the electromagnetic traveling through universe the influence ofamedium on the ofthe and usually path on ray the is small. there are several cases inwhich this influ frequency However, ence is well in inthe radio For measurable, particular very range. example, the deflection ofradio inthe field oftheSun is consider gravitational rays influenced the Solarcorona.currentand ably by Moreover, planned Doppler with microwaves in the Solar reach in the an experiments system accuracy 5 of 10 15 whichmakes it totakethe influence frequency Awlw necessary ofthe medium into account.