Evaluation of the State-of-the-Art Knowledge

During the last decade, active interest in optical mirage appears to have waned. The reasons for the apparent decline are believed to be two-fold. Firstly, on the basis of simple ray-tracing techniques, the mirage theories satisfactorily explain the various large-scale aspects of observations. Thus no disturbing contradictions between theory and observation have been found. Secondly, although atmospheric refraction remains of great interest to astronomy, optical communication, and optical ranging, the phenomenon of the mirage has so far failed to demonstrate a major use.

At the present time, there is no single theoretical model that explains all the aspects, both macroscopic and microscopic, of the mirage phenomenon. The absence of such a model must stand as evidence that shortcomings remain in current knowledge. These shortcomings are most eloquently discussed by Sir. C. V. Raman (1959), who suggests and actually demonstrates that any approach to explain the phenomenon must be based on wave-optics rather than ray-optics. The theory of wave-optics, as applied by Sir. C. V. Raman, suggests the presence of some intriguing aspects of the mirage that arise from the interference and focussing of wavefronts in selected regions of the refracting layer. Raman's experimental studies reveal that when a collimated pencil of light is incident obliquely on a heated plate in contact with air, the field of observation exhibits a dark region adjacent to the plate into which the incident radiation does not penetrate, followed by a layer in which there is an intense concentration of light and then again by a series of dark and bright bands of progressively diminishing intensity.

Further theoretical and experimental investigations are warranted in order to determine to what extent the brightening and brightness variations that arise from interference and focussing can add unusual effects to observations of phenomenon associated with abnormal refraction in the atmosphere.