«ألمع برق سرى أم ضوء مصباح»
Is it a lightning flash passing by, or the light of a lamp?
Such poetic questioning can heighten the praise of a radiant scene or face by making its brilliance comparable to lightning and lamplight. The speaker’s outward uncertainty dramatizes the intensity of what is seen.
The sequence is: observed radiance → an apparently uncertain question → praise of exceptional beauty. The question performs a literary function rather than requesting actual identification of the light source.