To make bright or to clarify; to give light to or illuminate something literally or metaphorically.
From 'em-' combined with 'brighten' (the causative verb of 'bright'). This double causative structure is less common than 'embright' but follows English morphological rules.
The double prefix 'em-brighten' is like saying 'super-brighten'—English lets you stack these intensity modifiers, which is why people instinctively say things are 'super-super-bright' rather than just 'very bright.'
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