Enlighten

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Middle English (in the sense ‘make luminous’; formerly also as inlighten ): in early use from Old English inlīhtan ‘to shine’; later from en-1, in-2 (as an intensifier) + lighten2 or the noun light1.

wiktionary

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Formerly also inlighten, from Middle English enlightenen, inlightnen, a hybrid formed from inlighten(“to enlighten, illuminate”), from Old English inlīhtan, onlīhtan, enlīhten(“to enlighten, illuminate, give light to, give sight to”) and lightnen(“to enlighten, illuminate”) (equivalent to light +‎ -en). Cognate with Dutch inlichten(“to enlighten, inform”), Old High German inliuhten(“to enlighten, illuminate”), Gothic 𐌹𐌽𐌻𐌹𐌿𐌷𐍄𐌾𐌰𐌽( inliuhtjan, “to enlighten, illuminate”). More at inlight, -en.

etymonline

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enlighten (v.)

late 14c., "to remove the dimness or blindness" (usually figurative, from one's eyes or heart); see en- (1) + lighten (v.2). From 1660s as "supply with intellectual light." Literal senses are later and less common in English: "put light in" is from 1580s; "shed light upon" is from 1610s. Related: Enlightened; enlightening. Old English had inlihtan "to illuminate, enlighten."