In a way that produces or reflects much light; in a cheerful or intelligent manner.
From Old English 'beorht' meaning 'light, bright.' The '-ly' suffix converts the adjective 'bright' to an adverb. The word has meant both literally luminous and figuratively intelligent/cheerful since medieval times.
Your eyes actually perceive brightness logarithmically, not linearly—doubling the light doesn't feel twice as bright to your brain. That's why sunset photos look so dull compared to real sunsets; your camera and eyes process light completely differently.
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