beaming smile; radiant smile
Describes a bright, cheerful, and open smile that conveys happiness or relief. Often used in literary or descriptive contexts.
Seeing her beaming smile made me happy too.
After the exam results were announced, he left the classroom with a radiant smile.
晴れやか is a na-adjective meaning 'bright' or 'radiant' and can modify other nouns, but 晴れやかな笑顔 is a fixed compound with its own entry.
Compound of 晴れやか (bright, radiant) and 笑顔 (smile). The exact historical derivation is uncertain, but the phrase is conventionally used to describe a particularly bright and cheerful smile.