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''A black dot on as a porcelain cup appp...

''A black dot on as a porcelain cup apppears dark. But when heated to a high temperature it becomes brighter than the rest of the cup.'' Why ?

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First it appears dark because it absorbs, more heat radiation than the white porcelain. But when heated it appears white because good absorbs and good emitters, in accordance with Kirchhoff.s law.
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