radiation
when the suns radiation fall on the earth and its objects they receive heat energy and hence get heated. Thus the suns heat reaches the earth by. the process of radiation
Absence of medium is the factor which shows that heat from the sun does reach the earth's surface by convection.
The heat of the sun reaches the earth only through radiation because radiation does not require any medium for the transfer of energy but convection and conduction required a medium to transfer heat energy.
We know that there is no medium present in vacuum so convection and conduction are impossible in a vacuum that's why the sun's heat does not reach the earth by conduction or convection.
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