a copper sphere is suspened in an evacuatd chamber at 300K. the sphere is maintained at 900 k by heating electrically. a total of 300w is needed to do dis. wen haf of d surface of copper is completely blackened to make dis portion completely absorbtive 600 w is needed to maintain same temp of sphere. the emissivity of copper is
Initially both the halves were radiatting 150W each (300/2)
On blackening one side, the other side will keep on iradiating 150W while blackened part will iradiate 450W (600-150).
So the emmisivity of copper = 150/450 = 1/3