$(1)$ 表面のふく射熱流束$E$は， \[\begin{align}E&=\varepsilon\sigma T^4\\　 &=0.25\times5.67\times10^{-8}\times(800+273.15)^4\\ &=1.88\times 10^4 \,\rm{W/m^2} \end{align}\] $(2)$ 表面のふく射熱流束$E$は， \[\begin{align}E&=\varepsilon\sigma T^4\\　 &=0.9\times 5.67\times 10^{-8}\times (800+273.15) ^4\\ &=6.77\times 10^4 \,\rm{W/m^2} \end{align}\]