A cylindrical cavity of diameter \(D\) and depth \(L\) is machined in a metal block, and conditions are such that the base and side surfaces of the cavity are maintained at \(T_{1}=950 \mathrm{~K}\) and \(T_{2}=650 \mathrm{~K}\), respectively. Approximating the surfaces as black, determine the emissive power of the cavity if \(L=25 \mathrm{~mm}\) and \(D=15 \mathrm{~mm}\).