A conical hole is machined in a metal whose emissivity is 0.5. The hole is 2.5 cm in diameter at the surface and 7.5 cm deep. If the metal block is heated to 800 oK, calculate the radiation emitted by the hole. Calculate the apparent emissivity of the hole defined as the ratio of the actual energy emitted to that energy which would be emitted by a black surface having an area equal to that of the opening and a temperature equal to that of the inside surfaces.