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The outside of this glassblowing oven is clearly discernible, as is the front surface which is slightly darker than the interior. The crucible, which is almost at the oven temperature, is more difficult to discern. Except for the cool glassblower's pipe, no features are visible within the oven because everything is at the same temperature and emits radiation identically. This oven provides a reasonable approximation of a blackbody. An even better approximation would be to seal the front of the oven, leaving only a small hole, so that the entire interior would be at exactly the same temperature. Any external light incident on the hole would be absorbed, and the only light radiated would come from the walls. Just such a technology was used for the early measurements on which Planck based his radiation law. (Unit: 5)