6:50 AM
@आर्यभट्ट The cylinder is acting as a black body radiator.
This works because the hole at the end of the cylinder is small compared to the internal volume of the cylinder. That means radiation created in the cylinder is absorbed and re-emitted by the walls of the cylinder many times before it eventually escapes out through the hole at the end and is emitted as black body radiation.
In this process of multiple absorption and re-emission the spectrum becomes a good approximation to the black body spectrum.
