Final answer:
The fireman feels a hot door due to the transfer of heat from the fire on the other side through thermal radiation.
Step-by-step explanation:
The fireman feels a hot door due to the transfer of heat from the fire on the other side. The heat is transferred through a process called thermal radiation. Thermal radiation is the transfer of heat energy in the form of electromagnetic waves, specifically infrared radiation. The fire emits infrared radiation, which is not visible to the human eye but is felt as heat. When the fireman feels the hot door, it indicates that there is a significant amount of radiant heat coming from the fire.