Which of the following is true regarding radiation from a furnace fire?

Radiation from a furnace fire does indeed travel in straight lines, which is consistent with the fundamental principles of thermal radiation. Radiation emits from a hot surface—the furnace fire in this case—broadly in all directions. Unlike convection, which requires a fluid medium to transfer heat, or conduction, which requires direct contact between materials, radiation can move through a vacuum without the need for any material medium, making it unique.

The straight-line travel of radiation allows for effective heat transfer over distance without interaction with other materials, unless intercepted by a surface where it can be absorbed or reflected. This characteristic is especially important in understanding the behavior of heat in systems, including furnaces, where radiant heat significantly contributes to thermal efficiency.

In assessing the other options, it is clear that radiation is indeed present in both oil and gas firing, contradicting the assertion that it is only present in oil. Furthermore, while it is true that radiant heat can penetrate certain materials, it does not directly pass through superheater tube walls in the same manner as the concept of radiation suggests. Lastly, radiation does not require a transfer medium, which is a fundamental physical characteristic that makes it different from other forms of heat transfer.