Final answer:
Observers at a safe distance from a nuclear bomb test feel the heat but not the x-rays because air molecules absorb x-rays due to their high photon energy that can ionize, while infrared radiation has lower photon energies and can pass through air.
Step-by-step explanation:
The reason that observers at a safe distance from an atmospheric test of a nuclear bomb feel its heat but receive none of its x-rays lies in the interaction between different types of electromagnetic radiation and the atmosphere. The correct answer is a) X-rays are absorbed by air molecules; infrared passes through.
The high photon energy of x-rays allows them to ionize air molecules, which leads to the absorption of x-rays. In contrast, infrared radiation has much lower photon energies that do not significantly alter atoms and molecules, therefore allowing it to pass through the air. This explains why air is opaque to x-rays but transparent to infrared radiation.
Transparent materials, including air, do not generally absorb visible light because there is no matching energy step in the atoms or molecules that could absorb the light. Higher frequency radiations like UV, x-ray, and gamma rays, however, have sufficient photon energy to ionize materials and are thus absorbed by air.
Infrared radiation is strongly absorbed by water because water molecules have many states with energy separations that fall within the infrared range. Air, however, is nearly transparent to many infrared frequencies, explaining why infrared radiation from a nuclear bomb can reach an observer without being absorbed to the same degree as x-rays.