Microwaves operate at a frequency of 2.40 GHz. Find the minimum thickness for a plastic tray with a refractive index of 1.45 that will cause enhanced reflection of microwaves incident normal to the tray.

Arigid tank is divided into two equal parts by a partition. one part of the tank contains 3 kg of compressed liquid water at 400 kpa and 60°c while the other part is evacuated. the partition is now removed, and the water expands to fill the entire tank. determine the entropy change of water during this process, if the final pressure in the tank is 40 kpa. use steam tables.