Assume that a spy satellite in orbit carries a telescope that can resolve objects on the ground as small as the width of a car’s license plate. if the satellite is in orbit at 400 km above the earth’s surface (which is typical for orbiting telescopes) and it focuses light of wavelength 505 nm , what minimum diameter of the mirror (or objective lens) would be needed (take the width of a typical license plate to be about 30.0 cm )?
answer is not 89 cm

Asheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200oc and is permitted to achieve a steady-state diffusion condition. the diffusion coefficient for nitrogen in steel at this temperature is 6 x 10- 11 m2 /s, and the diffusion flux is found to be 1.2 x 10-7 kg/m2 -s. also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3 . how far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3 ? assume a linear concentration profile.