The photograph shows an experimental device at sandia national laboratories in new mexico. this device is a mirror that focuses sunlight to heat sodium to a boil, which then heats helium gas in an engine. the engine does the work of driving a generator to produce electricity. the sodium unit and the engine are labeled in the photo. (a) what kind of mirror, concave or convex, is being used? (b) where is the sodium unit located relative to the mirror? express your answer in terms of the focal length of the mirror.