A speed that will permit a train or other equipment to stop within one half the range of vision short of a train, engine, railroad car, man or equipment fouling a track, stop signal or a derail or switch lined improperly. When a train or engine is required to move at restricted speed, the crew must keep a lookout for a broken rail and not exceed 20 mph:

Light from two different sources are incident upon the same side of a polarizing filter. light from one source is polarized with total intensity ip, while light from the other source is unpolarized with total intensity iu. as you rotate the polarizing filter through 360 degrees, the intensity of the transmitted light on the other side varies by a factor of 9. what is the relative intensity of the initial polarized source to the unpolarized source, or ip/iu?