I read in a history textbook that Roosevelt kept the Manhattan Project secret from Congress,most of his cabinet and his vice-president. So when did the general public first find out about both the atomic bomb and the Manhattan Project ? And what was their reaction ?
A very similar question was answered here just a day ago.
As for the reaction, it was a mixture of excitement (the war might be over soon) and dread (what does the future hold?), to sum it up rather broadly. I find these editorial cartoons and these headlines to be very nice ways to get a nice, intuitive sense of what the vibe "in the air" was at the time.