It's more about the appearance of working vs actually having work to do. I used to come in at 7:30 and leave at 4:30 because my commute was terrible other wise (2 hours each way). The few nights I stayed until 6-7pm I noticed the engineers that came in at 10-11am were all gone by 5:30pm but to most managers it looked like they were "working harder" than me when I left at 4:30.

Where I worked, official hours are 8.30 to 6. In Asia of course.

Is the official work week 47.5 hours?

In the U.S., while I think it's true that people tend to work closer to 50 hours, we pay lip service to the "40 hour work week".