Visual Conventions / Icons
On some newer equipment, on/off rocker switches are marked with a hyphen on one side and a circle on the other, while others have a hyphen on one side and nothing at all on the other. Since a hyphen could be a minus sign and a circle could be a zero, while nothing is vacancy, how does one decide which side is on and which is off? Wouldn't a minus and a plus (or a zero and a plus) be clearer? This matter is of major importance if the unit is misbehaving and one has to know whether the power is actually on.
Comments
To answer your question, the modern power icon you are describing is actually a 0/1. It is Binary; 0= off = down on the switch, 1 = on = up on the switch. I have also seen manufacturers rotate this so that it is a circle and dash making the icons less intuitive, a huge mistake in my book.
This is a pretty basic modern User Interface paradigm that has taken hold throughout the industry [both pro and consumer] over the last few years.
There are many great books on the subject of User Interface Design, and how almost everything today has some sort of interface.
*edit - here is a good one:
The Design of Everyday Things: Donald A. Norman