>>Sergey, Thekyr, ftfly Thank you for answering my question! I felt "upon" has various meanings.

on is more like a cup on a desk , touched upon is more like has some space between two things, like the light is upon the desk, i somehow feel like my poor german is making my english more poor now, reading my own sentense makes me feel confused . hope you could understand what i mean

You can also use "upon" like this: "Upon seeing him, she blushed." (Another Way to say "Whe she saw him, she blushed.") I think in this case you can't use "on".

The word "upon" has the same meaning as the word "on" but it sounds formal. There are some phrases where only "upon" must be used, for example: "once upon a time". Only "upon" is used to emphasize that there is a large number or amount of something: Mile upon mile of dusty road, hundreds upon hundreds of questions.