Are you talking about [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=121.msg552#msg552]Lens (thick lens) and Mirror[/url] (Please provide URL next time so that it is easier for me to find where might be the problem.)
The error message about [b]Scanner invalid character:  [/b] was coming from another message posted by other user.
It is fixed now.
Please try it again and let me know the result! Thanks!