[quote author=engrg1 link=topic=1155.msg9546#msg9546 date=1348062663]
Looking at the old simulation remind me of the times when I am using Prof Huang's lens applet to teach my students about drawing ray diagram :-) That is like 4 years ago.
the older applet is here

this a new version made in EJS

completed the dotted lines requested by teachers

teacher also asked
"Also regarding the object at focal length, I think there should be no
image so is it ok to display v = infinity?
Although I know in theory, 1/f = 1/u + 1/v but is it still valid when
there is no image?"

but i check there is no image