Dear professor Hwang,
The applet does not seems to work right. I tested it with 2 mirrors as following:
x              y              dx                dy
-0.352    -0.5          0.5                0.5
-0.310    0.025        0.5                0.5

ray speed =0.9, 18 rays
the refection rays look wrong. here is a screenshot.

Regards

Young Xu