Dear Fu-Kwun
Giving the two speeds is a lot better indeed. In this way it is obvious where it is all about.
The strange thing is that both with the sliders and the lower graph the axis is pointed downward, giving higher values when going down.
We are used to axis pointing upward, giving higher values when higher up. Is it possible to change that?

Another improvement would be to add the SUM of t1 and t2, to make it very clear also graphically where the real minimum is.

(Since you invite me to launch more ideas….)

Thirdly, new idea, is to take identically the same applet, but to change v so that it changes between 0,5 and 10 m/s for both “media” and the corresponding time is then in seconds, not ns. Why? This would correspond to situations like water/beach, water/riverbank, grass/asphalt, deep sand/ grass etc. . I am thinking of an active teaching method that goes as follows: there is a line on the playground, and on each side there is a mark, 10-20 m away from the line. Learners should go from one mark to the other, in one “medium” walking, in the other running, or in one walking, in the other on their knees. Or running, and hands and feet-running. To create two different speeds in any case. Which path to take? The simulation would make it clear one more time that it is not the shortest, but the fastest way that is valid.

In my previous mail, about the rounded edge: see attachment for a quick drawing: inside the rounded area there is water, outside is air (in principle). An object of dimensions AA’ looks  thicker (see dotted lines) when looking from outside. I would just take a dot A and B to drag around, and the light goes via the round border from A to B. The point on the border follows the rounded curve now. The calculations are the same basically I guess.

Fu-Kwun: The sum of t1+t2 was already shown in the lower plotingPanel.
The higher the green curve , the lower the sum of t1+t2.

Wim: Sorry, I did not understand it, because in my view the graph was upside down: I did not recognise the minimum where it was in your graph a "top" of the curve.
So the only thing that is really is still  needed before I can "sell" it to my teachers is putting the plot upside down (max above, minima below).
Thank you again.