The path of a ray of light between two points is the path that minimizes the travel time.
This java applet let you visualize the above statement.
There are two region with different color (one for air and one for water).
The white dot is the light source.

 The program show all possible paths for light emitted from the point source, reach the air/water interface then all heading towards blue/green dot.
The real path which arrive at either blue or green dot, is the path that has the smallest traveling time. (shown as white paths

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Usage:

1. Left-click mouse for more information.(Ratio of numbers (red) related to refractive index!)
   
2. Left-click and drag red, green or blue dots to move points.
   
3. Left-click on air/water label to switch medium.
   
4. Left-click on possible/real label (top left corner) to toggle.
   
5. Right-click mouse to start/pause animation.
   

Right click the top left corner will show all the real paths for light emitted from point source.
(Click it again to toggle between possible paths and real paths Fig.B