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 Fig. A).

Usage：

Click left mouse button toggle for more information.
(The ratio of those number shown in yellow is related to the index of refraction!)
You can click white/blue/green dots and drag the mouse button to move it around.

Right click the mouse button to start the animation again, Click it again to pause.

Click near label air/water will switch the medium.

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).